ENGINEERING
HYDROLOGY
1 |
Introduction to
Hydrology &
Weather Basics
Water is vital to life and development in all parts of the world. In
Third World countries where the agricultural sector plays a key role
1.1 | Introduction to
Hydrology
Hydrology
INTENDED LEARNING OUTCOMES
• Define Hydrology and understand
its application in engineering
• Differentiate Hydrology from
Hydraulics
in their economic growth, the management of water resources is
an item of high priority in their developmental activities. The basic
inputs in the evaluation of water resources are from hydrological
parameters and the subject of hydrology forms the core in the
evaluation and development of water resources. In the civil
engineering curriculum, this subject occupies an important
position.
What is hydrology?
• Hydrology means the science of water. It is the science that
deals with the occurrence, circulation and distribution of water of
the earth and earths atmosphere.
• Scientific Hydrology the study which is concerned with the
academic aspects.
• Engineering or Applied Hydrology a study concerned
with engineering applications. In a general sense
engineering hydrology deals with (i) estimation of water
resources, (ii) the study of processes such as precipitation,
runoff, evapotranspiration and their interaction and (iii) the
study of problems such as fl oods and droughts, and
strategies to combat them.
Hydrology in
engineering
Hydrology finds its greatest application in the design and
operation of water-resources engineering projects, such as
those for (i) irrigation, (ii) water supply, (iii) flood control, (iv)
water power, and (v) navigation. In all these projects
hydrological investigations for the proper assessment of the
following factors are necessary:
1. The capacity of storage structures such as reservoirs.
2. The magnitude of flood flows to enable safe disposal of the
excess flow.
3. The minimum flow and quantity of flow available at various
seasons.
4. The interaction of the flood wave and hydraulic structures,
such as levees, reservoirs, barrages and bridges.
The hydrological study of a project should necessarily precede
structural and other detailed design studies. It involves the
collection of relevant data and analysis of the data by applying
the principles and theories of hydrology to seek solutions to
practical problems
Hydraulics
• HYDRAULICS is defi ned as the study of the mechanical
behavior of water in physical systems. In engineering terms,
hydraulics is the analysis of how surface, and/or subsurface
fl ows move from one point to the next. Hydraulic analysis is
used to evaluate fl ow in rivers, streams, storm drain networks,
water aqueducts, waterlines, sewers, etc.
ENGINEERING
HYDROLOGY
1 |
Introduction to
Hydrology &
Weather Basics
Earth's water is always in movement, and the natural water cycle,
1.2 | Hydrologic Cycle
and the Human Impact
Hydrologic
cycle
INTENDED LEARNING OUTCOMES
• Describe the Hydrologic Cycle and
identify its different components
• Apply Water Budget Equation and
Balance in solving hydrological
problems
also known as the hydrologic cycle, describes the continuous
movement of water on, above, and below the surface of the Earth.
Water is always changing states between liquid, vapor, and ice,
with these processes happening in the blink of an eye and over
millions of years.
Where does all the Earth's water come from? Ancient, primordial
Earth was an incandescent globe made of magma, but all magmas
contain water. Water set free by magma began to cool down the
Earth's atmosphere, and eventually the environment became cool
enough so water could stay on the surface as a liquid. Volcanic
activity kept and still keeps introducing water into the atmosphere,
thus increasing the surface-water and groundwater volume of the
Earth.
Each path of the hydrologic cycle involves one or more of the
following aspects: (i) transportation of water, (ii) temporary storage
and (iii) change of state.
Note: Our information only covers the natural water cycle, which
does not take human activities into account. In today's world,
humans have a major impact on many components of the water
cycle.
back to the atmosphere even while falling. Another part
may be intercepted by vegetation, structures and other
such surface modifications from which it may be either
evaporated back to atmosphere or move down to the
ground surface.
A portion of the water that reaches the ground enters the
earths surface through infiltration, enhance the moisture
content of the soil and reach the groundwater body.
Vegetation sends a portion of the water from under the
ground surface back to the atmosphere through the
process of transpiration. The precipitation reaching the
ground surface after meeting the needs of infiltration and
evaporation moves down the natural slope over the
surface and through a network of gullies, streams and
A convenient starting point to describe the cycle is in the
rivers to reach the ocean. The groundwater may come to
oceans. Water in the oceans evaporate due to the heat
the surface through springs and other outlets after
energy provided by solar radiation. The water vapor moves
spending a considerably longer time than the surface flow.
upwards and forms clouds. While much of the clouds
The portion of the precipitation which by a variety of paths
condense and fall back to the oceans as rain, a part of the
above and below the surface of the earth reaches the
clouds is driven to the land areas by winds. There they
stream channel is called runoff. Once it enters a stream
condense and precipitate onto the land mass as rain, snow,
channel, runoff becomes stream flow.
hail, sleet, etc. A part of the precipitation may evaporate
components of the water cycle:
transportation & storage
The main components of the hydrologic cycle can be broadly
It is important to note that the total water resources of the earth
classified as transportation ( flow) components and storage
are constant and the sun is the source of energy for the
components as below:
hydrologic cycle. A recognition of the various processes such
TRANSPORTATION
COMPONENTS
STORAGE COMPONENTS
Precipitation
Evaporation
Transpiration
Infiltration
Runoff
Storage on the land surface
(Depression storage, ponds, lakes,
reservoirs, etc)
Soil moisture storage
Groundwater storage
Atmosphere
Schematically the interdependency of the transportation
components can be represented as in the figure on the right.
The quantities of water going through various individual paths
of the hydrological cycle in a given system can be described by
the continuity principle known as water budget equation or
hydrologic equation.
as evaporation, precipitation and groundwater flow helps one
to study the science of hydrology in a systematic way. Also, one
realizes that man can interfere with virtually any part of the
hydrologic cycle, e.g. through artificial rain, evaporation
suppression, change of vegetal cover and land use, extraction
• EVAPORATION. Evaporation is the process by which water
• PRECIPITATION. Precipitation is water released from clouds
changes from a liquid to a gas or vapor. Evaporation is the
in the form of rain, freezing rain, sleet, snow, or hail. It is the
primary pathway that water moves from the liquid state back
primary connection in the water cycle that provides for the
into the water cycle as atmospheric water vapor. Studies have
delivery of atmospheric water to the Earth. Most precipitation
shown that the oceans, seas, lakes, and rivers provide nearly
falls as rain.
90 percent of the moisture in the atmosphere via evaporation,
• SURFACE RUNOFF. Surface runoff is nothing more than
with the remaining 10 percent being contributed by plant
water “running off” the land surface. Surface runoff is affected
transpiration.
by both meteorological factors and the physical geology and
• CONDENSATION. Condensation is the process in which
topography of the land. Only about a third of the
water vapor in the air is changed into liquid water.
precipitation that falls over land runs off into streams and
Condensation is crucial to the water cycle because it is
rivers and is returned to the oceans. The other two-thirds is
responsible for the formation of clouds. These clouds may
evaporated, transpired, or soaks (infi ltrates) into groundwater.
produce precipitation, which is the primary route for water to
Surface runoff can also be diverted by humans for their own
return to the Earth's surface within the water cycle.
uses.
• TRANSPIRATION.
• GROUNDWATER STORAGE. The upper layer of the soil is
Transpiration is essentially
the unsaturated zone, where water is present in varying
evaporation of water from
amounts that change over time, but does not saturate the soil.
plant leaves. Studies have
Below this layer is the saturated zone, where all of the pores,
revealed that transpiration
cracks, and spaces between rock particles are saturated with
accounts for about 10
water. The term groundwater is used to describe this area.
percent of the moisture in
Another term for groundwater is "aquifer," although this term
the atmosphere, with
is usually used to describe water-bearing formations capable
oceans, seas, and other
of yielding enough water to supply peoples' uses.
bodies of water (lakes,
rivers, streams) providing nearly 90 percent, and a tiny
amount coming from sublimation (ice changing into water
vapor without fi rst becoming liquid).
move vertically and horizontally through the soil and
subsurface material. Some of the water may infi ltrate deeper,
recharging groundwater aquifers.
• PERCOLATION. Percolation is the movement of water
through the soil and porous or fractured rock.
• SUBLIMATION. Sublimation is the conversion between the
solid and the gaseous phases of matter, with no intermediate
liquid stage. For those of us interested in the water cycle,
sublimation is most often used to describe the process of
snow and ice changing into water vapor in the air without fi rst
melting into water. The opposite of sublimation is
"deposition", where water vapor changes directly into ice—
such a snowfl akes and frost.
• GROUNDWATER FLOW.
Water moves underground
downward and sideways, in great quantities, due to gravity
and pressure. Eventually it emerges back to the land surface,
into rivers, and into the oceans to keep the water cycle going.
People have been using groundwater for thousands of years
and continue to use it today, largely for drinking water and
irrigation.
• INFILTRATION. Infi ltration is the process by which water on
the ground surface enters the soil. Some water that infi ltrates
will remain in the shallow soil layer, where it will gradually
Where and how
much water is there
on earth?
This image shows blue spheres representing relative amounts
of Earth's water in comparison to the size of the Earth. These
images attempt to show three dimensions, so each sphere
represents "volume." They show that in comparison to the
volume of the globe, the amount of water on the planet is very
small.
The largest sphere represents all of Earth's water. Its diameter
is about 860 miles (the distance from Salt Lake City, Utah, to
Topeka, Kansas) and has a volume of about 332,500,000 cubic
miles (mi3) (1,386,000,000 cubic kilometers (km3)). This sphere
includes all of the water in the oceans, ice caps, lakes, rivers,
groundwater, atmospheric water, and even the water in you,
your dog, and your tomato plant.
The blue sphere over Kentucky represents the world's liquid
fresh water (groundwater, lakes, swamp water, and rivers).
Do you notice the "tiny" bubble over Atlanta, Georgia? That
one represents fresh water in all the lakes and rivers on the
planet.
Water budget
equation
In applying this continuity equation [Eq. (1.1)] to the paths of
the hydrologic cycle involving change of state, the volumes
considered are the equivalent volumes of water at a reference
temperature. In hydrologic calculations, the volumes are often
expressed as average depths over the catchment area. Thus,
For a given problem area, say a catchment, in an interval of
time Dt, the continuity equation for water in its various phases
is written as
Mass inflow mass outflow = change in mass storage
If the density of the inflow, outflow, and storage volumes are
the same
107
) = 1 m = 100
is 10⁷ m³ , it corresponds to a depth of (
10 × 106
cm. Rainfall, evaporation and often runoff volumes are
expressed in units of depth over the catchment.
While realizing that all the terms in a hydrological water
budget may not be known to the same degree of accuracy, an
Vi - V0 = ΔS
where:
for example, if the annual stream flow from a 10 km² catchment
(1.1)
expression for the water budget of a catchment for a time
interval Dt is written as
Vi = inflow volume of water into the
P - R - G - E - T = ΔS
problem area during the time
period
V₀ = outflow volume of water from the
problem area during the time period
ΔS = change in the storage of the water
volume over and under the given area
during the given period
where:
(1.2-a)
P = precipitation
R = surface runoff
G = net groundwater flow out of the
catchment
E = evaporation
The storage S consists of three components as
S = Ss + Ssm + Sg
where:
Ss = surface water storage
Ssm =
water in storage as soil moisture
Sg = water in storage as groundwater
Thus in Eq. (1.2-a) S = ΔSs + ΔSsm + ΔSg
All terms in Eq. (1.2-a) have the dimensions of volume.
Note that all these terms can be expressed as depth over
the catchment area (e.g. in centimeters), and in fact this is a
very common unit.
In terms of rainfall-runoff relationship, Eq. (1.2-a) can be
represented as
R=P-L
where:
(1.2-b)
L = Losses = water not available to runoff
due to infiltration (causing addition to
soil moisture and groundwater
storage), evaporation, transpiration and
surface storage.
World water
Balance
considered are the equivalent volumes of water at a reference
temperature. In hydrologic calculations, the volumes are often
expressed as average depths over the catchment area. Thus,
for example, if the annual stream flow from a 10 km² catchment
For a given problem area, say a catchment, in an interval of
time Dt, the continuity equation for water in its various phases
is written as
107
) = 1 m = 100
is 10⁷ m³ , it corresponds to a depth of (
10 × 106
cm. Rainfall, evaporation and often runoff volumes are
expressed in units of depth over the catchment.
Mass inflow mass outflow = change in mass storage
While realizing that all the terms in a hydrological water
If the density of the inflow, outflow, and storage volumes are
expression for the water budget of a catchment for a time
the same
interval Dt is written as
Vi - V0 = ΔS
where:
budget may not be known to the same degree of accuracy, an
(1.1)
Vi = inflow volume of water into the
problem area during the time
period
V₀ = outflow volume of water from the
problem area during the time period
ΔS = change in the storage of the water
volume over and under the given area
during the given period
In applying this continuity equation [Eq. (1.1)] to the paths of
the hydrologic cycle involving change of state, the volumes
P - R - G - E - T = ΔS
where:
(1.2-a)
P = precipitation
R = surface runoff
G = net groundwater flow out of the
catchment
E = evaporation
T = transpiration
ΔS = change in storage
• RESIDENCE TIME. The average duration of a particle of
water to pass through a phase of the hydrologic cycle is
known as the residence time of that phase. It could be
calculated by dividing the volume of water in the phase by
the average fl ow rate in that phase. For example, by
assuming that all the surface runoff to the oceans comes
from the rivers, the volume of water in the rivers of the
world = 0.00212 M km³, the average fl ow rate of water in
global rivers = 44700 km³/year. Hence residence time of
global rivers, Tr = 2120/44700 = 0.0474 year = 17.3 days.
1.2 | Hydrologic Cycle
and the Human Impact
human
impact
INTENDED LEARNING OUTCOMES
• Identify and examine human
activities that impact the
Hydrologic Cycle
Human activities that
impact the hydrological
cycle
• DEFORESTATION. Forests transport large quantities of water into the
atmosphere via plant transpiration. This replenishes the clouds and
instigates rain that maintains the forests. When deforestation occurs,
precious rain is lost from the area, fl owing away as river water and
causing permanent drying.
• URBANIZATION. Impervious surfaces associated with urbanization
alter the natural amount of water that takes each route. The
consequences of this change are a decrease in the volume of water
that percolates into the ground, and a resulting increase in volume
and decrease in quality of surface water.
• REFORESTATION AND AFFORESTATION. Tree growth can consume
more water than other shorter vegetation. According to the mass
balance principle, if more water is used by trees, less water will fl ow
into rivers and lakes or recharge the groundwater that people can
directly use.
• IRRIGATION. Irrigation is the artifi cial watering of land that
• CLOUD SEEDING. Cloud seeding is a weather modifi cation,
does not get enough water through rainfall. ... Unfortunately it
where you change the amount or type of precipitation that
removes water from rivers and can cause surface run-off and
falls from clouds through the usage of harmful substances.
leaching. The problem with irrigation is that it removes water
During this process, the substances that fall from the clouds
from its natural source and often causes leaching and run-off
are dispersed into the air, causing cloud condensation which
where it is used.
further affects climate conditions.
• AGRICULTURE. Causes reduced vegetation cover and soil
compaction from machinery use can reduce the amount of
water that drains into the soil and therefore increase run off.
this can increase soil erosion and the need for irrigation.
• INDUSTRY. Industries cause enormous water pollution: By
directly discharging their untreated effl uents into water
bodies like rivers and lakes. By letting their polluting effl uents
fl ow onto land so that they get absorbed into the soil and
pollute underground water.
• TRANSPORT. Air pollution caused by transportation have a
direct effect on water pollution. When particles like sulfur
dioxide get high into the air they can combine with rain to
produce acid rain. Acid rain can turn lakes acidic, killing fi shes
and other animals.
• DAMS. Dams change the timing, amount and chemical
composition of a river's fl ow, leading to dramatic changes to
groundwater-storing fl oodplains and wetlands. Such changes
can lead to the destruction of forests, which among other
things help regulate local climate.
ENGINEERING
HYDROLOGY
1 |
Introduction to
Hydrology &
Weather Basics
WATERSHED
1.3 | Philippine
Watersheds
philippine
watersheds
INTENDED LEARNING OUTCOMES
• Discuss the different definitions of
watershed according to Philippine
legislature
• Discuss the important properties
of watershed and its significance
in hydrologic analyses
• Identify the stream order using
Horton’s Law
• Draw a watershed’s boundaries
• “A body of land bounded above by a ridge or water divide
and below by the level at which water drains from it.” (Bruce,
2017)
• A high area of land where rain collects, some of it flowing
down to supply rivers, lakes, etc., at lower levels.
• An area of land on a slope which drains its water into a stream
and its tributaries.
• It is also called as catchment area or drainage basin.
Philippine
watersheds
• According to the River Basin COntrol Office (RBCO), the
philippines has 142 critical watersheds.
• The watershed referred to under the IRR of RA 7942 is
“critical watershed,” which means a drainage area of a
river system, lake or water reservoir supporting existing
and proposed hydroelectric power, domestic water
supply, geothermal power and irrigation works that
need immediate rehabilitation and protection to
minimize soil erosion, improve water yield and prevent
possible flooding.
• PD 705 defi nes watershed as “a land area drained by a
stream or fi xed body of water and its tributaries having
a common outlet for surface run-off.” Moreover, the
same law defi nes watershed reservation as “a forest
land reservation established to protect or improve the
conditions of the water yield thereof or reduce
sedimentation.”
Watershed
Delineation
All watershed delineation means is that you’re drawing line on
a map to identify a watershed’s boundaries. These are typically
drawn on topographic maps using information from contour
lines. Contour lines are lines of equal elevation, so any point
along a given contour line is the same elevation.
INFORMATION SOURCES:
NAMRIA - National Mapping Resource Information Agency
DENR - Department of Environment and Natural Resources
Local Government
DIGITAL ELEVATION MODELS (DEM’s)
DEM’s store topographic data in the form of grid cells.
Typically, these grid cells have a resolution of 30 meters and
elevation intervals of 1 meter. Using a DEM within a
Geographic Information System (GIS), digital terrain analysis
(DTA) such as calculating slopes, fl ow lengths, and delineation
of watershed boundaries and stream networks can be
performed.
Delineation STEps
STEP 1: Mark the outlet. This is generally our point of
interest for designing a structure or monitoring
location.
STEP 2: Mark the high points adjacent to the water body.
STEP 3: Connect the marks. Always cross a contour line
at right angles (perpendicular).
watershed geomorphology:
Principal watershed
characteristics
WATERSHED GEOMORPHOLOGY refers to the physical
characteristics of the watershed. Certain physical properties
signifi cantly affect the characteristics of runoff and as such are of
great interest in hydrologic analyses.
• DRAINAGE AREA. The drainage area (A) is the probably the single
• WATERSHED SLOPE. Flood magnitudes refl ect the momentum of the
most important watershed characteristic for hydrologic design. It
runoff. Slope is an important factor in the momentum. Both watershed
refl ects the volume of water that can be generated from rainfall. It is
and channel slope may be of interest. Watershed slope refl ects the
common in hydrologic design to assume a constant depth of rainfall
rate of change of elevation with respect to distance along the principal
occurring uniformly over the watershed. Under this assumption, the
fl ow path. Typically, the principal fl ow path is delineated, and the
volume of water available for runoff would be the product of rainfall
watershed slope (S) is computed as the difference in elevation (∆E)
depth and the drainage area. Thus the drainage area is required as
between the end points of the principalfl ow path divided by the
input to models ranging from simple linear prediction equations to
hydrologic length of the fl ow path (L):
complex computer models.
• WATERSHED LENGTH. The length (L) of a watershed is the second
watershed characteristic of interest. While the length increases as the
drainage increases, the length of a watershed is important in
hydrologic computations. Watershed length is usually defi ned as the
distance measured along the main channel from the watershed outlet
to the basin divide. Since the channel does not extend to the basin
divide, it is necessary to extend a line from the end of the channel to
the basin divide following a path where the greatest volume of water
would travel. The straight-line distance from the outlet point on the
watershed divide is not usually used to compute L because the travel
distance of fl oodwaters is conceptually the length of interest. Thus, the
length is measured along the principal fl ow path. Since it will be used
for hydrologic calculations, this length is more appropriately labeled
the hydrologic length.
S = ∆E / L
• WATERSHED SHAPE. Basin shape is not usually used directly in
hydrologic design methods; however, parameters that refl ect basin
shape are used occasionally and have a conceptual basis. Watersheds
have an infi nite variety of shapes, and the shape supposedly refl ects
the way that runoff will “bunch up” at the outlet. A circular watershed
would result in runoff from various parts of the watershed reaching the
outlet at the same time. An elliptical watershed having the outlet at one
end of the major axis and having the same area as the circular
watershed would cause the runoff to be spread out over time, thus
producing a smaller fl ood peak than that of the circular watershed.
• LAND USE AND SOIL CHARACTERISTICS. Land use and soil
characteristics affect both the volume and timing of runoff. During a
rainstorm, fl ow from an impervious, steeply sloped, and smooth,
surface make a little retardation and no loss to the fl ow. In comparison,
fl ow along a pervious grassy hill of the same size will produce
retardation and signifi cant loss to the fl ow due to infi ltration. A lot of
information about land use has been gathered over the years and is
available from maps or as GIS data sets. In some cases, a fi eld survey is
necessary to determine the various land uses within a watershed. Many
hydrological analyses deal with assessing the effect of land use
changes on runoff. For example, the development of a new residential
neighborhood is likely to have signifi cant impact on runoff
characteristics. Surface roughness, soil characteristics such as texture,
soil structure, soil moisture and hydrologic soil groups also affect the
runoff in various ways. For example; Soil properties affect the
infi ltration capacity. Soil particles are usually classifi ed as clay (d<0.002
mm), silt (0.002<d<0.02), or sand (d>0.02 mm). A particular soil is a
combination of clay, silt, and sand. Generally, soils with a signifi cant
portion of small particles have low infi ltration capacity, whereas sandy
soils have high infi ltration capacity.
Channel
geomorphology:
• CHANNEL LENGTH. The distance measured along the main channel
from the watershed outlet to the end of the channel, which is denoted
as Lc.
• CHANNEL SLOPE. (Sc = ΔEc / Lc), where ΔEc is the difference in
elevation between the points defining the upper and lower ends of the
channel and Lc is the length of the channel between the same two
points. If the channel slope is not uniform, a weighted slope may
provide and index that better reflects the effect of slope on the
hydrologic response of the watershed.
• DRAINAGE DENSITY. (D = Lt / A).The drainage density, D, is the ratio
of the total length of streams (Lt) with in a watershed to the total area of
the watershed; thus D has units of the reciprocal of length (1/L). A high
value of the drainage density would indicate a relatively high density of
streams and thus a rapid storm response. Values typically ranges from
1.5 to 6 mi/mi2.
• HORTON’S LAWS. Horton (from Horton’s infi ltration equation fame)
developed a set of “laws” that are indicators of the geomorphological
characteristics of watershed. The stream order is a measure of the
degree of stream branching within a watershed. Each length of stream
is indicated by its order (for example, fi rst-order, second-order, etc.). A
fi rst-order stream is an unbranched tributary, a second-order stream is
a tributary formed by two or more fi rst-order streams. A third-order
stream is a tributary formed by two or more second-order streams and
so on. In general, an nth order stream is a tributary formed by two or
more streams of order (n-1) and streams of lower order. For a
watershed, the principal order is defined as the order of the principal
channel. The figure below gives an example of stream ordering.
ENGINEERING
HYDROLOGY
1 |
Introduction to
Hydrology &
Weather Basics
Meteorology
METEOROLOGY is the science of the atmosphere. With
t h e i n c re a s i n g t e n d e n c y t o w a rd s p e c i a l i z a t i o n
characteristic of our time, the subject matter under the
general
heading of meteorology may be referred to
various subdivisions, or branches, depending partly on the
theoretical approach and partly on the application of
meteorology to human activities.
PHYSICAL METEOROLOGY deals with processes of a
purely physical nature, such as radiation, heat, evaporation,
condensation, precipitation, ice accretion, and optical,
acoustical and electrical phenomena.
CLIMATOLOGY or STATISTICAL METEOROLOGY
determines the statistical relations, mean values, normals,
frequencies, variations, distributions, etc., of the
meteorological elements.
H Y D R O M E T E O R O LO GY i s c o n c e r n e d w i t h
meteorological problems relating to water supply, fl ood
control, irrigation, etc.
The hydrologic characteristics of a region are determined
Hydrologic problems in which meteorology plays an
largely by its climate and its geological structure. Among
important role include determination of probable
the climatic factors that establish the hydrologic features of
maximum precipitation for spillway design, forecasts of
a region are the amount and distribution of precipitation;
precipitation for reservoir operations, and determination of
the occurrence of snow and ice; the effects of wind,
probable maximum winds over water surfaces for
temperature and humidity on evaporation and snowmelt.
evaluating resulting waves in connection with the design of
dams and levees.
Weather vs climate
The difference between weather and climate is a measure of time.
Weather is what conditions of the atmosphere are over a short
period of time, and climate is how the atmosphere "behaves" over
relatively long periods of time.
What Weather Means
What Climate Means
Weather is basically the way the atmosphere is behaving, mainly
Climate is the description of the long-term pattern of weather in a
particular area. Some scientists defi ne climate as the average
weather for a particular region and time period, usually taken over
30-years. It's really an average pattern of weather for a particular
region.
with respect to its effects upon life and human activities. The
difference between weather and climate is that weather consists of
the short-term (minutes to months) changes in the atmosphere.
Most people think of weather in terms of temperature, humidity,
precipitation, cloudiness, brightness, visibility, wind, and
atmospheric pressure, as in high and low pressure. In most places,
weather can change from minute-to-minute, hour-to-hour, day-today, and season-to-season.
There are really a lot of components to weather. Weather includes
sunshine, rain, cloud cover, winds, hail, snow, sleet, freezing rain,
flooding, blizzards, ice storms, thunderstorms, steady rains from a
cold front or warm front, excessive heat, heat waves and more.
When scientists talk about climate, they're looking at averages of
precipitation, temperature, humidity, sunshine, wind velocity,
phenomena such as fog, frost, and hail storms, and other measures
of the weather that occur over a long period in a particular place.
For example, after looking at rain gauge data, lake and reservoir
levels, and satellite data, scientists can tell if during a summer, an
area was drier than average. If it continues to be drier than normal
over the course of many summers, than it would likely indicate a
change in the climate.
the heat engine
1.4 | The Atmosphere
If our planet were just a polished rock, with no air or oceans, the sun would
heat up the surface to very high temperatures on the lit side, while the dark
side’s surface would fall to very low temperatures. Fortunately, the
The
atmosphere
INTENDED LEARNING OUTCOMES
• Discuss the composition,
characteristics and structure of the
atmosphere
• Define atmospheric stability
atmosphere, a gaseous blanket that traps heat and lets it go slowly, keeps
the earth at a reasonable temperature, but also turns the planet into a
genuine, although not very efficient, heat engine.
The earth as a whole receives incoming solar radiation and emits it back as
terrestrial radiation. The globally averaged net radiation is zero in the long‐
term mean. However, the net solar radiation drops off by about a factor of
four between the equator and the poles. The poles emit more radiation than
they receive from the sun, while the tropics receive more radiation than they
emit back (see fig. 1.1)
This difference causes the atmosphere to heat up more at the
equator than at the poles. It sets up an atmospheric conveyor
belt that transports the warm moist tropical air from its heat
source to a colder drier polar heat sink. The work done by the
atmospheric heat engine maintains the kinetic energy of the
general circulation against frictional dissipation, just as in a
Carnot cycle. The efficiency of this heat engine is rather small
and can be calculated using the following equation:
where η is the thermal efficiency and Tn the temperatures of
composition
The atmosphere (below 100 km) is roughly 78 % nitrogen and
21 % oxygen, by volume. Argon makes the remaining 1 %.
Other constituents are only present as traces and include, in
decreasing order of amount: Water vapour, Carbon dioxide,
Neon, Helium, Krypton, Hydrogen, Ozone.
Oxygen is a colorless, odourless, and tasteless element. It is
mostly seen as an inert diatomic gas at standard atmospheric
temperature and pressure. Nitrogen has the same
characteristics. Although scarce compared to nitrogen and
the source (H) and sink (C).
oxygen, water vapour, carbon dioxide, and ozone play an
Assuming the polar temperature to be ‐30 oC (243 K) and the
they absorb radiation and trap heat.
tropical temperature to be 30 oC (303 K), then ΔT is 60 K, and
ΔT/TH would give about 20 %. In actual fact, and all considered,
important role in the thermal structure of the atmosphere as
In addition to gaseous substances, the lower levels of the
the efficiency is really only about 1 %.
atmosphere contain quantities of solid particles. These
The earth being made of oceans and mountains, as well as
important in the process of condensation (gas to a liquid) and
being a rotating body, the simple assumption of a direct
thermal circulation transporting heat towards the poles
straight from the tropics is a gross oversimplification. In reality,
energy is transported in the atmosphere in many ways and at
different scales. This transfer of energy is responsible for the
occurrence of weather
particles can reduce the visibility through the air, and are also
sublimation (gas to a solid). If no solid particles were in the air,
it would be very difficult for cloud droplets to form and thus for
weather to occur.
thermal structure
The vertical distribution of temperature for the standard
atmosphere is shown in fig. 1.2. This profile is
representative of typical conditions in the middle latitudes.
Day to day or latitudinal variations will show a different
profile, but the main characteristics will remain.
The profile is divided into four distinct layers roughly
located at the heights listed below:
• Troposphere (0 to 10 km)
• Stratosphere (10 to 50 km)
• Mesosphere (50 to 80 km)
• Thermosphere (80 to 100 km)
The troposphere accounts for more than 80 % of the mass
of virtually all water vapour, clouds, and precipitation in the
earth’s atmosphere. In the troposphere, the temperature
decreases with increasing height, but temperature
inversions (temperature increasing with increasing height)
are possible near the surface.
The layer above, of constant or increasing temperature, is
The mesosphere and thermosphere are so high up and
called the stratosphere. Its vertical temperature profile
so devoid of air and water, that their effect on the weather
makes it very stable and dampens vertical motion. This
is practically non‐existent. Consequently, this course will
particular profile is due to the presence of ozone
only cover tropospheric phenomena
absorbing ultraviolet rays. Very little mixing occurs
between the troposphere and the stratosphere. Due to the
extreme stability of the stratosphere, particles that are
emitted directly into it (from volcanic eruptions, nuclear
explosions, etc.) will remain there for a long time. Even the
strongest thunderstorms will only penetrate the
stratosphere to a few kilometres before their updrafts are
capped. The stratosphere therefore acts as a reservoir for
certain types of atmospheric pollution.
The transition between the two layers, called the
tropopause, is marked by abrupt changes in water vapour
and ozone concentration. Water vapour decreases and
ozone increases as one moves closer to the stratosphere.
The average height of the tropopause (or depth of the
troposphere) is about 11 kilometres. It can vary
considerably, however, and be of the order of 17
kilometres over the equator and about 8 kilometres over
the poles. The tropopause is generally higher in the
summer than in the winter.
THE TROPOSPHERE
The troposphere is characterized by rather strong vertical
mixing and is where most weather phenomena take place.
It is further divided into two layers: the planetary
boundary layer and the free atmosphere.
The planetary boundary layer (PBL) is defined as the part of
the atmosphere that is strongly and directly influenced by
the presence of the surface of the earth. It responds to
surface forcings (solar heating, friction, evapotranspiration,
etc.) with a timescale of about an hour or less. The PBL is an
important part of the atmosphere, because it is where
almost all human activities take place, where most heating
gets into the atmosphere and where the ground affects
weather the most.
The daytime boundary layer is usually very turbulent, due
to ground level heating and winds. It can reach depths of
about 1500 m. At night‐time, surface cooling dampens the
turbulent activity of the boundary layer and its depth can
reduce to a few hundred metres.
The characteristics of the PBL greatly influence the
behaviour of atmospheric dispersion. Sunny summer
afternoon conditions will generally lead to a well‐mixed
and deep PBL. This will lead to the rapid and extensive
dilution of any pollutants released at the surface. On the
other hand, early morning, clear winter conditions will
often result in a very shallow boundary layer, resulting in
relatively high pollutant concentrations.
The free atmosphere is right above the boundary layer and
is not directly influence by local surface forcings. Although
air does flow between the boundary layer and the free
atmosphere, at night‐time or under special circumstances
the exchange can be very limited. Table 1.1 shows the
main differences between the two layers.
general
characteristics
• The lower atmosphere contains varying amounts of water
vapor, which determine its humidity.
• Condensation and sublimation within the atmosphere
cause clouds or fog, and the resulting liquid water
droplets or ice crystals may precipitate to the ground as
rain, sleet, snow, hail, dew, or frost.
• Because of the pull of gravity the density of the
atmosphere and the pressure exerted by air molecules
are greatest near the earth's surface.
variable that influence
the atmosphere
1. TEMPERATURE
2. PRESSURE
3. HUMIDITY
4. CLOUDINESS
5. WINDS
ATMOSPHERIC STABILITY
• Stability refers to a condition of equilibrium.
• When the atmosphere is stable, a parcel of air will want
to return to its original position after being raised or
lowered.
• Vertical motions are inhibited.
• Atmospheric stability is a measure of the atmosphere's
tendency to encourage or deter vertical motion, and
vertical motion is directly correlated to different types of
weather systems and their severity.
• The atmosphere is absolutely stable when the
environmental lapse rate is less than the moist adiabatic
lapse rate.
ENGINEERING
HYDROLOGY
1 |
Introduction to
Hydrology &
Weather Basics
Cloud physics is the study of the physical processes that lead to
1.5 | Introduction to
Cloud Physics
cloud
physics
INTENDED LEARNING OUTCOMES
• Describe how clouds are formed
and distributed
• Identify the different types of
clouds
the formation, growth and precipitation of atmospheric clouds.
Clouds consist of microscopic droplets of liquid water (warm
clouds), tiny crystals of ice (cold clouds), or both (mixed phase
clouds). Cloud droplets initially form by the condensation of water
vapor onto condensation nuclei when the super saturation of air
exceeds a critical value according to Köhler theory.
cloud formation
Terrestrial clouds can be found throughout most of the
homosphere, which includes the troposphere, stratosphere,
and mesosphere. Within these layers of the atmosphere, air
can become saturated as a result of being cooled to its dew
point or by having moisture added from an adjacent source. In
the latter case, saturation occurs when the dew point is raised
to the ambient air temperature.
extratropical cyclones tend to generate mostly cirriform and
stratiform clouds over a wide area unless the approaching
warm airmass is unstable, in which case cumulus congestus or
cumulonimbus clouds are usually embedded in the main
precipitating cloud layer. Cold fronts are usually faster moving
and generate a narrower line of clouds, which are mostly
stratocumuliform, cumuliform, or cumulonimbiform depending
on the stability of the warm airmass just ahead of the front.
Another agent is the convective upward motion of air caused
by daytime solar heating at surface level. Airmass instability
ADIABATIC COOLING
allows for the formation of cumuliform clouds that can produce
Adiabatic cooling occurs when one or more of three possible
occasions, convective lift can be powerful enough to penetrate
lifting agents – cyclonic/frontal, convective, or orographic –
the tropopause and push the cloud top into the stratosphere.
cause a parcel of air containing invisible water vapor to rise
and cool to its dew point, the temperature at which the air
becomes saturated. The main mechanism behind this process
is adiabatic cooling. As the air is cooled to its dew point and
becomes saturated, water vapor normally condenses to form
cloud drops. This condensation normally occurs on cloud
condensation nuclei such as salt or dust particles that are small
enough to be held aloft by normal circulation of the air.
Frontal and cyclonic lift occur when stable air is forced aloft at
weather fronts and around centers of low pressure by a
process called convergence. Warm fronts associated with
showers if the air is sufficiently moist. On moderately rare
A third source of lift is wind circulation forcing air over a
physical barrier such as a mountain (orographic lift). If the air is
generally stable, nothing more than lenticular cap clouds form.
However, if the air becomes sufficiently moist and unstable,
orographic showers or thunderstorms may appear.
NON-ADIABATIC COOLING
ADDING MOISTURE TO THE AIR
Along with adiabatic cooling that requires a lifting agent, three
Several main sources of water vapor can be added to the air as
major non-adiabatic mechanisms exist for lowering the
a way of achieving saturation without any cooling process:
temperature of the air to its dew point. Conductive, radiational,
water or moist ground, precipitation or virga, and transpiration
and evaporative cooling require no lifting mechanism and can
from plants
cause condensation at surface level resulting in the formation
of fog.
cloud DISTRIBUTION
CONVERGENCE ALONG LOW-PRESSURE ZONES
stability characteristics of the various air masses that are in
conflict.
Although the local distribution of clouds can be significantly
influenced by topography, the global prevalence of cloud
cover in the troposphere tends to vary more by latitude. It is
most prevalent in and along low pressure zones of surface
tropospheric convergence which encircle the Earth close to the
equator and near the 50th parallels of latitude in the northern
and southern hemispheres. The adiabatic cooling processes
that lead to the creation of clouds by way of lifting agents are
all associated with convergence; a process that involves the
horizontal inflow and accumulation of air at a given location, as
well as the rate at which this happens. Near the equator,
increased cloudiness is due to the presence of the lowpressure Intertropical Convergence Zone (ITCZ) where very
warm and unstable air promotes mostly cumuliform and
cumulonimbiform clouds. Clouds of virtually any type can form
along the mid-latitude convergence zones depending on the
stability and moisture content of the air. These extratropical
convergence zones are occupied by the polar fronts where air
masses of polar origin meet and clash with those of tropical or
subtropical origin. This leads to the formation of weathermaking extratropical cyclones composed of cloud systems that
may be stable or unstable to varying degrees according to the
DIVERGENCE ALONG HIGH PRESSURE ZONES
Divergence is the opposite of convergence. In the Earth's
troposphere, it involves the horizontal outflow of air from the
upper part of a rising column of air, or from the lower part of a
subsiding column often associated with an area or ridge of
high pressure. Cloudiness tends to be least prevalent near the
poles and in the subtropics close to the 30th parallels, north
and south. The latter are sometimes referred to as the horse
latitudes. The presence of a large-scale high-pressure
subtropical ridge on each side of the equator reduces
cloudiness at these low latitudes. Similar patterns also occur at
higher latitudes in both hemispheres.
TYPES OF CLOUDS
HIGH CLOUDS
1. CIRRUS. Detached clouds in the form of white, delicate
fi laments, mostly white patches or narrow bands. They may
have a fi brous (hair -like) and/or silky sheen appearance. Cirrus
clouds are always composed of ice crystals, and their
transparent character depends upon the degree of separation
of the crystals. As a rule when these clouds cross the sun's disk
they hardly diminish its brightness. Before sunrise and after
sunset, cirrus is often colored bright yellow or red. These clouds
are lit up long before other clouds and fade out much later.
2. CIRROSTRATUS. Transparent, whitish veil clouds with a fi brous
(hair -like) or smooth appearance. A sheet of cirrostratus which
is very extensive, nearly always ends by covering the whole sky.
A milky veil of fog (or thin Stratus) is distinguished from a veil of
Cirrostratus of a similar appearance by the halo phenomena
which the sun or the moon nearly always produces in a layer of
cirrostratus.
3. CIRROCUMULUS. Thin, white patch, sheet, or layered of clouds
without shading. They are composed of very small elements in
the form of more or less regularly arranged grains or ripples. In
general Cirrocumulus represents a degraded state of cirrus and
cirrostratus both of which may change into it and is an
uncommon cloud. There will be a connection with cirrus or
cirrostratus and will show some characteristics of ice crystal
clouds.
MID CLOUDS
1. ALTOSTRATUS. Gray or bluish cloud sheets or layers of striated
or fi brous clouds that totally or partially covers the sky. They are
thin enough to regularly reveal the sun as if seen through
ground glass. Altostratus clouds do not produce a halo
phenomenon nor are the shadows of objects on the ground
visible. Sometime virga is seen hanging from Altostratus, and at
times may even reach the ground causing very light
precipitation.
2. ALTOCUMULUS. White and/or gray patch, sheet or layered
clouds, generally composed of laminae (plates), rounded
masses or rolls. They may be partly fi brous or diffuse. When the
edge or a thin semitransparent patch of altocumulus passes in
front of the sun or moon a corona appears. This colored ring
has red on the outside and blue inside and occurs within a few
degrees of the sun or moon. The most common mid cloud,
more than one layer of Altocumulus often appears at different
levels at the same time. Many times Altocumulus will appear
with other cloud types.
3. NIMBOSTRATUS. The continuous rain cloud. Resulting from
thickening Altostratus, This is a dark gray cloud layer diffused by
falling rain or snow. It is thick enough throughout to blot out the
sun. The cloud base lowers into the low level of clouds as
precipitation continues. Also, low, ragged clouds frequently
occur beneath this cloud which sometimes merges with its
base.
LOW CLOUDS
1. CUMULUS. Detached, generally dense clouds and with sharp
outlines that develop vertically in the form of rising mounds, domes
or towers with bulging upper parts often resembling a caulifl ower.
The sunlit parts of these clouds are mostly brilliant white while their
bases are relatively dark and horizontal. Over land cumulus develops
on days of clear skies, and is due diurnal convection; it appears in the
morning, grows, and then more or less dissolves again toward
evening.
2. STRATUS. A generally gray cloud layer with a uniform base which
may, if thick enough, produce drizzle, ice prisms, or snow grains.
When the sun is visible through this cloud, its outline is clearly
discernible.Often when a layer of Stratus breaks up and dissipates
blue sky is seen.
3. CUMULONIMBUS. The thunderstorm cloud, this is a heavy and
dense cloud in the form of a mountain or huge tower. The upper
portion is usually smoothed, fi brous or striated and nearly always
fl attened in the shape of an anvil or vast plume. Under the base of
this cloud which is often very dark, there are often low ragged clouds
that may or may not merge with the base. They produce
precipitation, which sometimes is in the form of virga. Cumulonimbus
clouds also produce hail and tornadoes.
4. STRATOCUMULUS. Gray or whitish patch, sheet, or layered clouds
which almost always have dark tessellations (honeycomb
appearance), rounded masses or rolls. Except for virga they are nonfi brous and may or may not be merged. They also have regularly
arranged small elements with an apparent width of more than fi ve
degrees (three fi ngers - at arm's length).
ENGINEERING
HYDROLOGY
1 |
Introduction to
Hydrology &
Weather Basics
The main source of energy at the Earth’s surface is radiant energy
1.6 | Solar Radiation
from the Sun, termed solar radiation or insolation. It is the solar
radiation impinging on the Earth that fuels the heat engine driving
the hydrological cycle.
solar
radiation
INTENDED LEARNING OUTCOMES
• Discuss the sources of radiation
• Discuss the earth’s energy balance
radiation sources
All matter radiates energy in the form of electromagnetic
waves. The amount of energy and the wavelengths emitted are
dependent on the temperature of the matter. The hotter a
substance is, the greater the amount of emitted energy, and
the shorter the wavelength of the emission.
When it comes to fuelling the atmospheric heat engine, the
only source of energy that counts is the sun. The sun, a huge
nuclear reactor, emits electromagnetic radiation in all direction,
and the earth intercepts a small fraction of it. Radiation emitted
by the sun is referred to as short wave radiation. It ranges
from ultraviolet (0.12 µm) to near infrared (4 µm) and includes
visible light (0.34 to 0.7 µm). The earth, on the other hand,
being many times colder than the sun, emits long wave
radiation (i.e. infrared or IR). Except for the effect of distance,
radiation from the sun reaches the outer limit of the
atmosphere undepleted.
Electromagnetic radiation is not heat. Only if a substance
absorbs the radiation will it heat up and raise its temperature.
For a substance or a body to remain at constant temperature,
the heat absorbed must balance the heat emitted. As we will
see in the next section, the earth and its atmosphere radiate
back as much as they take in
earth’s energy budget
Gases are selective in the wavelengths that they absorb and
permit certain bands of wavelengths to pass through
unhindered in what are called windows. Other bands of
wavelengths are absorbed and cause the temperature of the
substance to rise. Air molecules, haze, dust, cloud particles
scatter and absorb radiation so that only a part of the energy
actually reaches the earth’s surface, either directly or indirectly
(see fig. 2.1).
Ozone is concentrated in a layer called the ozonosphere
extending from about 10 km up to 50 km. It absorbs most of
the solar ultraviolet (UV) radiation. If this ultraviolet radiation
were to reach the surface it would kill all life on earth.
The absorption of ultraviolet radiation by ozone raises the
The remainder is absorbed by the earth, which heats up and
radiates back in longer, infrared (IR) wavelengths.
temperature of the high atmosphere to near zero Celsius. As
Some of the outgoing IR radiation from the earth’s surface is
the radiation penetrates further into the ozonosphere,
absorbed by the carbon dioxide and water vapour of the
absorption gradually depletes the ultraviolet rays and the
atmosphere. The amount of heating that this creates decreases
temperature of the atmosphere steadily falls to a minimum at a
with altitude as the radiation is depleted. Cloud, if present,
height that averages around 11 km (the tropopause). The
absorbs a great deal of terrestrial radiation and radiates it both
remainder of the solar radiation continues through the
back to earth and out to space. Some of the terrestrial radiation
atmosphere with only a small amount of it being absorbed by
passes directly out to space through windows, and the
other atmospheric constituents. Some of it is reflected back to
atmosphere itself radiates out to space. The outgoing radiation
space from the atmosphere, cloud tops, or the earth’s surface.
balances the incoming radiation from the sun, so that the
earth’s average temperature remains nearly constant.
Because the atmosphere is transparent to solar radiation
this effect, the mean global surface temperature would be
(except for UVs) but absorbs long wave radiation from the
about 30 degrees colder than the current average
earth, the earth’s surface has a higher temperature on
temperature (~15 degrees). The addition of so‐called
average than it would have if there were no atmosphere.
greenhouse gases into the atmosphere would likely
Although a bit of a misnomer, this heating mechanism is
change this balance and could produce a rise in the
called the greenhouse effect. It is estimated that, without
average temperature of the earth.
ENGINEERING
HYDROLOGY
1 |
Introduction to
Hydrology &
Weather Basics
A spatial imbalance between radiative inputs and outputs exists for the
earth-ocean-atmosphere system. The earth loses energy at all latitudes
1.7 | General Circulation
due to outgoing infrared (IR) radiation. Near the tropics, more solar
radiation enters than IR leaves, hence there is a net input of radiative
energy. Near Earth’s poles, incoming solar radiation is too weak to
general
circulation
INTENDED LEARNING OUTCOMES
• Discuss the general and thermal
circulations
totally offset the IR cooling, allowing a net loss of energy. The result is
differential heating, creating warm equatorial air and cold polar air (Fig.
11.1a).
A spatial imbalance between radiative inputs and outputs exists for the
earth-ocean-atmosphere system. The earth loses energy at all latitudes
due to outgoing infrared (IR) radiation. Near the tropics, more solar
radiation enters than IR leaves, hence there is a net input of radiative
energy. Near Earth’s poles, incoming solar radiation is too weak to
totally offset the IR cooling, allowing a net loss of energy. The result is
differential heating, creating warm equatorial air and cold polar air (Fig.
11.1a).
A spatial imbalance between radiative inputs and outputs exists for the
earth-ocean-atmosphere system. The earth loses energy at all latitudes
due to outgoing infrared (IR) radiation. Near the tropics, more solar
radiation enters than IR leaves, hence there is a net input of radiative
energy. Near Earth’s poles, incoming solar radiation is too weak to
totally offset the IR cooling, allowing a net loss of energy. The result is
differential heating, creating warm equatorial air and cold polar air (Fig.
11.1a).
Consider a hypothetical rotating planet with no contrast
between continents and oceans. The climatological average
(average over 30 years; see the Climate chapter) winds in
such a simplified planet would have characteristics as
sketched in Figs. 11.3. Actual winds on any day could differ
from this climatological average due to transient weather
systems that perturb the average flow. Also, monthlyaverage conditions tend to shift toward the summer
hemisphere (e.g., the circulation bands shift northward
during April through September).
NEAR THE SURFACE
Near-surface average winds are sketched in Fig. 11.3a. At
low latitudes are broad bands of persistent easterly winds
(U ≈ –7 m s
–1
) called trade winds, named because the
easterlies allowed sailing ships to conduct transoceanic
trade in the old days.
These trade winds also blow toward the equator from both
hemispheres, and the equatorial belt of convergence is
called the intertropical convergence zone (ITCZ). On
average, the air at the ITCZ is hot and humid, with low
pressure, strong upward air motion, heavy convective
(thunderstorm) precipitation, and light to calm winds except
in thunderstorms. This equatorial trough (low-pressure belt)
was called the doldrums by sailors whose sailing ships were
becalmed there for many days.
At 30° latitude are belts of high surface pressure called
subtropical highs (Fig. 11.3a). In these belts are hot, dry, cloudfree air descending from higher in the troposphere. Surface
winds in these belts are also calm on average. In the old days,
becalmed sailing ships would often run short of drinking water,
causing horses on board to die and be thrown overboard.
Hence, sailors called these miserable places the horse
latitudes. On land, many of the world’s deserts are near these
latitudes.
In mid-latitudes are transient centers of low pressure (midlatitude cyclones, L) and high pressure (anticyclones, H). Winds
around lows converge (come together) and circulate
cyclonically — counterclockwise in the N. Hemisphere, and
clockwise in the S. Hemisphere. Winds around highs diverge
(spread out) and rotate anticyclonically — clockwise in the N.
Hemisphere, and counterclockwise in the S. Hemisphere. The
cyclones are regions of bad weather (clouds, rain, high
humidity, strong winds) and fronts. The anticyclones are
regions of good weather (clear skies or fair-weather clouds, no
precipitation, dry air, and light winds).
The high- and low-pressure centers move on average from west to
east, due to easterly inertia from the trade winds being carried
east, driven by large-scale winds from the west. Although these
upward in the thunderstorm convection. Diverging from this belt
westerlies dominate the general circulation at mid-latitudes, the
are winds that blow toward the north in the N. Hemisphere, and
surface winds are quite variable in time and space due to the sum
toward the south in the S. Hemisphere. As these winds move away
of the westerlies plus the transient circulations around the highs
from the equator, they turn to have an increasingly westerly
and lows.
component as they approach 30° latitude.
Near 60° latitude are belts of low surface pressure called subpolar
Near 30° latitude in each hemisphere is a persistent belt of strong
lows. Along these belts are light to calm winds, upward air motion,
westerly winds at the tropopause called the subtropical jet. This jet
clouds, cool temperatures, and precipitation (as snow in winter).
meanders north and south a bit. Pressure here is very high, but not
Near each pole is a climatological region of high pressure called a
as high as over the equator.
polar high. In these regions are often clear skies, cold dry
In mid-latitudes at the tropopause is another belt of strong westerly
descending air, light winds, and little snowfall. Between each polar
winds called the polar jet. The centerline of the polar jet meanders
high (at 90°) and the subpolar low (at 60°) is a belt of weak easterly
north and south, resulting in a wave-like shape called a Rossby
winds, called the polar easterlies
wave (or planetary wave), as sketched in Fig. 11.1c. The
equatorward portions of the wave are known as low-pressure
troughs, and poleward portions are known as high-pressure ridges.
UPPER-TROPOSPHERE
The stratosphere is strongly statically stable, and acts like a lid to
the troposphere. Thus, vertical circulations associated with our
weather are mostly trapped within the troposphere. These vertical
circulations couple the average near-surface winds with the average
upper-tropospheric (near the tropopause) winds described here
(Fig. 11.3b).
In the tropics is a belt of very strong equatorial high pressure along
the tops of the ITCZ thunderstorms. Air in this belt blows from the
These ridges and troughs are very transient, and generally shift
from west to east relative to the ground.
Near 60° at the tropopause is a belt of low to medium pressure. At
each pole is a low-pressure center near the tropopause, with winds
at high latitudes generally blowing from the west causing a cyclonic
circulation around the polar low. Thus, contrary to near-surface
conditions, the near-tropopause average winds blow from the west
at all latitudes (except near the equator).
VERTICAL CIRCULATIONS
circulation is during February and March, and the ITCZ is centered at
Vertical circulations of warm rising air in the tropics and descending air in
the subtropics are called Hadley cells or Hadley circulations (Fig. 11.4).
At the bottom of the Hadley cell are the trade winds. At the top, near the
roughly 6°S, but varies with longitude. The major Hadley cell transports
significant heat away from the tropics, and also from the summer to the
winter hemisphere.
tropopause, are divergent winds. The updraft portion of the Hadley
During the transition months (April-May and October-November)
circulation often contains thunderstorms and heavy precipitation at the
between summer and winter, the Hadley circulation has nearly symmetric
ITCZ. This vigorous convection in the troposphere causes a high
Hadley cells in both hemispheres (Fig. 11.4b). During this transition, the
tropopause (15 - 18 km altitude) and a belt of heavy rain in the tropics.
intensities of the Hadley circulations are weak.
The summer- and winter-hemisphere Hadley cells are strongly
When averaged over the whole year, the strong but reversing major
asymmetric (Fig. 11.4). The major Hadley circulation (denoted with
Hadley circulation partially cancels itself, resulting in an annual average
subscript “M”) crosses the equator, with rising air in the summer
circulation that is somewhat weak and looks like Fig. 11.4b. This weak
hemisphere and descending air in the winter hemisphere. The updraft is
annual average is deceiving, and does not reflect the true movement of
often between 0° and 15° latitudes in the summer hemisphere, and has
heat, moisture, and momentum by the winds. Hence, climate experts
average core vertical velocities of 6 mm s
prefer to look at months JJA and DJF separately to give seasonal
–1
. The broader downdraft is
often found between 10° and 30° latitudes in the winter hemisphere,
with average velocity of about –4 mm s
–1
in downdraft centers.
Connecting the up- and downdrafts are meridional wind components of
averages.
In the winter hemisphere, one or more jet streams circle the earth at mid-
3 m s –1 at the cell top & bottom.
latitudes while meandering north and south as Rossby waves (Fig. 11.1c).
The major Hadley cell changes direction and shifts position between
circulation called a Ferrel cell. At high latitudes is a modest polar cell.
summer and winter. During June-July-August-September, the average
solar declination angle is 15°N, and the updraft is in the Northern
Hemisphere (Fig. 11.4a). Out of these four months, the most well-defined
circulation occurs in August and September. At this time, the ITCZ is
centered at about 9°N, but varies with longitude.
During December-January-February-March, the average solar
declination angle is 14.9°S, and the major updraft is in the Southern
Hemisphere (Fig. 11.4c). Out of these four months, the strongest
When averaged around latitude bands, the net effect is a weak vertical
In the summer hemisphere, all the circulations are weaker. There are
minor Hadley and Ferrel cells (Fig. 11.4). Summer-hemisphere
circulations are weaker because the temperature contrast between the
tropics and poles are weaker.
MONSOONAL CIRCULATIONS
Monsoon circulations are continental-scale circulations driven
by continent-ocean temperature contrasts, as sketched in Figs.
11.5. In summer, high-pressure centers (anticyclones) are over
the relatively warm oceans, and low-pressure centers
(cyclones) are over the hotter continents. In winter, lowpressure centers are over the cool oceans, and high-pressure
centers are over the colder continents.
These monsoon circulations represent average conditions over
a season. The actual weather on any given day can be variable,
and can deviate from these seasonal averages.
Our Earth has a complex arrangement of continents and
oceans. As a result, seasonally-varying monsoonal circulations
are superimposed on the seasonally-varying planetary-scale
circulation to yield a complex and varying global-circulation
pattern.
THERMAL CIRCULATIONS
THERMAL CIRCULATION a circulation generated by pressure
At sunset, the land surface stops receiving radiation from the Sun
gradients produced by differential heating. They tend to be shallow
(Figure 7-5). As night continues the land surface begins losing heat
and do not extend up through the depth of the troposphere.
energy at a much faster rate than the water surface. After a few
Examples of thermal circulations are: sea breeze, land breeze,
hours, air temperature and pressure contrasts begin to develop
monsoons, mountain and valley breezes.
between the land and ocean surfaces. The land surface being
Sea and land breezes are types of thermal circulation systems that
develop at the interface of land and ocean. At this interface, the
dissimilar heating and cooling characteristics of land and water
initiate the development of an atmospheric pressure gradient
which causes the air in these areas to fl ow.
During the daytime land heats up much faster than water as it
receives solar radiation from the Sun (Figure 7-4). The warmer air
over the land then begins to expand and rise forming a low. At the
same time, the air over the ocean becomes a cool high because of
water's slower rate of heating. Air begins to flow as soon as there is
a significant difference in air temperature and pressure across the
land to sea gradient. The development of this pressure gradient
causes the heavier cooler air over the ocean to move toward the
land and to replace the air rising in the thermal low. This localized
air flow system is called a sea breeze. Sea breeze usually begins in
midmorning and reaches its maximum strength in the later
afternoon when the greatest temperature and pressure contrasts
exist. It dies down at sunset when air temperature and pressure
once again become similar across the two surfaces.
cooler than the water becomes a thermal high pressure area. The
ocean becomes a warm thermal low. Wind flow now moves from
the land to the open ocean. This type of localized air flow is called a
land breeze.
Mountain and valley breezes are common in regions with great
topographic relief (Figure 7-6 and 7-7). A valley breeze develops
during the day as the Sun heats the land surface and air at the
valley bottom and sides (Figure 7-6). As the air heats it becomes
less dense and buoyant and begins to fl ow gently up the valley
sides. Vertical ascent of the air rising along the sides of the
mountain is usually limited by the presence of a temperature
inversion layer. When the ascending air currents encounter the
inversion they are forced to move horizontally and then back down
to the valley fl oor. This creates a self-contained circulation system. If
conditions are right, the rising air can condense and form into
cumulus-type clouds.
During the night, the air along the mountain slopes begins to cool
quickly because of longwave radiation loss (Figure 7-7). As the air
cools, it becomes more dense and begins to flow downslope
causing a mountain breeze. Convergence of the draining air occurs
at the valley floor and forces the air to move vertically upward. The
upward movement is usually limited by the presence of a
temperature inversion which forces the air to begin moving
horizontally. This horizontal movement completes the circulation
cell system. In narrowing terrain, mountain winds can accelerate in
speed because of the venturi effect. Such winds can attain speeds
as high has 150 kilometers per hour.
Monsoons are regional scale wind systems that predictably change
cooling rapidly as longwave radiation is emitted to space. The
direction with the passing of the seasons. Like land/sea breezes,
ocean surface retains its heat energy longer because of water's
these wind systems are created by the temperature contrasts that
high specific heat and subsurface mixing. The winter monsoons
exist between the surfaces of land and ocean. However, monsoons
bring clear dry weather and winds that flow from land to sea.
are different from land/sea breezes both spatially and temporally.
Monsoons occur over distances of thousands of kilometers, and
their two dominant patterns of wind fl ow act over an annual time
scale.
During the summer, monsoon winds blow from the cooler ocean
surfaces onto the warmer continents. In the summer, the continents
become much warmer than the oceans because of a number of
factors. These factors include:
•
Specific heat differences between land and water.
•
Greater evaporation over water surfaces.
•
Subsurface mixing in ocean basins which redistributes heat
energy through a deeper layer.
Precipitation is normally associated with the summer monsoons.
Onshore winds blowing inland from the warm ocean are very high
in humidity, and slight cooling of these air masses causes
condensation and rain. In some cases, this precipitation can be
greatly intensified by orographic uplift. Some highland areas in Asia
receive more than 10 meters of rain during the summer months.
In the winter, the wind patterns reverse as the ocean surfaces are
now warmer. With little solar energy available, the continents begin
ENGINEERING
HYDROLOGY
1 |
Introduction to
Hydrology &
Weather Basics
general distribution
1.8 | Temperature and
Humidity
If the Earth was a homogeneous body without the present land/
temperature
this being composed of a mosaic of land and water. This mosaic
INTENDED LEARNING OUTCOMES
• Describe the general distribution
and variances of temperature on
earth
• Apply statistical treatment on
temperature measurements
ocean distribution, its temperature distribution would be strictly
latitudinal ( Figure 7m-1). However, the Earth is more complex than
causes latitudinal zonation of temperature to be disrupted
spatially.
The following two factors are important in influencing the
distribution of temperature on the Earth's surface:
Mainly because of specific heat, land surfaces behave quite
• The latitude of the location determines how much solar
differently from water surfaces. In general, the surface of
radiation is received. Latitude influences the angle of
any extensive deep body of water heats more slowly and
incidence and duration of daylength.
cools more slowly than the surface of a large land body.
• Surface properties - surfaces with high albedo absorb
less incident radiation. In general, land absorbs less
insolation that water because of its lighter color. Also,
even if two surfaces have the same albedo, a surface's
specific heat determines the amount of heat energy
required for a specific rise in temperature per unit mass.
The specific heat of water is some five times greater than
that of rock and the land surface (see Table 7m-1 below).
As a result, water requires the input of large amounts of
energy to cause a rise in its temperature.
Other factors influencing the way land and water surfaces
heat and cool include:
• Solar radiation warms an extensive layer in water, o n
land just the immediate surface is heated.
• Water is easily mixed by the process of convection.
• Evaporation of water removes energy from water's
surface.
AIR TEMPERATURE
AIR TEMPERATURE
TEMPERATURE VARIATION
DIURNAL VARIATION is the change in temperature from
SEASONAL VARIATION is the change in temperature due
day to night brought about by the daily rotation of the
to seasonal variation of the angle of incident solar radiation
Earth. The Earth receives heat during the day by solar
between hemispheres brought about by the tilting of the
radiation and warms up. During the night, the Earth loses
Earth‟s axis during its orbit around the sun. The Northern
energy and cools down even until sunrise.
Hemisphere is warmer in June, July, and August because it
receives more solar energy than does the Southern
Hemisphere. During December, January, and February, the
opposite is true; the Southern Hemisphere receives more
solar energy and is warmer.
VARIATION WITH LATITUDE is the change in temperature
temperature changes since it contains some water and also
due to the variation in the angle of incident solar radiation
insulates against heat transfer between the ground and the
in contact with the geographical surface profi le of the
atmosphere.
Earth. Since the Earth is essentially spherical, the sun is
more nearly overhead in equatorial regions than at higher
latitudes. Equatorial regions, therefore, receive the most
radiant energy and are warmest.
VARIATION WITH ALTITUDE. Temperature varies as one
move vertically upward from the Earth‟s surface.
Temperature normally decreases with increasing altitude
throughout the troposphere. This decrease of temperature
with altitude is defined as lapse rate. An increase in
temperature with altitude is defined as an inversion, i.e.,
lapse rate is inverted.
An inversion often develops near the ground on clear, cool
nights when wind is light. The ground radiates and cools
much faster than the overlying air. Air in contact with the
ground becomes cold while the temperature a few
hundred feet above changes very little. Thus, temperature
VARIATION WITH TOPOGRAPHY is the change in
increases with height. Inversions may also occur at any
temperature not related to movement or shape of the
altitude when conditions are favorable. Inversions are
earth are temperature variations induced by water and
common in the stratosphere.
terrain. Water absorbs and radiates energy with less
temperature change than does land. Large, deep water
bodies tend to minimize temperature changes, while
continents favor large changes. Wet soil such as in swamps
and marshes is almost as effective as water in suppressing
temperature changes. Thick vegetation tends to control
TEMPERATURE MEASUREMENT
Temperature is measured with thermometers that may be
mean of the daily minimum and maximum readings). The
calibrated to a variety of temperature scales:
world‟s average surface air temperature is about 15ºC.
• Celsius scale – measure temperature using the Celsius
scale
• Kelvin scale – measure thermodynamics temperature
• Rankine scale – a shifted Fahrenheit scale, used when
working in thermodynamic related disciplines such as
combustion.
A thermometer is an instrument that operates on the
principle of thermal expansion of the material used, e.g.
liquids like mercury and alcohol, metallic materials, etc.
The global temperature records and measurements are
typically acquired using the satellite or ground
instrumental temperature measurements, the usually
compiled using a database or c The true daily mean,
obtained from a thermograph, is approximated by the
mean of 24 hourly readings (which is not the same as the
general distribution
1.8 | Temperature and
Humidity
The most humid cities on earth are generally located closer to the
humidity
Singapore have very high humidity all year round because of their
equator, near coastal regions. Cities in South and Southeast Asia
are among the most humid. Kuala Lumpur, Manila, Jakarta, and
proximity to water bodies and the equator and often overcast
weather. Some places experience extreme humidity during their
rainy seasons combined with warmth giving the feel of a lukewarm
sauna, such as Kolkata, Chennai and Cochin in India, and Lahore in
Pakistan.
INTENDED LEARNING OUTCOMES
• Describe the general distribution
and types of humidity on earth
• Identify the devices that measures
humidity
types of humidity
ABSOLUTE HUMIDITY is is the total mass of water vapor present in
a given volume or mass of air. It does not take temperature into
RELATIVE HUMIDITY of an air-water mixture is defi ned as the ratio
of the partial pressure of water vapor (pH2 0) in the mixture to the
equilibrium vapor pressure of water (p*
) over a fl at surface of pure
H0
2
water at a given temperature:
consideration. Absolute humidity in the atmosphere ranges from
RH =
near zero to roughly 30 grams per cubic metre when the air is
saturated at 30 °C (86 °F).
Absolute humidity is the mass of the water vapor (mH20), divided by
the volume of the air and water vapor mixture (Vnet), which can be
expressed as:
pH2 0
p*
H2 0
Relative humidity is normally expressed as a percentage; a higher
percentage means that the air-water mixture is more humid.
Relative humidity is an important metric used in weather forecasts
AH =
mH2 0
Vnet
.
and reports, as it is an indicator of the likelihood of precipitation,
dew, or fog. In hot summer weather, a rise in relative humidity
increases the apparent temperature to humans (and other animals)
The absolute humidity changes as air temperature or pressure
by hindering the evaporation of perspiration from the skin. For
changes, if the volume is not fixed. This makes it unsuitable for
example, according to the Heat Index, a relative humidity of 75% at
chemical engineering calculations, e.g. in drying, where
air temperature of 80.0 °F (26.7 °C) would feel like 83.6 °F ±1.3 °F
temperature can vary considerably. As a result, absolute humidity in
(28.7 °C ±0.7 °C).
chemical engineering may refer to mass of water vapor per unit
mass of dry air, also known as the humidity ratio or mass mixing
ratio (see "specific humidity" below), which is better suited for heat
and mass balance calculations. Mass of water per unit volume as in
the equation above is also defined as volumetric humidity. Because
of the potential confusion, British Standard BS 1339 suggests
avoiding the term "absolute humidity". Units should always be
carefully checked. Many humidity charts are given in g/kg or kg/kg,
but any mass units may be used.
SPECIFIC HUMIDITY or moisture content is the ratio of the mass of
water vapor to the total mass of the air parcel. Specifi c humidity is
approximately equal to the mixing ratio, which is defi ned as the
ratio of the mass of water vapor in an air parcel to the mass of dry
air for the same parcel. As temperature decreases, the amount of
water vapor needed to reach saturation also decreases. As the
temperature of a parcel of air becomes lower it will eventually reach
the point of saturation without adding or losing water mass.
measurement
A device used to measure humidity is called a psychrometer or
hygrometer. A humidistat is a humidity-triggered switch, often
used to control a dehumidifier.
There are various devices used to measure and regulate humidity.
Calibration standards for the most accurate measurement include
the gravimetric hygrometer, chilled mirror hygrometer, and
electrolytic hygrometer. The gravimetric method, while the most
accurate, is very cumbersome. For fast and very accurate
measurement the chilled mirror method is effective. For process online measurements, the most commonly used sensors nowadays
are based on capacitance measurements to measure relative
humidity, frequently with internal conversions to display absolute
humidity as well. These are cheap, simple, generally accurate and
relatively robust. All humidity sensors face problems in measuring
dust-laden gas, such as exhaust streams from dryers.
Humidity is also measured on a global scale using remotely placed
satellites. These satellites are able to detect the concentration of
water in the troposphere at altitudes between 4 and 12 kilometres.
Satellites that can measure water vapor have sensors that are
sensitive to infrared radiation. Water vapor specifically absorbs and
re-radiates radiation in this spectral band. Satellite water vapor
imagery plays an important role in monitoring climate conditions
(like the formation of thunderstorms) and in the development of
weather forecasts.
types of humidity
ABSOLUTE HUMIDITY is is the total mass of water vapor present in
a given volume or mass of air. It does not take temperature into
RELATIVE HUMIDITY of an air-water mixture is defi ned as the ratio
of the partial pressure of water vapor (pH2 0) in the mixture to the
equilibrium vapor pressure of water (p*
) over a fl at surface of pure
H0
2
water at a given temperature:
consideration. Absolute humidity in the atmosphere ranges from
RH =
near zero to roughly 30 grams per cubic metre when the air is
saturated at 30 °C (86 °F).
Absolute humidity is the mass of the water vapor (mH20), divided by
the volume of the air and water vapor mixture (Vnet), which can be
expressed as:
pH2 0
p*
H2 0
Relative humidity is normally expressed as a percentage; a higher
percentage means that the air-water mixture is more humid.
Relative humidity is an important metric used in weather forecasts
AH =
mH2 0
Vnet
.
and reports, as it is an indicator of the likelihood of precipitation,
dew, or fog. In hot summer weather, a rise in relative humidity
increases the apparent temperature to humans (and other animals)
The absolute humidity changes as air temperature or pressure
by hindering the evaporation of perspiration from the skin. For
changes, if the volume is not fixed. This makes it unsuitable for
example, according to the Heat Index, a relative humidity of 75% at
chemical engineering calculations, e.g. in drying, where
air temperature of 80.0 °F (26.7 °C) would feel like 83.6 °F ±1.3 °F
temperature can vary considerably. As a result, absolute humidity in
(28.7 °C ±0.7 °C).
chemical engineering may refer to mass of water vapor per unit
mass of dry air, also known as the humidity ratio or mass mixing
ratio (see "specific humidity" below), which is better suited for heat
and mass balance calculations. Mass of water per unit volume as in
the equation above is also defined as volumetric humidity. Because
of the potential confusion, British Standard BS 1339 suggests
avoiding the term "absolute humidity". Units should always be
carefully checked. Many humidity charts are given in g/kg or kg/kg,
but any mass units may be used.
SPECIFIC HUMIDITY or moisture content is the ratio of the mass of
water vapor to the total mass of the air parcel. Specifi c humidity is
approximately equal to the mixing ratio, which is defi ned as the
ratio of the mass of water vapor in an air parcel to the mass of dry
air for the same parcel. As temperature decreases, the amount of
water vapor needed to reach saturation also decreases. As the
temperature of a parcel of air becomes lower it will eventually reach
the point of saturation without adding or losing water mass.
ENGINEERING
HYDROLOGY
2 |
Precipitation
The term precipitation denotes all forms of water that reach the earth from
the atmosphere. The usual forms are rainfall, snowfall, hail, frost and dew. Of
2.1 | Occurrence
formation of
precipitation
INTENDED LEARNING OUTCOMES
• Discuss how precipitation is
formed.
all these, only the first two contribute significant amounts of water. Rainfall
being the predominant form of precipitation causing stream flow, especially
the flood flow in a majority of rivers in India, unless otherwise stated the term
rainfall is used in this book synonymously with precipitation. The magnitude
of precipitation varies with time and space. Differences in the magnitude of
rainfall in various parts of a country at a given time and variations of rainfall
at a place in various seasons of the year are obvious and need no
elaboration. It is this variation that is responsible for many hydrological
problems, such as floods and droughts. The study of precipitation forms a
major portion of the subject of Hydrometeorology.
For precipitation to form: (i) the atmosphere must have moisture, (ii) there
must be sufficient nuclei present to aid condensation, (iii) weather conditions
must be good for condensation of water vapor to take place, and (iv) the
products of condensation must reach the earth. Under proper weather
conditions, the water vapor condenses over nuclei to form tiny water
droplets of sizes less than 0.1 mm in diameter. The nuclei are usually salt
particles or products of combustion and are normally available in plenty.
Wind speed facilitates the movement of clouds while its turbulence retains
the water droplets in suspension. Water droplets in a cloud are somewhat
similar to the particles in a colloidal suspension. Precipitation results when
water droplets come together and coalesce to form larger drops that can
drop down. A considerable part of this precipitation gets evaporated back to
the atmosphere. The net precipitation at a place and its form depend upon a
number of meteorological factors, such as the weather elements like wind,
temperature, humidity and pressure in the volume region enclosing the
clouds and the ground surface at the given place.
1. RAIN is the principal form of precipitation in India. The term rainfall is
used to describe precipitations in the form of water drops of sizes larger
2.1 | Occurrence
forms of
precipitation
INTENDED LEARNING OUTCOMES
• Describe the different forms of
precipitation.
• Identify the forms of precipitation
that Philippines experiences.
than 0.5 mm. The maximum size of a raindrop is about 6 mm. Any drop
larger in size than this tends to break up into drops of smaller sizes
during its fall from the clouds. On the basis of its intensity, rainfall is
classifi ed as:
2. SNOW is another important form of precipitation. Snow consists of ice
crystals which usually combine to form fl akes. When fresh, snow has an
initial density varying from 0.06 to 0.15 g/cm3 and it is usual to assume
an average density of 0.1 g/ cm3 .
3. DRIZZLE is a fi ne sprinkle of numerous water droplets of size less than
0.5 mm and intensity less than 1 mm/h is known as drizzle. In this the
drops are so small that they appear to fl oat in the air.
4. GLAZE. When rain or drizzle comes in contact with cold ground at
around 0º C, the water drops freeze to form an ice coating called glaze
or freezing rain.
5. SLEET is frozen raindrops of transparent grains which form when rain
falls through air at subfreezing temperature.
6. HAIL is a showery precipitation in the form of irregular pellets or lumps
of ice of size more than 8 mm. Hails occur in violent thunderstorms in
which vertical currents are very strong.
RAINS
• Overcast sky with continuous or steady precipitation that may last
2.1 | Occurrence
several hours.
• Has a water droplets of 0.5 mm or greater in size but if widely scattered
classification of
rain & rainshower
(philippines)
INTENDED LEARNING OUTCOMES
• Describe the different
classification of rain and
rainshower
• Interpret weather forecast given
by PAGASA.
the drops may be smaller.
• Associated with meso-scale (synoptic) system or macro-scale (large
scale) system like TC's, Easterly Waves, Monsoons, Fronts and ITCZ.
RAINSHOWERS
• Precipitation of short duration but usually of greater
intensity from convective clouds(primarily cumulus or
cumulonimbus)
• Characterized by sudden start and sudden end of
precipitation, rapid change in intensity.
For the formation of clouds and subsequent precipitation, it is necessary that
the moist air masses cool to form condensation. This is normally
2.1 | Occurrence
weather
systems
INTENDED LEARNING OUTCOMES
• Describe the different weather
systems associated with
precipitation.
• Determine the classification of a
tropical cyclone.
accomplished by adiabatic cooling of moist air through a process of being
lifted to higher altitudes. Some of the terms and processes connected with
the weather systems associated with precipitation are given below.
1. FRONT. A front is the interface between two distinct air masses. Under
certain favorable conditions when a warm air mass and cold air mass
meet, the warmer air mass is lifted over the colder one with the formation
of a front. The ascending warmer air cools adiabatically with the
consequent formation of clouds and precipitation.
2. CYCLONE. A cyclone is a large low pressure region with circular wind
motion. Two types of cyclones are recognized: tropical cyclones and
extratropical cyclones.
CLASSIFICATION OF TROPICAL CYCLONES (PAGASA)
Tropical cyclones derive their energy from the latent heat of
condensation which made them exist only over the oceans and die out
• TROPICAL CYCLONE. A tropical cyclone, also called cyclone in
rapidly on land. One of its distinguishing features is its having a central
India, hurricane in USA and typhoon in South-East Asia, is a wind
sea-level pressure of 900 mb or lower and surface winds often
system with an intensely strong depression with MSL pressures
exceeding 100 knots. They reach their greatest intensity while located
sometimes below 915 mbars The normal areal extent of a
cyclone is about 100-200 km in diameter. The isobars are closely
spaced and the winds are anti-clockwise in the northern
hemisphere. The centre of the storm, called the eye, which may
extend to about 1050 km in diameter, will be relatively quiet.
However, right outside the eye, very strong winds/reaching to as
much as 200 kmph exist. The wind speed gradually decreases
over warm tropical waters and they begin to weaken as they move
inland. The intensity of tropical cyclones vary, thus , we can classify them
based upon their degree of intensity.
The classification of tropical cyclones according to the strength of the
associated windsas adopted by PAGASA as of 01 May 2015 are as
follows:
towards the outer edge. The rainfall will normally be heavy in the
TROPICAL DEPRESSION (TD) - a tropical cyclone with maximum
entire area occupied by the cyclone.
sustained winds of up to 61 kilometers per hour (kph) or less than 33
nautical miles per hour (knots) .
• EXTRATROPICAL CYCLONE. These are cyclones formed in
locations outside the tropical zone. Associated with a frontal
system, they possess a strong counter-clockwise wind circulation
in the northern hemisphere. The magnitude of precipitation and
wind velocities are relatively lower than those of a tropical
cyclone. However, the duration of precipitation is usually longer
and the areal extent also is larger.
3. ANTICLONES. These are regions of high pressure, usually of large
TROPICAL STORM (TS) - a tropical cyclone with maximum wind speed
of 62 to 88 kph or 34 - 47 knots.
SEVERE TROPICAL STORM (STS) , a tropical cyclone with maximum
wind speed of 89 to 117 kph or 48 - 63 knots.
TYPHOON (TY) - a tropical cyclone with maximum wind speed of 118 to
220 kph or 64 - 120 knots.
areal extent. The weather is usually calm at the centre. Anticyclones
SUPER TYPHOON (STY) - a tropical cyclone with maximum wind speed
cause clockwise wind circulations in the northern hemisphere. Winds
exceeding 220 kph or more than 120 knots.
are of moderate speed, and at the outer edges, cloudy and
precipitation conditions exist.
1. CONVECTIVE PRECIPITATION is typical of the tropics and is brought
about by heating of the air at the interface with the ground. This heated
2.1 | Occurrence
types of
precipitation
INTENDED LEARNING OUTCOMES
• Describe the different types of
precipitation.
air expands with a resultant reduction in weight. During this period,
increasing quantities of water vapor are taken up; the warm moistureladen air becomes unstable; and pronounced vertical currents are
developed. Dynamic cooling takes place, causing condensation and
precipitation. Convective precipitation may be in the form of light
showers or storms of extremely high intensity.
2. OROGRAPHIC PRECIPITATION results results from the mechanical
lifting of moist horizontal air currents over natural barriers such as
mountain ranges. Factors that are important in this process include
land elevation, local slope, orientation of land slope, and distance
from the moisture source.
4. ARTIFICIALLY INDUCED. Rainmaking is the act of attempting
to artifi cially induce or increase precipitation, usually to stave off
drought. According to the clouds' different physical properties,
this can be done using airplanes or rockets to sow to the clouds
with catalysts such as dry ice, silver iodide and salt powder, to
3. CYCLONIC PRECIPITATION is associated with the movement
of air masses from high-pressure regions to low-pressure
regions. These pressure differences are created by the unequal
heating of the earth‟s surface. It may be classified as frontal or
non-frontal. If the air masses are moving so that warm air
replaces colder air, the front is known as warm front; if, on the
other hand, cold air displaces warm air, the front is cold. If the
front is not in motion, it is said to be a stationary front.
make clouds rain or increase precipitation, to remove or lessen
farmland drought, to increase reservoir irrigation water or water
supply capacity, or to increase water levels for power
generation
ENGINEERING
HYDROLOGY
2 |
Precipitation
Precipitation is expressed in terms of the depth to which rainfall water would
stand on an area if all the rain were collected on it. Thus 1 cm of rainfall over
2.2 | Measurement
measurement
of precipitation
INTENDED LEARNING OUTCOMES
• Describe how precipitation is
measured.
• Identify and describe instruments
used in measuring rain.
• Solve for the optimum number of
raingauge station in a given area.
a catchment area of l km 2 represents a volume of water equal to 10 4 m3 .
In the case of snowfall, an equivalent depth of water is used as the depth of
precipitation. The precipitation is collected and measured in a raingauge.
Terms such as pluviometer, ombrometer and hyetometer are also
sometimes used to designate a raingauge.
A raingauge essentially consists of a cylindrical-vessel assembly kept in the
open to collect rain. The rainfall catch of the raingauge is affected by its
exposure conditions. To enable the catch of raingauge to accurately
represent the rainfall in the area surrounding the raingauge standard
settings are adopted. For siting a raingauge the following considerations are
important:
• The ground must be level and in the open and the instrument must
present a horizontal catch surface.
• The gauge must be set as near the ground as possible to reduce wind
effects but it must be sufficiently high to prevent splashing, flooding,
etc.
• The instrument must be surrounded by an open fenced area of at least
5.5 m ´ 5.5 m. No object should be nearer to the instrument than 30 m
or twice the height of the obstruction.
raingauges
Raingauges can be broadly classified into two categories
as (i) nonrecording raingauges and (ii) recording gauges.
NONRECORDING GAUGE
• 8-INCH RAINGAUGE. The inside diameter of the
collector is exactly 8 inches above a funnel that
conducts rain into a cylindrical measuring tube or
receiver. The volume of the collector is 10 times the
volume of the measuring tube. Therefore the actual
depth of rainfall is increased ten times on being
collected in the smaller measuring tube.
To measure the amount of rainfall accumulated in the
measuring tube, (a) a thin measuring stick with the
magnifi ed scale printed on its face is used. The
precisely dimensioned (b) measuring tube has a
capacity representative of only 2 inches (50.8
millimeters) on fl at level ground. Rainfall exceeding this
amount spills into the (d) overfl ow can but can be easily
measured by pouring it into the measuring tube for
total rainfall.
RECORDING GAUGE
• TIPPING-BUCKET RAINGAUGE is a type of rainfall
recording instrument. It is an upright cylinder that has
funnel-shaped collector. The precipitation collected by
the collector empties into one side of a "tipping
bucket", an inverted triangular contraption partitioned
transversely at its center, and is pivoted about a
horizontal axis. Once one compartment is fi lled with
rain, it tips, spilling out the water and placing the other
half of the bucket under the funnel. The tipping
activates a mercury switch causing an electrical current
to move the pen in the recorder. Each tipping is equal
to one-half millimeter of rainfall.
• WEIGHING-BUCKET TYPE. In this raingauge the catch
from the funnel empties into a bucket mounted on a
weighing scale. The weight of the bucket and its
contents are recorded on a clock-work-driven chart. The
clockwork mechanism has the capacity to run for as
long as one week. This instrument gives a plot of the
accumulated rainfall against the elapsed time, i.e. the
mass curve of rainfall. In some instruments of this type
the recording unit is so constructed that the pen
reverses its direction at every preset value, say 7.5 cm (3
in.) so that a continuous plot of storm is obtained.
• NATURAL-SYPHON TYPE. This type of recording
• TELEMETERING RAINGAUGES. These raingauges are
raingauge is also known as fl oat-type gauge. Here the
of the recording type and contain electronic units to
rainfall collected by a funnel-shaped collector is led into
transmit the data on rainfall to a base station both at
a fl oat chamber causing a fl oat to rise. As the fl oat rises,
regular intervals and on interrogation. The tipping-
a pen attached to the fl oat through a lever system
bucket type raingauge, being ideally suited, is usually
records the elevation of the fl oat on a rotating drum
adopted for this purpose. Any of the other types of
driven by a clockwork mechanism. A syphon
recording raingauges can also be used equally
arrangement empties the fl oat chamber when the fl oat
effectively. Telemetering gauges are of utmost use in
has reached a pre-set maximum level.
gathering rainfall data from mountainous and generally
inaccessible places.
A typical chart from this type of raingauge is shown in
Fig. 2.6. This chart shows a rainfall of 53.8 mm in 30 h.
The vertical lines in the pen-trace correspond to the
sudden emptying of the fl oat chamber by syphon
action which resets the pen to zero level. It is obvious
that the natural syphon-type recording raingauge gives
a plot of the mass curve of rainfall.
radar measurement of rainfall
The meteorological radar is a powerful instrument for measuring
details of heavy flood-producing rains, a 10-cm radar is used while
the areal extent, location and movement of rain storms. Further, the
for light rain and snow a 5-cm radar is used. The hydrological range
amounts of rainfall over large areas can be determined through the
of the radar is about 200 km. Thus a radar can be considered to be
radar with a good degree of accuracy.
a remote-sensing super gauge covering an areal extent of as much
The radar emits a regular succession of pulses of electromagnetic
radiation in a narrow beam. When raindrops intercept a radar
beam, it has been shown that
Pr =
space. Present-day developments in the field include (i) On-line
processing of radar data on a computer and (ii) Doppler-type
radars for measuring the velocity and distribution of raindrops.
CZ
r2
(2.1)
where Pr = average echopower, Z = radar-echo factor, r = distance
to target volume and C = a constant. Generally the factor Z is
related to the intensity of rainfall as
Z = al b
as 100,000 km2 . Radar measurement is continuous in time and
(2.2)
where a and b are coefficients and I = intensity of rainfall in mm/h.
The values a and b for a given radar station have to be determined
by calibration with the help of recording gauges. At typical
equation for Z is
Z = 200I 1.60
Meteorological radars operate with wavelengths ranging from 3 to
10 cm, the common values being 5 and 10 cm. For observing
raingauge network
Since the catching area of a raingauge is very small compared to
the areal extent of a storm, it is obvious that to get a representative
picture of a storm over a catchment the number of raingauges
should be as large as possible, i.e. the catchment area per gauge
should be small. On the other hand, economic considerations to a
large extent and other considerations, such as topography,
accessibility, etc. to some extent restrict the number of gauges to
be maintained. Hence one aims at an optimum density of gauges
from which reasonably accurate information about the storms can
be obtained. Towards this the World Meteorological Organisation
(WMO) recommends the following densities.
• In flat regions of temperate, Mediterranean and tropical zones:
Ideal
1 station for 600900 km2
Acceptable
1 station for 9003000 km2
• In mountainous regions of temperate, Mediterranean and
topical zones:
Ideal
1 station for 100250 km2
Acceptable
1 station for 251000 km2
• In arid and polar zones: 1 station for 150010,000 km2
depending on the feasibility.
Ten per cent of raingauge stations should be equipped with selfrecording gauges to know the intensities of rainfall.
adequacy of raingauge statations
If there are already some raingauge stations in a catchment, the
In calculating N from Eq.(2.3) it is usual to take ε = 10%. It is seen
optimal number of stations that should exist to have an assigned
that if the value of ε is small, the number of raingauge stations will
percentage of error in the estimation of mean rainfall is obtained by
be more. According to WMO recommendations, at least 10% of the
statistical analysis as
total raingauges should be of self-recording type.
N=(
Cv
ε
)2
(2.3)
where N = optimal number of stations, ε = allowable degree of
error in the estimate of the mean rainfall and Cv = coefficient of
variation of the rainfall values at the existing m stations (in per cent).
If there are m stations in the catchment each recording rainfall
values P1, P2, . . . , Pi, . . . , Pm in a known time, the coefficient of
variation Cv is calculated as:
Cv =
100 × σm−1
P̄
m
where
σm−1 =
[
∑1 (Pi − P̄)2
m−1
= standard deviation
Pi = precipitation magnitude in the i th station
1 m
P̄ = (
Pi) = mean precipitation
m ∑
1
ENGINEERING
HYDROLOGY
2 |
Precipitation
Before using the rainfall records of a station, it is necessary to first
2.3 | Precipitation Data
Analysis
preparation
of data
INTENDED LEARNING OUTCOMES
• Estimate missing data from rainfall
measurements.
• Discuss the test for consistency of
rainfall data.
check the data for continuity and consistency. The continuity of a
record may be broken with missing data due to many reasons such
as damage or fault in a raingauge during a period. The missing
data can be estimated by using the data of the neighbouring
stations. In these calculations the normal rainfall is used as a
standard of comparison. The normal rainfall is the average value of
rainfall at a particular date, month or year over a specified 30-year
period. The 30-year normals are recomputed every decade. Thus
the term normal annual percipitation at station A means the
average annual precipitation at A based on a specified 30-years of
record.
estimation of missing data
test for consistency of record
If the conditions relevant to the recording of a raingauge station
precipitation values at station X beyond the period of change of
have undergone a significant change during the period of record,
regime (point 63 in Fig. 2.7) is corrected by using the relation
inconsistency would arise in the rainfall data of that station. This
Pcx = Px
inconsistency would be felt from the time the significant change
took place. Some of the common causes for inconsistency of record
are: (i) shifting of a raingauge station to a new location, (ii) the
neighbourhood of the station undergoing a marked change, (iii)
change in the ecosystem due to calamities, such as forest fires, land
slides, and (iv) occurrence of observational error from a certain
date. The checking for inconsistency of a record is done by the
double-mass curve technique. This technique is based on the
principle that when each recorded data comes from the same
parent population, they are consistent.
A group of 5 to 10 base stations in the neighbourhood of the
problem station X is selected. The data of the annual (or monthly or
seasonal mean) rainfall of the station X and also the average rainfall
of the group of base stations covering a long period is arranged in
the reverse chronological order (i.e. the latest record as the first
entry and the oldest record as the last entry in the list). The
accumulated precipitation of the station X (i.e. ΣPx ) and the
accumulated values of the average of the group of base stations
(i.e. ΣPav ) are calculated starting from the latest record. Values of
ΣPx are plotted against ΣPav for various consecutive time periods
(Fig. 2.7). A decided break in the slope of the resulting plot
indicates a change in the precipitation regime of station X. The
where
Me
Ma
(2.6)
Pcx = corrected precipitation at any time period t1at
station X
Px = original recorded precipitation at any time
period t1 at station X
Mc = corrected slope of the double-mass curve
Ma = original slope of the double-mass curve
In this way the older records are brought to the new regime of the
station. It is apparent that the more homogeneous the base station
records are, the more accurate will be the corrected values at
station X. A change in the slope is normally taken as significant only
where it persists for more than five years. The double-mass curve is
also helpful in checking systematic arithmetical errors in
transferring rainfall data from one record to another.
2.3 | Precipitation Data
Analysis
Presentation of
rainfall data
INTENDED LEARNING OUTCOMES
• Understand the different ways of
presenting rainfall data.
mass curve of rainfall
The mass curve of rainfall is a plot of the accumulated precipitation against
time, plotted in chronological order. Records of float type and weighing
bucket type gauges are of this form. A typical mass curve of rainfall at a
station during a storm is shown in Fig. 2.9. Mass curves of rainfall are very
useful in extracting the information on the duration and magnitude of a
storm. Also, intensities at various time intervals in a storm can be obtained
by the slope of the curve. For nonrecording raingauges, mass curves are
prepared from a knowledge of the approximate beginning and end of a
storm and by using the mass curves of adjacent recording gauge stations as
a guide.
hyetograph
A hyetograph is a plot of the intensity of rainfall against the time
precipitation received in the period. The time interval used
interval. The hyetograph is derived from the mass curve and is
depends on the purpose, in urban-drainage problems small
usually represented as a bar chart (Fig. 2.10). It is a very convenient
durations are used while in flood-flow computations in larger
way of representing the characteristics of a storm and is particularly
catchments the intervals are of about 6 h.
important in the development of design storms to predict extreme
floods. The area under a hyetograph represents the total
Point rainfall
Point rainfall, also known as station rainfall refers to the rainfall data
of a station. Depending upon the need, data can be listed as daily,
weekly, monthly, seasonal or annual values for various periods.
Graphically these data are represented as plots of magnitude vs
chronological time in the form of a bar diagram. Such a plot,
however, is not convenient for discerning a trend in the rainfall as
there will be considerable variations in the rainfall values leading to
rapid changes in the plot. The trend is often discerned by the
method of moving averages, also known as moving means.
MOVING AVERAGE. Moving average is a technique for
smoothening out the high frequency fl uctuations of a time series
and to enable the trend, if any, to be noticed. The basic principle is
that a window of time range m years is selected. Starting from the
fi rst set of m years of data, the average of the data for m years is
calculated and placed in the middle year of the range m. The
window is next moved sequentially one time unit (year) at a time
and the mean of the m terms in the window is determined at each
window location. The value of m can be 3 or more years; usually an
odd value. Generally, the larger the size of the range m, the greater
is the smoothening. There are many ways of averaging (and
consequently the plotting position of the mean) and the method
described above is called Central Simple Moving Average.
Example 2.4 describes the application of the method of moving
averages.
As indicated earlier, raingauges represent only point sampling of
the a real distribution of a storm. In practice, however, hydrological
2.3 | Precipitation Data
Analysis
analysis requires a knowledge of the rainfall over an area, such as
Mean precipitation
over and area
To convert the point rainfall values at various stations into an
INTENDED LEARNING OUTCOMES
• Compute the average value of
rainfall over a catchment area.
over a catchment.
average value over a catchment the following three methods are in
use: (i) Arithmetical-mean method, (ii) Thiessen-polygon method,
and (iii) Isohyetal method.
Arithmetical-mean method
Thiessen-mean method
Isohyetal method
The areal distribution characteristics of a storm of given duration is
reflected in its depth-area relationship. A few aspects of the
2.3 | Precipitation Data
Analysis
depth-area
duration
relationships
INTENDED LEARNING OUTCOMES
interdependency of depth, area and duration of storms are
discussed below.
depth-area relation
For a rainfall of a given duration, the average depth decreases with
the area in an exponential fashion given by
• Describe the relationship between
land area and rainfall depth.
P̄ = P0 exp (−K A n)
where P̄ = average depth in cm over an area A km2, P0 = highest
amount of rainfall in cm at the storm centre and K and n are
constants for a given region. Since it is very unlikely that the storm
centre coincides over a raingauge station, the exact determination
of P0 is not possible. Hence in the analysis of large area storms the
highest station rainfall is taken as the average depth over an area
of 25 km2.
maximum depth-area-duration curves
In many hydraulic-engineering applications such as those
concerned with floods, the probability of occurrence of a particular
2.3 | Precipitation Data
Analysis
frequency of
point rainfall
INTENDED LEARNING OUTCOMES
• Compute the probability of
rainfall.
extreme rainfall, e.g. a 24-h maximum rainfall, will be of
importance. Such information is obtained by the frequency analysis
of the point-rainfall data. The rainfall at a place is a random
hydrologic process and a sequence of rainfall data at a place when
arranged in chronological order constitute a time series. One of
the commonly used data series is the annual series composed of
annual values such as annual rainfall. If the extreme values of a
specified event occurring in each year is listed, it also constitutes
an annual series. Thus for example, one may list the maximum 24-h
rainfall occurring in a year at a station to prepare an annual series
of 24-h maximum rainfall values. The probability of occurrence of
an event in this series is studied by frequency analysis of this
annual data series. A brief description of the terminology and a
simple method of predicting the frequency of an event is
described in this section and for details the reader is referred to
standard works on probability and statistical methods. The analysis
of annual series, even though described with rainfall as a reference
is equally applicable to any other random hydrological process,
e.g. stream flow.
First, it is necessary to correctly understand the terminology used in
a. The probability of an event of exceedence probability P
frequency analysis. The probability of occurrence of an event of a
occurring 2 times in n successive years is
random variable (e.g. rainfall) whose magnitude is equal to or in
P2,n =
excess of a specified magnitude X is denoted by P. The recurrence
interval (also known as return period) is defined as
T = 1/P
b. The probability of the event not occurring at all in n successive
years is
(2.11)
This represents the average interval between the occurrence of a
rainfall of magnitude equal to or greater than X. Thus if it is stated
that the return period of rainfall of 20 cm in 24 h is 10 years at a
certain station A, it implies that on an average rainfall magnitudes
equal to or greater than 20 cm in 24 h occur once in 10 years, i.e. in
a long period of say 100 years, 10 such events can be expected.
However, it does not mean that every 10 years one such event is
likely, i.e. periodicity is not implied. The probability of a rainfall of
20 cm in 24 h occurring in anyone year at station A is 1/ T = 1/10 =
0.l.
If the probability of an event occurring is P, the probability of the
event not occurring in a given year is q = (1P). The binomial
distribution can be used to find the probability of occurrence of the
event r times in n successive years. Thus
Pr,n =n Cr Pr q n−r =
n!
P r q n−r
(n − r)!r!
(2.12)
where P r, n = probability of a random hydrologic event (rainfall) of
given magnitude and exceedence probability P occurring r times in
n successive years. Thus, for example,
n!
P 2q n−2
(n − 2)!2!
P0,n = q n = (1 − P)n
c. The probability of the event occurring at least once in n
successive years
P1 = 1 − q n = 1 − (1 − P)n
(2.13)
plotting position
In the design of major hydraulic structures such as spillways
2.3 | Precipitation Data
Analysis
probable maximum
precipitation (pmp)
in large dams, the hydrologist and hydraulic engineer
would like to keep the failure probability as low as possible,
i.e. virtually zero. This is because the failure of such a major
structure will cause very heavy damages to life, property,
economy and national morale. In the design and analysis of
such structures, the maximum possible precipitation that
can reasonably be expected at a given location is used.
This stems from the recognition that there is a physical
upper limit to the amount of precipitation that can fall over
INTENDED LEARNING OUTCOMES
• Describe the maximum amount of
precipitation over a specified area
in a given time.
a specified area in a given time.
world’s greatest observed rainfall
ENGINEERING
HYDROLOGY
3 |
Evaporation,
Transpiration,
Interception, and
Depression Storage
3.1 | Evaporation From
Free Surface
Measurement
evaporation
In Engineering Hydrology runoff due to a storm event is often the
major subject of study. All abstractions from precipitation, viz.
those due to evaporation, transpiration, infiltration, surface
detention and storage, are considered as losses in the production
of runoff.
Evaporation is the process in which a liquid changes to the
gaseous state at the free surface, below the boiling point through
the transfer of heat energy. Consider a body of water in a pond.
The molecules of water are in constant motion with a wide range of
instantaneous velocities. An addition of heat causes this range and
INTENDED LEARNING OUTCOMES
• Discuss the evaporation process
and the factors affecting it.
• Identify devices used in measuring
evaporation.
• Use the different methods in
estimating evaporation.
average speed to increase. When some molecules possess
suffi cient kinetic energy, they may cross over the water surface.
Similarly, the atmosphere in the immediate neighborhood of the
water surface contains water molecules within the water vapor in
motion and some of them may penetrate the water surface. The
net escape of water molecules from the liquid state to the gaseous
state constitutes evaporation. Evaporation is a cooling process in
that the latent heat of vaporization (at about 585 cal/g of
evaporated water) must be provided by the water body. The rate of
evaporation is dependent on (i) the vapor pressures at the water
surface and air above, (ii) air and water temperatures, (iii) wind
speed, (iv) atmospheric pressure, (v) quality of water, and (vi) size
of the water body.
evaporation process
• VAPOR PRESSURE. The rate of evaporation is proportional to the
beyond which any further increase in the wind speed has no
difference between the saturation vapour pressure at the water
infl uence on the evaporation rate. This critical windspeed value is
temperature, e w and the actual vapour pressure in the air, ea .
a function of the size of the water surface. For large water bodies
Thus
highspeed turbulent winds are needed to cause maximum rate of
EL = C(ew − ea)
(3.1)
where EL = rate of evaporation (mm/day) and C = a constant; ew
and ea are in mm of mercury. Equation (3.1) is known as Daltons
law of evaporation after John Dalton (1802) who fi rst
recognized this law. Evaporation continues till ew = ea . If ew > ea
condensation takes place.
• TEMPERATURE. Other factors remaining the same, the rate of
evaporation increases with an increase in the water temperature.
Regarding air temperature, although there is a general increase in
the evaporation rate with increasing temperature, a high
correlation between evaporation rate and air temperature does
not exist. Thus for the same mean monthly temperature it is
possible to have evaporation to different degrees in a lake in
different months.
• WIND. Wind aids in removing the evaporated water vapour from
the zone of evaporation and consequently creates greater scope
for evaporation. However, if the wind velocity is large enough to
remove all the evaporated water vapour, any further increase in
wind velocity does not infl uence the evaporation. Thus the rate of
evaporation increases with the wind speed up to a critical speed
evaporation.
• ATMOSPHERIC PRESSURE. Other factors remaining same, a
decrease in the barometric pressure, as in high altitudes,
increases evaporation.
• SOLUBLE SALTS. When When a solute is dissolved in water, the
vapor pressure of the solution is less than that of pure water and
hence causes reduction in the rate of evaporation. The percent
reduction in evaporation approximately corresponds to the
percentage increase in the specifi c gravity. Thus, for example,
under identical conditions evaporation from sea water is about
2-3% less than that from fresh water.
• HEAT STORAGE IN WATER BODIES. Deep water bodies have
more heat storage than shallow ones. A deep lake may store
radiation energy received in summer and release it in winter
causing less evaporation in summer and more evaporation in
winter compared to a shallow lake exposed to a similar situation.
However, the effect of heat storage is essentially to change the
seasonal evaporation rates and the annual evaporation rate is
seldom affected.
evaporimeters
Estimation of evaporation is of utmost importance in many
• CLASS A EVAPORATION PAN. It is a standard pan of 1210 mm
hydrologic problems associated with planning and operation of
diameter and 255 mm depth used by the US Weather Bureau and
reservoirs and irrigation systems. In arid zones, this estimation is
is known as Class A Land Pan. The depth of water is maintained
particularly important to conserve the scarce water resources.
between 18 cm and 20 cm (Fig. 3.1). The pan is normally made of
However, the exact measurement of evaporation from a large body
unpainted galvanised iron sheet. Monel metal is used where
of water is indeed one of the most difficult tasks.
corrosion is a problem. The pan is placed on a wooden platform
The amount of water evaporated from a water surface is estimated
by the following methods: (i) using evaporimeter data, (ii) empirical
evaporation equations, and (iii) analytical methods.
Evaporimeters are water-containing pans which are exposed to the
atmosphere and the loss of water by evaporation measured in them
at regular intervals. Meteorological data, such as humidity, wind
movement, air and water temperatures and precipitation are also
noted along with evaporation measurement.
of 15 cm height above the ground to allow free circulation of air
below the pan. Evaporation measurements are made by
measuring the depth of water with a hook gauge in a stilling well.
• ISI STANDARD PAN. This pan evaporimeter specifi ed by IS:
• COLORADO SUNKEN PAN. This pan, 920 mm square and 460
5973-1970, also known as modified Class A Pan, consists of a
mm deep is made up of unpainted galvanized iron sheet and
pan 1220 mm in diameter with 255 mm of depth. The pan is
buried into the ground within 100 mm of the top (Fig. 3.3). The
made of copper sheet of 0.9 mm thickness, tinned inside and
chief advantage of the sunken pan is that radiation and
painted white outside (Fig. 3.2). A fi xed point gauge indicates the
aerodynamic characteristics are similar to those of a lake.
level of water. A calibrated cylindrical measure is used to add or
However, it has the following disadvantages: (i) diffi cult to detect
remove water maintaining the water level in the pan to a fi xed
leaks, (ii) extra care is needed to keep the surrounding area free
mark. The top of the pan is covered fully with a hexagonal wire
from tall grass, dust, etc., and (iii) expensive to install.
netting of galvanized iron to protect the water in the pan from
birds. Further, the presence of a wire mesh makes the water
temperature more uniform during day and night. The evaporation
from this pan is found to be less by about 14% compared to that
from unscreened pan. The pan is placed over a square wooden
platform of 1225 mm width and 100 mm height to enable
circulation of air underneath the pan.
• US GEOLOGICAL SURVEY FLOATING PAN. With a view to
In view of the above, the evaporation observed from a pan
simulate the characteristics of a large body of water, this square
has to be corrected to get the evaporation from a lake under
pan (900 mm side and 450 mm depth) supported by drum fl oats
similar climatic and exposure conditions. Thus a coefficient is
in the middle of a raft (4.25 m ´ 4.87 m) is set afl oat in a lake. The
introduced as
water level in the pan is kept at the same level as the lake leaving
a rim of 75 mm. Diagonal baffl es provided in the pan reduce the
surging in the pan due to wave action. Its high cost of installation
and maintenance together with the diffi culty involved in
performing measurements are its main disadvantages.
Lake evaporation = CP x pan evaporation
in which CP = pan coefficient. The values of CP in use for
different pans are given in Table 3.l.
• PAN COEFFICIENT CP . Evaporation pans are not exact models of
large reservoirs and have the following principal drawbacks:
1. They differ in the heat-storing capacity and heat transfer from
the sides and bottom. The sunken pan and floating pan aim
to reduce this deficiency. As a result of this factor the
evaporation from a pan depends to a certain extent on its
size. While a pan of 3 m diameter is known to give a value
which is about the same as from a neighboring large lake, a
pan of size 1.0 m diameter indicates about 20% excess
evaporation than that of the 3 m diameter pan.
2. The height of the rim in an evaporation pan affects the wind
action over the surface. Also, it casts a shadow of variable
magnitude over the water surface.
3. The heat-transfer characteristics of the pan material is
different from that of the reservoir.
• EVAPORATION STATIONS. It is usual to instal evaporation pans
in such locations where other meteorological data are also
simultaneously collected. The WMO recommends the minimum
network of evaporimeter stations as below:
1. Arid zonesOne station for every 30,000 km2 ,
2. Humid temperate climatesOne station for every 50,000 km2 ,
and
3. Cold regionsOne station for every 100,000 km2 .
empirical evaporation equations
A large number of empirical equations are available to estimate
• ROHWER’S FORMULA (1931). Rohwer’s formula considers a
lake evaporation using commonly available meteorological data.
correction for the effect of pressure in addition to the wind-speed
Most formulae are based on the Daltontype equation and can be
effect and is given by
expressed in the general form
EL = 0.771(1.465 − 0.000732pa)(0.44 + 0.0733u0)(ew − ea)
EL = K f (u)(ew − ea)
(3.2)
(3.4)
in which EL , ew , and ea are as defi ned earlier in Eq. (3.2), pa =
mean barometric reading in mm of mercury, u0 = mean wind
where EL = lake evaporation in mm/day, ew = saturated vapor
velocity in km/h at ground level, which can be taken to be the
actual vapor pressure of over-lying air at a specified height in mm
In using the empirical equations, the saturated vapor pressure at a
pressure at the water surface temperature in mm of mercury, ea =
of mercury, f (u) = wind-speed correction function and K = a
coefficient. The term ea is measured at the same height at which
wind speed is measured. Two commonly used empirical
evaporation formulae are:
EL = KM (ew − ea)(1 +
16
given temperature (ew ) is found from a table of ew vs temperature in
°C. Often, the wind-velocity data would be available at an elevation
other than that needed in the particular equation. However, it is
known that in the lower part of the atmosphere, up to a height of
about 500 m above the ground level, the wind velocity can be
• MEYER’S FORMULA (1915)
u9
velocity at 0.6 m height above ground.
)
(3.3)
in which EL , ew , ea are as defi ned in Eq. (3.2), u9 = monthly mean
wind velocity in km/ h at about 9 m above ground and KM =
coeffi cient accounting for various other factors with a value of
0.36 for large deep waters and 0.50 for small, shallow waters.
assumed to follow the 1/7 power law as
uh = Ch 1/7
(3.5)
where uh = wind velocity at a height h above the ground and C =
constant. This equation can be used to determine the velocity at
any desired level if uh is known.
analytical methods of evaporation estimation
WATER BUDGET METHOD
analytical methods of evaporation estimation
ENERGY-BUDGET METHOD
ENGINEERING
HYDROLOGY
3 |
Evaporation,
Transpiration,
Interception, and
Depression Storage
TRANSPIRATION
3.2 | Transpiration and
Evaporation
evapotranspiration
• The process by which water leaves the body of a living plant
and reaches the atmosphere as water vapour. The water is
taken up by the plant-root system and escapes through the
leaves.
• The important factors affecting transpiration are: atmospheric
vapour pressure, temperature, wind, light intensity and
characteristics of the plant, such as the root and leaf systems.
For a given plant, factors that affect the free-water evaporation
INTENDED LEARNING OUTCOMES
• Differentiate transpiration from
evaporation.
also affect transpiration.
• For a given plant, factors that affect the free-water evaporation
also affect transpiration. However, a major difference exists
between transpiration and evaporation. Transpiration is
• Identify the factors affecting
transpiration.
essentially confined to daylight hours and the rate of
• Discuss the significance of
evapotranspiration in hydrology.
Evaporation, on the other hand, continues all through the day
• Identify devices used in measuring
evapotranspiration in the field.
transpiration depends upon the growth periods of the plant.
and night although the rates are different.
evapotranspiration
• While transpiration takes place, the land area in which plants
stand also lose moisture by the evaporation of water from soil
and water bodies. In hydrology and irrigation practice, it is
found that evaporation and transpiration processes can be
considered advantageously under one head as
evapotranspiration.
• The term consumptive use is also used to denote this loss by
evapotranspiration.
• For a given set of atmospheric conditions, evapotranspiration
obviously depends on the availability of water. If sufficient
moisture is always available to completely meet the needs of
v e g e t at i o n f u l l y c o v e r i n g t h e a rea , t h e re s u l t i n g
evapotranspiration is called potential evapotranspiration
(PET). Potential evapotranspiration no longer critically
depends on the soil and plant factors but depends essentially
on the climatic factors.
• The real evapotranspiration occurring in a specific situation is
called actual evapotranspiration (AET).
• Field capacity is the maximum quantity of water that the soil can
retain against the force of gravity. Any higher moisture input to a
soil at field capacity simply drains away.
outflow, E act = actual evapotranspiration (AET) and ΔS = change
in the moisture storage. This water budgeting can be used to
• Permanent wilting point is the moisture content of a soil at which
the moisture is no longer available in sufficient quantity to sustain
the plants. At this stage, even though the soil contains some
moisture, it will be so held by the soil grains that the roots of the
plants are not able to extract it in sufficient quantities to sustain
the plants and consequently the plants wilt.
calculate E act by knowing or estimating other elements of Eq.
(3.12). Generally, the sum of Rs and Go can be taken as the stream
flow at the basin outlet without much error.
• Except in a few specialized studies, all applied studies in
hydrology use PET (not AET) as a basic parameter in various
estimations related to water utilizations connected with
• The field capacity and permanent wilting point depend upon the
soil characteristics. The difference between these two moisture
contents is called available water, the moisture available for plant
growth.
• If the water supply to the plant is adequate, soil moisture will be at
the field capacity and AET will be equal to PET.
• If the water supply is less than PET, the soil dries out and the ratio
AET/PET would then be less than unity.
• The decrease of the ratio AET/PET with available moisture
depends upon the type of soil and rate of drying. Generally, for
clayey soils, AET/PET = 1.0 for nearly 50% drop in the available
moisture. As can be expected, when the soil moisture reaches the
permanent wilting point, the AET reduces to zero (Fig. 3.5). For a
catchment in a given period of time, the hydrologic budget can
be written as
P − Rs − Go − Eact = ΔS
where P = precipitation, Rs = surface runoff, Go = subsurface
(3.12)
evapotranspiration process. It is generally agreed that PET is a
good approximation for lake evaporation. As such, where pan
evaporation data is not available, PET can be used to estimate
lake evaporation.
measurement of evapotranspiration
The measurement of evapotranspiration for a given vegetation type
• FIELD PLOTS. In special plots all the elements of the water
can be carried out in two ways: either by using lysimeters or by the
budget in a known interval of time are measured and the
use of field plots.
evapotranspiration determined as
• LYSIMETERS. A lysimeter is a special watertight tank containing a
block of soil and set in a fi eld of growing plants. The plants grown
in the lysimeter are the same as in the surrounding fi eld.
Evapotranspiration is estimated in terms of the amount of water
required to maintain constant moisture conditions within the tank
measured either volumetric ally or gravimetrically through an
arrangement made in the lysimeter. Lysimeters should be
designed to accurately
reproduce the soil
conditions, moisture
content, type and size of
the vegetation of the
surrounding area. They
should be so buried that
the soil is at the same
level inside and outside
the container. Lysimeter
studies are timeconsuming
expensive.
and
Evapotranspiration = [precipitation + irrigation input
runoff
increase in soil storage groundwater loss]
Measurements are usually confi ned to precipitation, irrigation
input, surface runoff and soil moisture. Groundwater loss due to
deep percolation is diffi cult to measure and can be minimised by
keeping the moisture condition of the plot at the fi eld capacity.
This method provides fairly reliable results.
ENGINEERING
HYDROLOGY
3 |
Evaporation,
Transpiration,
Interception, and
Depression Storage
• Interception is the removal of water that wets and adheres to plant foliage,
buildings, and other objects above ground surface. This water is subsequently
3.3 | Interception &
Depression Storage
interception
removed from the surface through evaporation.
• Interception can be as high as 2 mm during a single rainfall event, but typically
removes about 0.5 mm during a single rainfall/storm event.
• The quantity of water removed through interception is usually not significant for
an isolated storm, but, when added over a period of time, it can be significant. It
is thought that as much as 25 percent of the total annual precipitation for certain
heavily forested areas of the Pacific Northwest of the United States is lost
through interception during the course of a year.
• Interception is the process by which water is captured on vegetation (leaves,
INTENDED LEARNING OUTCOMES
bark, grasses, crops, etc.) during a precipitation event. Intercepted precipitation
• Discuss the process of interception
and its main components.
through evaporation. Interception losses generally occur during the first part of a
• Describe factors affecting
interception.
is not available for runoff or infiltration, but instead is returned to the atmosphere
precipitation event and the interception loss rate trends toward zero rather
quickly.
• Interception losses are described by the following
Interception loss prevents water from reaching the
equation: Li = S + KEt , where Li is the total volume of
ground surface and is regarded as a primary water
water intercepted, S is the interception storage, K is the
ration of the surface area of the leaves to the area of the
entire canopy, E is the rate of evaporation during the
precipitation event, and t is time.
• As the Horton equation suggests, the total interception is
dependent on the storm duration, as longer duration
storms allow more evaporation from the canopy during
the storm event. The intensity of the storm also plays a
role in canopy interception however, there is debate as to
whether intensity increases or decreases interception
storage in canopy.
• There are many other factors that influence interception
potential. Interception varies widely by season as
deciduous trees lose much of their canopy storage
potential during winter months.
• There are three main components of interception:
Interception loss, throughfall, and stemfall.
1. INTERCEPTION LOSS: The water that is retained
by vegetation surfaces that is later evaporated into
the atmosphere, or absorbed by the plant.
loss.
2. THROUGHFALL: The water which falls through
spaces in the vegetation canopy, or which drips
from the leaves, twigs and stems and falls to the
ground.
3. STEMFLOW: The water which trickles along the
stems and branches and down the main stem or
trunk to the ground surface.
factors affecting interception
• INTERCEPTION STORAGE: The ability of vegetation
interception storage is large, so the frequency of re-
surfaces to collect and retain Precipitation, capacity will
wetting is more significant than the duration and amount
be highest at the onset of rainfall when the vegetation is
of rainfall.
dry, when water is held by surface tension.
• TYPE AND MORPHOLOGY OF THE VEGETATION
• EVAPORATION: Even when the interception storage
COVER: Different vegetation types have different
capacity is exceeded water may be lost by evaporation
interception storage capacities, aerodynamic roughness
off leaf surfaces, which increases in windy conditions,
characteristics, rate of evaporation from their surfaces.
though the interception storage capacity may be
Interception losses are generally greater from trees than
reduced with increased windspeed.
other types of vegetation (grasses and agricultural
• DURATION OF RAINFALL: Influences interception by
determining the balance between reduced storage of
water on vegetation surfaces and increased evaporative
loss over time.Total interception losses increase with
duration of rainfall (but only gradually), though the
relative importance of interception decreases with
time.The importance of interception decreases with time,
due to duration of rainfall and changes in the
interception storage capacity.
• RAINFALL FREQUENCY: The highest levels of
interception loss occur when the leaves are dry and
crops)due to the greater aerodynamic roughness of trees
in promoting increased evaporation in wet conditions or
to their higher interception capacities (in some cases)
especially when wetted and dried frequently.
Interception losses are greater from coniferous forests
than from deciduous woodlands.
• Depression storage is the term applied to water that is lost because it becomes
trapped in the numerous small depressions that are characteristic of any natural
3.3 | Interception &
Depression Storage
depression
storage
surface.
• When water temporarily accumulates in a low point with no possibility for
escape as runoff, the accumulation is referred to as depression storage.
• The amount of water that is lost due to depression storage varies greatly with the
land use. A paved surface will not detain as much water as a recently furrowed
field.
• The relative importance of depression storage in determining the runoff from a
given storm depends on the amount and intensity of precipitation in the storm.
Typical values for depression storage range from 1 to 8 mm (0.04 to 0.3 in) with
INTENDED LEARNING OUTCOMES
some values as high as 15 mm (0.6 in) per event. As with evaporation and
• Define depression storage.
design.
• Identify factors affecting the
depression storage.
transpiration, depression storage is generally not directly calculated in highway
• If the soil surface has a low infiltration capacity and low hydraulic conductivity,
and if the topography allows for surface storage, then water may be stored at the
surface in small pools or depressions. These water-filled depressions, called
vernal pools, are often seasonal features that form because of perched water
tables. These depression storage areas may become hydrologically connected
during high water conditions and develop a flow network to deliver water to
streams or other surface water bodies.
• Depression storage refers to small low points in
undulating terrain that can store precipitation that
otherwise would become runoff. The precipitation stored
in these depressions is then either removed through
infiltration into the ground or by evaporation. Depression
storage exists on both pervious and impervious surfaces.
• The volume of water in depression storage at any time
during a precipitation event can be approximated as
(Linsley 1982):
V = Sd(1 − e −kPe)
(4.1)
where V is the volume of water in depression storage, Sd
is the maximum storage capacity of the depression, Pe is
the rainfall excess, and k is a constant equal to 1/Sd.
• FACTORS AFFECTING DEPRESSION STORAGE: Nature
of terrain, slope, type of soil surface, land surface,
antecedent rainfall, time.
ENGINEERING
HYDROLOGY
REFERENCE:
Rainfall-Runoff Processes
by David G. Tarboton
4 |
Surface and
Subsurface Runoff
Phenomenon
An important question in hydrology is how much stream flow occurs in a river
in response to a given amount of rainfall. To answer this question we need to
4.1 | Rainfall-Runoff
Processes
know where water goes when it rains, how long does water reside in a
runoff
processes
more generally surface water input – runoff processes. The term, "surface
INTENDED LEARNING OUTCOMES
• Understand the process involved
in the transformation of surface
water input to runoff at the earth
surface.
watershed, and what pathway does water take to the stream channel. These
are the questions addressed in the study of rainfall – runoff processes, or
water input" is used in preference to rainfall or precipitation to be inclusive
of snowmelt as a driver for runoff.
Answering the question of how much runoff is generated from surface water
inputs requires partitioning water inputs at the earth surface into
components that infiltrate and components that flow overland and directly
enter streams. The pathways followed by infiltrated water need to be
understood. Infiltrated water can follow subsurface pathways that take it to
the stream relatively quickly, in which case it is called interflow or
subsurface stormflow. Infiltrated water can also percolate to deep
groundwater, which may sustain the steady flow in streams over much
longer time scales that is called baseflow. Infiltrated water can also remain
in the soil to later evaporate or be transpired back to the atmosphere. The
• Discuss the physical factors at the
land surface that affect runoff.
paths taken by water determine many of the characteristics of a landscape,
• Discuss the current understanding
of runoff processes
modeling the rainfall – runoff process is therefore important in many flood
the occurrence and size of floods, the uses to which land may be put and the
strategies required for wise land management. Understanding and
and water resources problems. Figure 1 illustrates schematically many of the
processes involved in the generation of runoff.
runoff processes
The paths water can take in moving to a stream are illustrated in Figure 1.
Precipitation may be in the form of rain or snow. Vegetation may
intercept some fraction of precipitation. Precipitation that penetrates the
vegetation is referred to as throughfall and may consist of both
precipitation that does not contact the vegetation, or that drops or drains
off the vegetation after being intercepted. A large fraction of intercepted
water is commonly evaporated back to the atmosphere. There is also flux
of water to the atmosphere through transpiration of the vegetation and
evaporation from soil and water bodies. The surface water input
available for the generation of runoff consists of throughfall and
snowmelt. This surface water input may accumulate on the surface in
depression storage, or flow overland towards the streams as overland
flow, or infiltrate into the soil, where it may flow laterally towards the
stream contributing to interflow. Infiltrated water may also percolate
through deeper soil and rock layers into the groundwater. The water
table is the surface below which the soil and rock is saturated and at
pressure greater than atmospheric. This serves as the boundary between
the saturated zone containing groundwater and unsaturated zone. Water
added to the groundwater is referred to as groundwater recharge.
Immediately above the water table is a region of soil that is close to
saturation, due to water being held by capillary forces. This is referred to
as the capillary fringe. Lateral drainage of the groundwater into streams
is referred to as baseflow, because it sustains streamflow during rainless
periods. Subsurface water, either from interflow or from groundwater
may flow back across the land surface to add to overland flow. This is
referred to as return flow. Overland flow and shallower interflow
processes that transport water to the stream within the time scale of
approximately a day or so are classified as runoff. Water that percolates
to the groundwater moves at much lower velocities and reaches the
stream over longer periods of time such as weeks, months or even years.
The terms quick flow and delayed flow are also used to describe and
distinguish between runoff and baseflow. Runoff includes surface runoff
(overland flow) and subsurface runoff or subsurface stormflow (interflow).
runoff GENERATION MECHANISMS
Figure 2 depicts a cross section through a hillslope that exposes in more
detail the pathways infiltrated water may follow. Infiltrated water may
flow through the matrix of the soil in the inter-granular pores and small
structural voids. Infiltrated water may also flow through larger voids
referred to as macropores. Macropores include pipes that are open
passageways in the soil caused by decaying roots and burrowing
animals. Macropores also include larger structural voids within the soil
matrix that serve as preferential pathways for subsurface flow. The
permeability of the soil matrix may differ between soil horizons and this
may lead to the build up of a saturated wedge above a soil horizon
interface. Water in these saturated wedges may flow laterally through the
soil matrix, or enter macropores and be carried rapidly to the stream as
subsurface stormflow in the form of interflow.
Recent research in hillslope hydrology involving tracers, especially in
humid catchments has found that the dominant contributor to stormflow
in the stream is pre-event water (averaging 75% world wide, Buttle,
1994). Pre-event water is water that was present in the hillslope before
the storm as identified by a distinct isotopic or chemical composition.
Another consensus emerging from recent research is that interflow
involving preferential flow through macropores is a ubiquitous
phenomenon in natural soils. Rapid lateral flow through a network of
macropores and the effusion of old water into stream channels is the
primary mechanism for runoff generation in many humid regions where
overland flow is rarely observed. This mechanism has been linked to
nonlinear threshold type behavior in hillslope runoff response. Figure 3
shows how runoff ratio, the fraction of precipitation that appears as
runoff, is dependent upon soil moisture content. Soil moisture content
needs to exceed a threshold before any significant runoff occurs. Figure
4 shows the relationship between depth to groundwater and runoff at
two different hillslope locations (Seibert et al., 2003) that also shows
threshold behavior, with runoff being more tightly related to depth to
groundwater near the stream than further up a hillslope.
With this background on the pathways followed by infiltrated water we
can examine the mechanisms involved in the generation of runoff
(Figure 6). Each mechanism has a different response to rainfall or
snowmelt in the volume of runoff produced, the peak discharge rate,
Natural soils contain heterogeneities that lead to variability in the
and the timing of contributions to streamflow in the channel. The relative
infiltration process itself. Infiltrating water follows preferential pathways
importance of each process is affected by climate, geology, topography,
and macropores and may result in increases in moisture content at depth
soil characteristics, vegetation and land use. The dominant process may
before saturation or similar increases in moisture content higher in the
vary between large and small storms.
soil profile. Figure 5a shows a photograph of a soil where dye has been
used to trace infiltration pathways in experiments reported by Weiler and
Naef (2003). Figure 5b shows the dye intensity objectively classified from
the photograph following excavation of the plot following a dye
sprinkling experiment.
In Figure 6a the infiltration excess overland flow mechanism is illustrated.
There is a maximum limiting rate at which a soil in a given condition can
absorb surface water input. This was referred to by Robert E. Horton
(1933), one of the founding fathers of quantitative hydrology, as the
infiltration capacity of the soil, and hence this mechanism is also called
Horton overland flow. Infiltration capacity is also referred to as
infiltrability. When surface water input exceeds infiltration capacity the
excess water accumulates on the soil surface and fills small depressions.
Water in depression storage does not directly contribute to overland
flow runoff; it either evaporates or infiltrates later. With continued surface
water input, the depression storage capacity is filled, and water spills
over to run down slope as an irregular sheet or to converge into rivulets
of overland flow. The amount of water stored on the hillside in the
process of flowing down slope is called surface detention. The transition
from depression storage to surface detention and overland flow is not
sharp, because some depressions may fill and contribute to overland
flow before others. Figure 7 illustrates the response, in terms of runoff
from a hillside plot due to rainfall rate exceeding infiltration capacity with
the filling of depression storage and increase in, and draining of, water in
surface detention during a storm. Note, in Figure 7, that infiltration
capacity declines during the storm, due to the pores being filled with
water reducing the capillary forces drawing water into pores.
Due to spatial variability of the soil properties affecting infiltration
capacity and due to spatial variability of surface water inputs, infiltration
excess runoff does not necessarily occur over a whole drainage basin
during a storm or surface water input event. Betson (1964) pointed out
that the area contributing to infiltration excess runoff may only be a small
portion of the watershed. This idea has become known as the partialarea concept of infiltration excess overland flow and is illustrated in
Figure 6b.
Infiltration excess overland flow occurs anywhere that surface water input
in the depth to the shallow water table. This variability of the extent of
exceeds the infiltration capacity of the surface. This occurs most
surface saturation is referred to as the variable source area concept
frequently in areas devoid of vegetation or possessing only a thin cover.
(Hewlett and Hibbert, 1967) and is illustrated in Figures 8 and 9.
Semi-arid rangelands and cultivated fields in regions with high rainfall
intensity are places where this process can be observed. It can also be
seen where the soil has been compacted or topsoil removed. Infiltration
Geometrical considerations dictate that near stream saturated zones will
be most extensive in locations with concave hillslope profiles and wide
excess overland flow is particularly obvious on paved urban areas.
flat valleys. However, saturated overland flow is not restricted to near-
In most humid regions infiltration capacities are high because vegetation
flow lines converge in slope concavities (hillslope hollows) and water
protects the soil from rain-packing and dispersal, and because the
arrives faster than it can be transmitted down slope as subsurface flow;
supply of humus and the activity of micro fauna create an open soil
(2) at concave slope breaks where the hydraulic gradient inducing
structure. Under such conditions surface water input intensities generally
subsurface flow from upslope is greater than that inducing down slope
do not exceed infiltration capacities and infiltration excess runoff is rare.
transmission; (3) where soil layers conducting subsurface flow are locally
Overland flow can occur due to surface water input on areas that are
thin; and (4) where hydraulic conductivity decreases abruptly or
already saturated. This is referred to as saturation excess overland flow,
gradually with depth and percolating water accumulates above the low-
illustrated in Figure 6c. Saturation excess overland flow occurs in
conductivity layers to form perched zones of saturation that reach the
locations where infiltrating water completely saturates the soil profile
surface.
until there is no space for any further water to infiltrate. The complete
saturation of a soil profile resulting in the water table rising to the surface
is referred to as saturation from below. Once saturation from below
occurs at a location all further surface water input at that location
stream areas. Saturation from below can also occur (1) where subsurface
Return flow (qr in Figure 6c) is subsurface water that returns to the
surface to add to overland flow. Return flow also occurs at places where
the soil thins, for example rock outcrops and may manifest in the form of
becomes overland flow runoff.
springs.
In humid areas streams are typically gaining streams (gaining water by
In areas with high infiltration capacities, interflow, or subsurface storm
drainage of baseflow from the groundwater into the stream) with the
groundwater table near the surface coincident or close to the stream
water surface elevation. This means that the water table near streams is
close to the ground surface, especially in flat topography, making these
near stream areas in flat topography particularly susceptible to saturation
from below. The extent of the area subject to saturation from below
varies in time, both at seasonal and event time scales due to fluctuations
flow is usually the dominant contributor to streamflow, especially on
steeper terrain or more planar hillslopes where saturation excess is less
likely to occur. A number of processes are involved in rapid subsurface
stormflow. These include transmissivity feedback, lateral flow at the soil
bedrock interface and groundwater ridging.
Transmissivity feedback (Weiler and McDonnell, 2003) is illustrated in
Figure 10 and occurs when water infiltrates rapidly along preferential
pathways and causes the groundwater to rise to the point where highly
permeable soil layers or macropore networks become activated and
transmit water rapidly downslope. Much of the water that drains from the
soil matrix into the macropore network is preevent water. This
mechanism results in a nonlinear threshold like response as illustrated in
Figures 3 and 4.
Lateral flow at the soil bedrock interface (Weiler and McDonnell, 2003)
illustrated in Figure 11, occurs in steep terrain with relatively thin soil
cover and low permeability bedrock, where water moves to depth
rapidly along preferential infiltration pathways and perches at the soilbedrock interface. Since moisture content near the bedrock interface is
often close to saturated, the addition of only a small amount of new
water (rainfall or snowmelt) is required to produce saturation at the soilbedrock or soil-impeding layer interface. Rapid lateral flow occurs at the
permeability interface through the transient saturated zone. Once rainfall
inputs cease, there is a rapid dissipation of positive pore water pressures
and the system reverts back to a slow drainage of matrix flow.
ENGINEERING
HYDROLOGY
4 |
Surface and
Subsurface Runoff
Phenomenon
Consider a concentrated storm producing a fairly uniform rainfall
4.2 | Hydrograph
of duration, D over a catchment. After the initial losses and
infiltration losses are met, the rainfall excess reaches the stream
through overland and channel flows. In the process of translation a
hydrograph
certain amount of storage is built up in the overland and channelflow phases. This storage gradually depletes after the cessation of
the rainfall. Thus there is a time lag between the occurrence of
rainfall in the basin and the time when that water passes the
gauging station at the basin outlet. The runoff measured at the
stream-gauging station will give a typical hydrograph as shown in
Fig. 6.1. The duration of the rainfall is also marked in this figure to
INTENDED LEARNING OUTCOMES
• Discuss the characteristics and
components of a hydrograph.
• Identify the factors affecting flood
hydrographs.
• Identify methods of base-flow
separation.
• Compute for the effective rainfall.
• Discuss the unit hydrograph
theory, its application, uses and
limitations.
indicate the time lag in the rainfall and runoff. The hydrograph of
this kind which results due to an isolated storm is typically
singlepeaked skew distribution of discharge and is known
variously as storm hydrograph, flood hydrograph or simply
hydrograph. It has three characteristic regions:
(i)
the rising limb AB, joining point A, the starting point of
the rising curve and point B, the point of inflection,
(ii)
the crest segment BC between the two points of
inflection with a peak P in between,
(iii) the falling limb or depletion curve CD starting from the
second point of inflection C.
The interactions of various storms and catchments are in
general extremely complex. If one examines the record of
a large number of flood hydrographs of a stream, it will be
found that many of them will have kinks, multiple peaks,
etc. resulting in shapes much different from the simple
single-peaked hydrograph of Fig. 6.1. These complex
hydrographs are the result of storm and catchment
peculiarities and their complex interactions. While it is
theoretically possible to resolve a complex hydrograph
into a set of simple hydrographs for purposes of
hydrograph analysis, the requisite data of acceptable
quality are seldom available. Hence, simple hydrographs
resulting from isolated storms are preferred for
The hydrograph is the response of a given catchment to a
rainfall input. It consists of flow in all the three phases of
runoff, viz. surface runoff, interflow and base flow, and
embodies in itself the integrated effects of a wide variety of
catchment and rainfall parameters having complex
interactions.
• Two different storms in a given catchment produce
hydrographs differing from each other.
• Identical storms in two catchments produce
hydrographs that are different.
hydrograph studies.
factors affecting flood hydrographs
The factors that affect the shape of the hydrograph can be
• SHAPE OF THE BASIN. The shape of the basin
broadly grouped into climatic factors and physiographic
infl uences the time taken for water from the remote parts
factors. Each of these two groups contains a host of factors
of the catchment to arrive at the outlet. Fan-shaped, i.e.
and the important ones are listed in Table 6.1. Generally,
nearly semi-circular shaped catchments give high peak
the climatic factors control the rising limb and the
and narrow hydrographs while elongated catchments
recession limb is independent of storm characteristics,
give broad and low-peaked hydrographs. Figure 6.2
being determined by catchment characteristics only. Many
shows schematically the hydrographs from three
of the factors listed in Table 6.1 are interdependent.
catchments having identical infi ltration characteristics
Further, their effects are very varied and complicated. As
due to identical rainfall over the catchment. In catchment
such only important effects are listed below in qualitative
A the hydrograph is skewed to the left, i.e. the peak
terms only.
occurs relatively quickly. In catchment B, the hydrograph
is skewed to the right, the peak occurring with a relatively
longer lag. Catchment C indicates the complex
hydrograph produced by a composite shape.
• SIZE. In small catchments the overland fl ow phase is
• DRAINAGE DENSITY. The drainage density is defi ned as
predominant over the channel fl ow. Hence the land use
the ratio of the total channel length to the total drainage
and intensity of rainfall have important role on the peak
area. A large drainage density creates situation
fl ood. On large basins these effects are suppressed as
conducive for quick disposal of runoff down the
the channel fl ow phase is more predominant. The peak
channels. This fast response is refl ected in a pronounced
discharge is found to vary as An where A is the catchment
peaked discharge. In basins with smaller drainage
area and n is an exponent whose value is less than unity,
densities, the overland fl ow is predominant and the
being about 0.5. The time base of the hydrographs from
resulting hydrograph is squat with a slowly rising limb
larger basins will be larger than those of corresponding
(Fig. 6.3).
hydrographs from smaller basins. The duration of the
surface runoff from the time of occurrence of the peak is
proportional to Am , where m is an exponent less than
unity and is of the order of magnitude of 0.2.
• SLOPE. The slope of the main stream controls the
velocity of fl ow in the channel. As the recession limb of
the hydrograph represents the depletion of storage, the
stream channel slope will have a pronounced effect on
this part of the hydrograph. Large stream slopes give rise
to quicker depletion of storage and hence result in
steeper recession limbs of hydrographs. This would
obviously result in a smaller time base. The basin slope is
important in small catchments where the overland fl ow is
relatively more important. In such cases the steeper slope
of the catchment results in larger peak discharges.
• LAND USE. Vegetation and forests increase the
a direct proportional effect on the volume of
infi ltration and storage capacities of the soils. Further,
runoff. The effect of duration is refl ected in the
they cause considerable retardance to the overland fl ow.
rising limb and peak fl ow. Ideally, if a rainfall of
Thus the vegetal cover reduces the peak fl ow. This effect
given intensity i lasts suffi ciently long enough, a
is usually very pronounced in small catchments of area
state of equilibrium discharge proportional to iA
less than 150 km2 . Further, the effect of the vegetal
is reached.
cover is prominent in small storms. In general, for two
-
If the storm moves from upstream of the
catchments of equal area, other factors being identical,
catchment to the downstream end, there will be
the peak discharge is higher for a catchment that has a
a quicker concentration of fl ow at the basin
lower density of forest cover. Of the various factors that
outlet. This results in a peaked hydrograph.
control the peak discharge, probably the only factor that
-
If the storm movement is up the catchment, the
can be manipulated is land use and thus it represents the
resulting hydrograph will have a lower peak and
only practical means of exercising long-term natural
longer time base. This effect is further
control over the fl ood hydrograph of a catchment.
accentuated by the shape of the catchment, with
• CLIMATIC FACTORS. Among climatic factors the
intensity, duration and direction of storm movement
are the three important ones affecting the shape of a
fl ood hydrograph.
-
For a given duration, the peak and volume of the
surface runoff are essentially proportional to the
intensity of rainfall.
-
In very small catchments, the shape of the
hydrograph can also be affected by the intensity.
-
The duration of storm of given intensity also has
long and narrow catchments having hydrographs
most sensitive to the storm-movement direction.
components of a hydrograph
METHODS OF BASE-FLOW SEPARATION
EFFECTIVE RAINFALL (ER)
UNIT HYDROGRAPH
APPLICATION OF UNIT HYDROGRAPH
use and limitations of unit hydrograph
ENGINEERING
HYDROLOGY
4 |
Surface and
Subsurface Runoff
Phenomenon
4.3 | Flood Prediction
and Control
Flood
prediction
INTENDED LEARNING OUTCOMES
• Discuss the causes of flood and
how it is monitored and predicted.
Definition & nature
The “Manual of Operational Procedures on Flood Forecasting and Warning”
states:
“From a strict hydrological sense, flood is defined as a rise, usually brief, in
the water level in a stream to a peak from which the water level recedes at a
slower rate (UNESCO-WMO 1974). The episodic behavior of a river that
may be considered flood is then termed "flood event" (Linsley, 1942) which
is described as a flow of water in a stream constituting a distinct
progressive rise, culminating in a crest, together with the recession that
follows the crest (Linsley, 1942)."
From the foregoing technical definition, flood simply denotes a progressive
abnormal increase in the elevation of the surface level of streamfiow until it
reaches a maximum height from which the level slowly drops to what is its normal
level. The sequence described all takes place within a certain period of time.
The definition merely describes a characteristic behavior. It does not include the
element of "flooding" or inundation as implied by the popular notion of flood. The
technical definition is rather inadequate. Thus, considering the intents and
purposes of flood forecasting and warning, the definition seems rather restrictive
in its connotation for the public. Hence, for operational purposes, the Flood
Forecasting Branch, the hydrological service of PAGASA has adopted a more
extensive definition.
Flood is "an abnormal progressive rise in the water level of a stream that
may result in the overflowing by the water of the normal confines of the
stream with the subsequent inundation of areas which are not normally
submerged".
Causes of flood
Types of flood
MINOR FLOODING:
- Inundation may or may not be due to overbanking.
- When there is no bank overflow, flooding is simply due to the
accumulation of excessive surface run-off in low lying flat
areas.
- Floodwaters are usually confined to the flood plain of the
river along the channel, on random low-lying areas and
depressions in the terrain.
- Floodwater is usually shallow and there may not be a
perceptible flow.
MAJOR FLOODING:
- There is a highly perceptible current as the flood spreads to
other areas.
FLASH FLOOD
- While floods take some time, usually from 12 to 24 hours or
even longer, to develop after the occurrence of intense rainfall,
there is a particular type which develops after no more than six
hours and, frequently, after an even less time. These are what
are known as "flash floods".
- Flash floods develop in hilly and mountainous terrains where
the slope of the river is rather steep. The rapid development of
the flood is due to the extremely short concentration time of
the drainage catchment. This means that precipitation falling
on a point in the catchment farthest from the river takes only a
- Flooding is caused by the overflowing of rivers and lakes; by
short time to reach the river channel and become part of
serious breaks in dikes, levees, dams and other protective
streamflow. Thus, the amount of streamflow rapidly increases
structures; by uncontrollable releases of impounded water in
and, consequently, the rise in water level. When the flow
reservoirs and by the accumulation of excessive runoff.
capacity of the stream is exceeded, the channel overflows and
- Floodwaters cover a wide contiguous area and spread rapidly
to adjoining areas of relatively lower elevation.
- Flooding is relatively deep in most parts of the stricken areas.
the result is a flash flood.
monitoring & prediction
Modern flood forecasting is now based on the standard procedure of
The preparation, issuance and dissemination of an adequate and timely
monitoring and analysing the hydrological and meteorological
warning is the ultimate purpose of flood forecasting. Timeliness is an
conditions in a river basin. While the tools and methods of monitoring
essential requirement for a flood warning. A sufficient lead time enables
may have been modernized with the use of sensitive, telemeterized
the ultimate user to take the necessary precautionary countermeasures.
gauging instruments to effect better observation and faster transmission
of data, it is still basically an attempt to paint a bread picture of what is
currently happening, hydrologically and meteorologically, in a river
A flooding situation is not a daily occurrence. However, flood forecasting
operations must, of necessity, be a continuous activity. It is carried out
basin.
from day to day even when the possibility of a flood is highly
Flood predictions require several types of data:
pinpoint the beginning of a potential flood-generating situation.
- The amount of rainfall occurring on a real-time basis.
- The rate of change in river stage on a real-time basis, which can help
indicate the severity and immediacy of the threat.
- Knowledge about the type of storm producing the moisture, such as
duration, intensity and areal extent, which can be valuable for
determining possible severity of the flooding.
- Knowledge about the characteristics of a river's drainage basin, such
as soil-moisture conditions, ground temperature, snowpack,
topography, vegetation cover, and impermeable land area, which
can help to predict how extensive and damaging a flood might
become.
The Philippine Atmospheric Geophysical and Astronomical Services
Administration (PAGASA) collects and interprets rainfall data throughout
the Philippines and issues flood advisories, bulletins and warnings as
appropriate.
improbable. This mode of operation enables flood forecasters to
Like storm bulletins which are issued only during the presence of tropical
cyclones, flood forecast and warning bulletins are prepared only when a
potential flooding situation is definitely present. They are issued
regularly at specified hours of the day for the duration of the flooding
period until the flood recedes or when all hazards and dangers
associated with the phenomenon are no longer present.
Flood routing
4.3 | Flood Prediction
and Control
Flood routing is the technique of determining the flood hydrograph at a
Flood routing
& control
routing. In these applications two broad categories of routing can be
INTENDED LEARNING OUTCOMES
• Identify the different methods of
flood routing.
• Discuss the different methods of
flood control.
section of a river by utilizing the data of flood flow at one or more upstream
sections. The hydrologic analysis of problems such as flood forecasting, flood
protection, reservoir design and spillway design invariably include flood
recognised. These are: reservoir routing and channel routing.
RESERVOIR ROUTING. In reservoir routing the effect of a fl ood wave entering
a reservoir is studied. Knowing the volume-elevation characteristic of the
reservoir and the outfl ow-elevation relationship for the spillways and other
outlet structures in the reservoir, the effect of a fl ood wave entering the
reservoir is studied to predict the variations of reservoir elevation and outfl ow
discharge with time. This form of reservoir routing is essential:
(i) in the design of the capacity of spillways and other reservoir outlet
structures, and
(ii) in the location and sizing of the capacity of reservoirs to meet
specific requirements.
CHANNEL ROUTING. In channel routing the change in the shape of a
hydrograph as it travels down a channel is studied. By considering a channel
reach and an input hydrograph at the upstream end, this form of routing aims
to predict the fl ood hydrograph at various sections of the reach. Information
on the fl ood-peak attenuation and the duration of high-water levels obtained
by channel routing is of utmost importance in fl ood-forecasting operations
and fl ood-protection works.
ROUTING METHODS:
1. HYDROLOGIC ROUTING. These methods employ essentially the equation
of continuity.
2. HYDRAULIC ROUTING. These employ the continuity equation together
with the equation of motion of unsteady fl ow. The basic differential
equations used in the hydraulic routing, known as St. Venant equations
afford a better description of unsteady fl ow than hydrologic methods.
Flood control
The term flood control is commonly used to denote all the
measures adopted to reduce damages to life and property by
floods. Currently, many people prefer to use the term flood
management instead of flood control as it reflects the activity more
realistically. As there is always a possibility, however remote it may
be, of an extremely large flood occurring in a river the complete
control of the flood to a level of zero loss is neither physically
possible nor economically feasible. The flood control measures that
are in use can be classified as:
1. Structural Measures:
• Storage and detention reservoirs
• Flood ways (new channels)
• Watershed management
• Levees (flood embankments)
• Channel improvement
2. Non-structural Measures:
• Flood plain zoning
• Evacuation and relocation
• Flood forecast/warning
• Flood insurance
Flood control : structural measures
1. STORAGE RESERVOIR. Storage reservoirs offer one of the most reliable
embankments require considerable care and maintenance. In the event of
and effective methods of fl ood control. Ideally, in this method, a part of the
being overtopped, they fail and the damage caused can be enormous. In
storage in the reservoir is kept apart to absorb the incoming fl ood. Further,
fact, the sense of protection offered by a levee encourages economic
the stored water is released in a controlled way over an extended time so
activity along the embankment and if the levee is overtopped the loss
that downstream channels do not get fl ooded. As most of the present-day
would be more than what would have been if there were no levees.
storage reservoirs have multipurpose commitments, the manipulation of
Confi nement of fl ood banks of a river by levees to a narrower space leads
reservoir levels to satisfy many confl icting demands is a very diffi cult and
to higher fl ood levels for a given discharge. Further, if the bed levels of the
complicated task. It so happens that many storage reservoirs while
river also rise, as they do in aggrading rivers, the top of the levees have to
reducing the fl oods and fl ood damages do not always aim at achieving
be raised at frequent time intervals to keep up its safety margin.
optimum benefi ts in the fl ood-control aspect. To achieve complete fl ood
control in the entire length of the river, a large number of reservoirs at
Masonry structures used to confi ne the river in a manner similar to levees
strategic locations in the catchment will be necessary.
are known as flood walls. These are used to protect important structures
2. DETENTION RESERVOIR. A detention reservoir consists of an obstruction
to a river with an uncontrolled outlet. These are essentially small structures
and operate to reduce the fl ood peak by providing temporary storage and
by restriction of the outfl ow rate.
3. LEVEES. Levees, also known as dikes or fl ood embankments are earthen
banks constructed parallel to the course of the river to confi ne it to a fi xed
course and limited cross-sectional width. The heights of levees will be
higher than the design fl ood level with suffi cient free board. The
confi nement of the river to a fi xed path frees large tracts of land from
inundation and consequent damage.
Levees are one of the oldest and most common methods of fl oodprotection works adopted in the world. Also, they are probably the
cheapest of structural fl ood-control measures. While the protection offered
by a levee against food damage is obvious, what is not often appreciated is
the potential damage in the event of a levee failure. The levees, being earth
against fl oods, especially where the land is at a premium.
4. FLOODWAYS. Floodways are natural channels into which a part
evapotranspiration and reduction in soil erosion; all leading to
of the fl ood will be diverted during high stages. A fl oodway can
moderation of the peak fl ows and increasing of dry weather
be a natural or man-made channel and its location is controlled
fl ows. Watershed treatment is nowadays an integral part of
essentially by the topography. Generally, wherever they are
fl ood management. It is believed that while small and medium
feasible, fl oodways offer an economical alternative to other
fl oods are reduced by watershed management measures, the
structural fl ood-control measures.
magnitude of extreme fl oods are unlikely to be affected by
5. CHANNEL IMPROVEMENT. The works under this category
involve:
- Widening or deepening of the channel to increase the crosssectional area.
- Reduction of the channel roughness, by clearing of
vegetation from the channel perimeter.
- Short circuiting of meander loops by cutoff channels, leading
to increased slopes.
All these three methods are essentially short-term measures and
require continued maintenance.
6. WATERSHED MANAGEMENT. Watershed Watershed
management and land treatment in the catchment aims at
cutting down and delaying the runoff before it gets into the
river. Watershed management measures include developing
the vegetative and soil cover in conjunction with land treatment
works like check dams, contour bunding, zing terraces etc.
These measures are towards improvement of water infi ltration
capacity of the soil and reduction of soil erosion. These
t r e a t m e n t s c a u s e i n c r e a s e d i n fi l t r a t i o n , g r e a t e r
these measures.
Flood control : non-structural measures
1. FLOOD PLAIN ZONING. When the river discharges are very high, it is to
2. FLOOD FORECASTING AND WARNING. Forecasting of fl oods suffi ciently
be expected that the river will overfl ow its banks and spill into fl ood plains.
in advance enables a warning to be given to the people likely to be
In view of the increasing pressure of population this basic aspects of the
affected and further enables civil authorities to take appropriate
river are disregarded and there are greater encroachment of fl ood plains
precautionary measures. It thus forms a very important and relatively
by man leading to distress.
inexpensive non-structural fl ood management measure. However, it must
be realised that a fl ood warning is meaningful only if it is given suffi ciently
Flood plain management identifi es the fl ood prone areas of a river and
in advance. Further, erroneous warnings will cause the populace to lose
regulates the land use to restrict the damage due to fl oods. The locations
confi dence and faith in the system. Thus the dual requirements of reliability
and extent of areas likely to be affected by fl oods of different return
and advance notice are the essential ingredients of a fl ood-forecasting
periods are identifi ed and development plans of these areas are prepared
system.
in such a manner that the resulting damages due to fl oods are within
acceptable limits of risk.
3. EVACUATION AND RELOCATION. Evacuation of communities along with
their live stocks and other valuables in the chronic fl ood affected areas and
relocation of them in nearby safer locations is an area specifi c measure of
fl ood management. This would be considered as non-structural measure
when this activity is a temporary measure confi ned to high fl oods.
However, permanent shifting of communities to safer locations would be
termed as structural measure. Raising the elevations of buildings and
public utility installations above normal fl ood levels is termed as fl ood
proofi ng and is sometimes adopted in coastal areas subjected to severe
cyclones.
4. FLOOD INSURANCE. Flood insurance provides a mechanism for
spreading the loss over large numbers of individuals and thus modifi es the
impact of loss burden. Further, it helps, though indirectly, fl ood plain
zoning, fl ood forecasting and disaster preparedness activities.
ENGINEERING
HYDROLOGY
5 |
Infiltration,
Percolation, and
Groundwater Storage
When rain falls upon the ground it first of all wets the vegetation or
5.1 | Infiltration and
Percolation
Infiltration
& percolation
INTENDED LEARNING OUTCOMES
• Discuss the infiltration process.
• Discuss how infiltration capacity is
measured and classified.
the bare soil. When the surface cover is completely wet,
subsequent rain must either penetrate the surface layers if the
surface is permeable, or run off the surface towards a stream
channel if the surface is impermeable.
If the surface layers are porous and have minute passages
available for the passage of water droplets, the water infiltrates
into the subsurface soil. Soil with vegetation growing on it is always
permeable to some degree. Once infiltrating water has passed
through the surface layers, it percolates downwards under the
influence of gravity until it reaches the zone of saturation at the
phreatic surface.
Different types of soil allow water to infiltrate at different rates.
Each soil type has a different infiltration capacity, f, measured in
mm/h or in./h. For example, it can be imagined that rain falling on
a gravelly or sandy soil will rapidly infiltrate and, provided the
phreatic surface is below the ground surface, even heavy rain will
not produce surface runoff. Similarly a clayey soil will resist
infiltration and the surface will become covered with water even in
light rains. The rainfall rate, i, also obviously affects how much rain
will infiltrate and how much will run off.
Infiltration
Infiltration is the fl ow of water into the ground through the soil
surface. When water is applied at the surface of a soil, four moisture
zones in the soil can be identifi ed.
ZONE 1:
At the top, a thin layer of saturated zone is created.
ZONE 2:
Beneath zone 1, there is a transition zone.
ZONE 3:
Next lower zone is the transmission zone where the
downward motion of the moisture takes place. The
moisture content in this zone is above field capacity but
below saturation. Further, it is characterized by
unsaturated flow and fairly uniform moisture content.
ZONE 4:
The last zone is the wetting zone. The soil moisture in
this zone will be at or near field capacity and the
moisture content decreases with the depth. The
boundary of the wetting zone is the wetting front where
a sharp discontinuity exists between the newly wet soil
and original moisture content of the soil. Depending
upon the amount of infiltration and physical properties
of the soil, the wetting front can extend from a few
centimeters to metres.
Infiltration capacity is the maximum rate at which the ground can
absorb water.
Field capacity is the volume of water that the ground can hold.
Infiltration capacity
The maximum rate at which a given soil at a given time can absorb
water is defined as the infiltration capacity. It is designated as fp
and is expressed in units of cm/h.
The actual rate of infiltration f can be expressed as:
f = fp when i ≥ fp and f = i when i < fp
(3.20)
where i = intensity of rainfall. The infiltration capacity of a soil is
high at the beginning of a storm and has an exponential decay as
the time elapses.
The infiltration capacity of an area is dependent on a large number
of factors, chief of them are:
• Characteristics of the soil (Texture, porosity and hydraulic
conductivity)
• Condition of the soil surface
• Current moisture content
• Vegetative cover
• Soil temperature
CHARACTERISTICS OF SOIL:
The type of soil, viz. sand, silt or clay, its texture, structure,
permeability and underdrainage are the important characteristics
under this category. A loose, permeable, sandy soil will have a
larger infiltration capacity than a tight, clayey soil. A soil with good
underdrainage, i.e. the facility to transmit the infiltered water
downward to a groundwater storage would obviously have a higher
infiltration capacity. When the soils occur in layers, the transmission
capacity of the layers determines the overall infiltration rate.
Also, a dry soil can absorb more water than one whose pores are
already full. The land use has a significant influence on fp . For
example, a forest soil rich in organic matter will have a much higher
value of fp under identical conditions than the same soil in an urban
area where it is subjected to compaction.
SURFACE OF ENTRY:
FLUID CHARACTERISTICS:
At the soil surface, the impact of raindrops causes the fines in the
Water infiltrating into the soil will have many impurities, both in
soil to be displaced and these in turn can clog the pore spaces in
solution and in suspension. The turbidity of the water, especially the
the upper layers of the soil. This is an important factor affecting the
clay and colloid content is an important factor and such suspended
infiltration capacity. Thus a surface covered with grass and other
particles block the fine pores in the soil and reduce its infiltration
vegetation which can reduce this process has a pronounced
capacity. The temperature of the water is a factor in the sense that it
influence on the value of fp.
affects the viscosity of the water by which in turn affects the
infiltration rate. Contamination of the water by dissolved salts can
affect the soil structure and in turn affect the infiltration rate.
Measurement of infiltration
Infiltration characteristics of a soil at a given location can be
may take 23 hours. The surface of the soil is usually protected
estimated by:
by a perforated disc to prevent formation of turbidity and its
• Using flooding types infiltrometers
settling on the soil surface.
• Measurement of subsidence of free water in a large basin or
pond
• Rainfall simulator
• Hydrograph analysis
FLOODING-TYPE INFILTROMETER:
Flooding-type infiltrometers are experimental devices used to
obtain data relating to variation of infiltration capacity with time.
Two types of flooding type infiltrometers are in common use. They
are (a) Tube-type (or Simple) infiltrometer and (b) Doublering
infiltrometer.
a. SIMPLE (TUBE TYPE) INFILTROMETER. This is a simple
instrument consisting essentially of a metal cylinder, 30 cm
diameter and 60 cm long, open at both ends. The cylinder is
driven into the ground to a depth of 50 cm. Water is poured
into the top part to a depth of 5 cm and a pointer is set to
mark the water level. As infi ltration proceeds, the volume is
made up by adding water from a burette to keep the water
level at the tip of the pointer. Knowing the volume of water
added during different time intervals, the plot of the
infi ltration capacity vs time is obtained. The experiments are
continued till a uniform rate of infi ltration is obtained and this
A major objection to the simple infi ltrometer as above is that
the infi ltered water spreads at the outlet from the tube and
as such the tube area is not representative of the area in
which infi ltration is taking place.
b. DOUBLE-RING INFILTROMETER. This most
commonly used infi ltrometer is designed to
overcome the basic objection of the tube
infi ltrometer, viz. the tube area is not representative
of the infi ltrating area. In this, two sets of
concentrating rings with diameters of 30 cm and 60
cm and of a minimum length of 25 cm are used. The
two rings are inserted into the ground and water is
applied into both the rings to maintain a constant
depth of about 5.0 cm. The outer ring provides water
jacket to the infi ltering water from the inner ring and
hence prevents the spreading out of the infi ltering
water of the inner ring. The water depths in the inner
and outer rings are kept the same during the
observation period. The measurement of the water
volume is done on the inner ring only. The
experiment is carried out till a constant infi ltration
rate is obtained. A perforated disc to prevent
formation of turbidity and settling of fi nes on the soil
surface is provided on the surface of the soil in the
inner ring as well as in the annular space.
As the flooding-type infiltrometer measures the infiltration
characteristics at a spot only, a large number of pre-planned
experiments are necessary to obtain representative infiltration
characteristics for an entire watershed. Some of the chief
disadvantages of flooding-type infiltrometers are:
• the raindrop impact effect is not simulated;
• the driving of the tube or rings disturbs the soil structure; and
• the results of the infiltrometers depend to some extent on their
size with the larger meters giving less rates than the smaller
ones; this is due to the border effect.
RAINFALL SIMULATOR:
In a small plot of land, of about 2 m ´ 4 m size, is provided with a
series of nozzles on the longer side with arrangements to collect
and measure the surface runoff rate. The specially designed nozzles
produce raindrops falling from a height of 2 m and are capable of
producing various intensities of rainfall. Experiments are conducted
under controlled conditions with various combinations of intensities
and durations and the surface runoff rates and volumes are
measured in each case. Using the water budget equation involving
the volume of rainfall, infiltration and runoff, the infiltration rate and
its variation with time are estimated. If the rainfall intensity is higher
than the infiltration rate, infiltration capacity values are obtained.
Rainfall simulator type infiltrometers give lower values than flooding
type infiltrometers. This is due to effect of the rainfall impact and
turbidity of the surface water present in the former.
HYDROGRAPH ANALYSIS:
Reasonable estimation of the infiltration capacity of a small
watershed can be obtained by analyzing measured runoff
hydrographs and corresponding rainfall records. If sufficiently good
rainfall records and runoff hydrographs corresponding to isolated
storms in a small watershed with fairly homogeneous soils are
available, water budget equation can be applied to estimate the
abstraction by infiltration. In this the evapotranspiration losses are
estimated by knowing the land cover/use of the watershed.
Classification of infiltration capacities
The steady state infiltration capacity, being one of the main
parameters in this soil classification, is divided into four
infiltration classes as mentioned below.
Infiltration indices
In hydrological calculations involving floods it is found convenient
W-INDEX. In an attempt to refi ne the φ-index the initial losses are
to use a constant value of infiltration rate for the duration of the
separated from the total abstractions and an average value of
storm. The defined average infiltration rate is called infiltration
infi ltration rate, called W-index, is defi ned as:
index and two types of indices are in common use, the φ-index
W=
and W-index.
P − R − Ia
te
P
=
total storm precipitation (cm)
φ-INDEX is the average rainfall above which the rainfall volume is
R
=
total storm runoff (cm)
equal to the runoff volume. The φ-index is derived from the rainfall
Ia
=
Initial losses (cm)
te
=
duration of the rainfall excess, i.e. the total
where:
hyetograph with the knowledge of the resulting runoff volume. The
initial loss is also considered as infi ltration. The φ value is found by
time in which the rainfall intensity is greater
treating it as a constant infi ltration capacity. If the rainfall intensity is
than W (in hours)
less than φ, then the infi ltration rate is equal to the rainfall intensity;
W
however, if the rainfall intensity is larger than φ the difference
between the rainfall and infi ltration in an interval of time represents
the runoff volume. The amount of rainfall in excess of the index is
called rainfall excess. In connection with runoff and fl ood studies it
is also known as effective rainfall. The φ-index thus accounts for
the total abstraction
and
enables
magnitudes to be
estimated for a
given rainfall
hyetograph.
=
defined average rate of infiltration (cm).
Since Ia rates are difficult to obtain, the accurate estimation of W
-index is rather difficult. The minimum value of the W -index
obtained under very wet soil conditions, representing the constant
minimum rate of infiltration of the catchment, is known as Wmin . It is
to be noted that both the φ-index and W -index vary from storm to
storm.
ENGINEERING
HYDROLOGY
6 |
Role of Hydrology in
Water Resource
Management
6.1 | Water Governance
in the Philippines
policy, legal and
institutional
regulatory frameworks
INTENDED LEARNING OUTCOMES
• Identify the different legal
frameworks the govern the water
sector in the Philippines.
national level
LEGAL FRAMEWORKS THAT GOVERN THE WATER SECTOR IN THE
PHILIPPINES:
1. PD 1067, Water Code (1976)
2. PD 424, (1974)
3. PD 198, Provincial Water Utilities Act (1973)
4. PD 522, Prescribing Sanitation Requirements for the Traveling
Public (1974)
5. RA 7586, National Integrated Protected Area System Act
(1992)
6. RA 8041, National Water Crisis Act (1995)
7. RA 8371, Indigenous Peoples Rights Act (1997)
8. RA 9275, Clean Water Act (2004)
9. RA 8435, Agricultural and Fisheries Modernization Act
• The Water Code of the Philippines is the over- arching law that
• Identify the different agencies
involved in water governance and
discuss their roles.
governs water access, allocation and use. It stipulates rules on
• Discuss the challenges in the
governance of water in the
Philippines.
administrative and enforcement of these rules. It defines
appropriation and utilization of all waters; control, conservation and
protection of waters, watershed and related land resources; and,
requirements for application of water permits and conditions of its
use. It also sets charges per rate of withdrawal based on the permits.
During periods of drought or water scarcity, the Water Code
prioritizes the use of water for domestic use, followed by irrigation,
and other uses.
• The PD 424 created the National Water Resources Council, which is
now the National Water Resources Board (NWRB).
• The Provincial Water Utilities Act created the Local Water Utilities
Administration or LWUA in the national level and provided for the
establishment of Water Districts in provincial cities and municipalities.
• The National Water Crisis Act paved the way for the privatization of
the Metropolitan Waterworks and Sewerage System (MWSS, which
handles water supply in the National Capital Region and outlying
provinces) in 1997 and other water supply systems around the country
• The Indigenous Peoples Rights Act is an act to recognize, protect and
promote the rights of Indigenous Cultural Communities/Indigenous
The LWUA is a government-owned and controlled corporation (GOCC)
Peoples, creating a National Commission on Indigenous Peoples,
with a specialized lending function mandated by law to promote and
establishing implementing mechanisms, appropriating funds therefor,
oversee the development of water supply systems in provincial cities
and for other purposes..
and municipalities outside of Metropolitan Manila.
A Water District (WD) is a local corporate entity that operates and
maintains a water supply system in one or more provincial cities or
municipalities. It is established on a local option basis and, like LWUA,
is classified as a government-owned and controlled corporation or
GOCC.
• The National Integrated Protected Areas System Act providesthe
legal framework for the establishment and management of protected
areas (PAs) in the Philippines, and that the use and enjoyment of these
protected areas must be consistent with the principles of biological
diversity and sustainable development.
NIPAS is the classification and management of all designated PAs, in
order to maintain essential ecological processes and life support
systems, preserve genetic diversity, ensure sustainable use of
resources found therein, and maintain their natural conditions to the
greatest extent possible.
• The Philippine Clean Water Act aims to protect the country’s water
bodies from pollution from land-based sources (industries and
commercial establishments, agriculture and community/household
activities). It provides for a comprehensive and integrated strategy to
prevent and minimize pollution through a multi-sectoral and
participatory approach involving all the stakeholders. The Department
of Environment and Natural Resources is the primary agency
responsible for the implementation of this act.
• The Agriculture and Fisheries Modernization Act (AFMA) promotes
the development and adoption of modern, appropriate and costeffective and environmentally safe agricultural and fisheries
machineries and equipment to enhance farm productivity and
efficiency in order to achieve food security and safety and increase
farmers’ income. The acts states that the government shall adopt a
rational approach in the use of resources including water.
EXAMPLES OF CONFLICTS BETWEEN THE LAWS:
• The Water Code stipulates that the state owns all the
water in the country, the Indigenous Peoples Rights Act
protects the rights of indigenous peoples with respect
to resources contained in their ancestral domain.
Therefore, water is not freely shared with the other
community members.
• The National Integrated Protected Area System (NIPAS)
Act provides for watershed protection so water supply
can be sustained. This overlaps with the provisions of
the Indigenous Peoples Rights Act.
• The Agricultural and Fisheries Modernization Act
(AFMA) provides that all watersheds that are sources
of water for existing and potential irrigable areas and
recharge areas of major aquifers identified by the
Department of Agriculture and the Department of
Environment and Natural Resources shall be preserved
as such at all times, which is consistent with the NIPAS
Act.
AGENCIES INVOLVED IN WATER resourceS:
• The Metropolitan Waterworks and Sewerage Services (MWSS) and its
within their respective territorial jurisdictions (Section 18). Thus,
two concessionaires (after it was privatized in 1997) for Metro Manila,
conflicts with respect to the powers of the water agencies including
servicing 62.68 percent of its total population.
NWRB vis-à-vis the local governments have arisen.
• The Local Water Utilities Administration (LWUA) and its water district
• The National Water Resources Board (NWRB) is the coordinating and
offices for other cities and municipalities, servicing 58 percent of the
regulating agency for all water resources management development
total urban population within its area of responsibility.
activities. It is tasked with the formulation and development of policies
• The Departments of Interior and Local Government (DILG) and Public
on water utilization and appropriation, the control and supervision of
Works and Highway (DPWH) and local governments which manage
water utilities and franchises, and the regulation and rationalization of
community water systems (usually involving point sources and piped
water rates.
systems with communal faucets), servicing 86.85 percent of the
country’s rural population.
• The Department of Environment and Natural Resources (DENR)
• National Irrigation Administration (NIA)
• National Power Corporation (NAPOCOR)
formulates policies for the enforcement of environmental protection
• Department of Energy (DOE)
and pollution control regulations. It is primarily responsible for the
• Philippine Atmospheric, Geophysical and Astronomical
preservation of watershed areas and ensures water quality with respect
to rivers, streams and other sources of water.
• The Department of Health (DOH) is responsible for drinking water
quality regulation and supervision of general sanitation activities.
• The Local Government Units (LGUs) are also resource regulators, as the
Local Government Code of 1991 (Republic Act No. 7160) devolved to
local governments the power to discharge functions and
responsibilities of national agencies and offices such as the provision
of basic services and facilities including water supply systems (Section
17). It also gave the local governments the right to an equitable share
of the proceeds from the use and development of national wealth and
resources (which can be interpreted as to include water resources)
Administration (PAGASA)
• Department of Foreign Affairs (DFA)
• Metropolitan Manila DEvelopment Authority(MMDA)
• Department of Tourism (DOT)
• Laguna Lake Development Authority (LLDA)
challenges in water governance:
• Multiple and fragmented water institutions in the
country, the nested and interlocking mechanisms
in spatial water governance.
• Water agencies are not connected vertically nor
horizontally. And does not have sufficient human
and financial resources and presence at the local
level to be effective in their mandates.
• Conflicts between national policies and local
agreements and conflicts between customary rules
and formal state laws.
• Legal documents for water are a source of
confusion, and water data for planning are
insufficient
ENGINEERING
HYDROLOGY
6 |
Role of Hydrology in
Water Resource
Management
6.2 | Agencies Involved
in the Collection of
Hydrological Data
collection of
hydrologic data in
the Philippines
INTENDED LEARNING OUTCOMES
• Identify the agencies involved in
the collection of Hydrological
Data.
The main goal of the collection of hydrologic data in hydrology is
to provide a set of sufficient good quality data that can be used in
decision-making in all aspects of water resources management,
operations, and in research.
national mapping and resource
information authority (namria)
An agency of the Philippine government under the Department of
Environment and Natural Resources responsible for providing the
public with mapmaking services and acting as the central mapping
agency, depository, and distribution facility of natural resources
data in the form of maps, charts, texts, and statistics.
national power
corporation (NPC)
A Philippine government-owned and controlled
corporation that is mandated to provide electricity to all
rural areas of the Philippines by 2025 (known as
"missionary electrification"), to manage water resources for
power generation, and to optimize the use of other power
generating assets. It provides stream flow data from the
watersheds and dams under its management.
national irrigation
administration (nia)
A government-owned and controlled corporation primarily
responsible for irrigation development and management.
It is primarily responsible in gathering and reporting water
discharge data used in irrigation.
national water
resources board
(NWRB)
The NWRB is an attached agency of the Department of
Environment and Natural Resources responsible for
ensuring the exploitation, utilization, development,
conservation and protection of the country's water
resource, consistent with the principles of "Integrated
Water Resource Management".
philippine atmospheric, geophysical and
astronomical services and administration (pagasa)
The National Meteorological and Hydrological Services
• Engage in studies of geophysical and astronomical
(NMHS) agency of the Philippines mandated to provide
phenomena essential to the safety and welfare of the
protection against natural calamities and to insure the
people;
safety, well-being and economic security of all the people,
• Undertake researches on the structure, development and
and for the promotion of national progress by undertaking
motion of typhoons and formulate measures for their
scientific and technological services in meteorology,
moderation; and
hydrology, climatology, astronomy and other geophysical
sciences.
FUNCTIONS:
• Maintains a nationwide network pertaining to
observation and forecasting of weather and flood and
other conditions affecting national safety, welfare and
economy;
• Undertake activities relative to observation, collection,
assessment and processing of atmospheric and allied
data for the benefit of agriculture, commerce and
industry;
• Maintain effective linkages with scientific organizations
here and abroad and promote exchange of scientific
information and cooperation among personnel engaged
in atmospheric, geophysical, astronomical and space
studies.
ENGINEERING
HYDROLOGY
6 |
Role of Hydrology in
Water Resource
Management
6.3 |
Water Resource
Planning, Design,
Management,
and Protection
Water resource
Management
The importance of hydrology is increasing because of the global
growth of water needs and the rise of water scarcity, which
together cause greater risk and unreliability in water resources
management.
• Planing is where the sources and consumers are identified, the
overall "architecture" of a proposed system is laid out, including
its topology and connectivity.
• Design is where sizes of facilities are fixed.
• Operational policy determines the operation of the system under
INTENDED LEARNING OUTCOMES
a selected forecasted set of typical and/or critical conditions,
• Discuss the concept of Water
Resource Management.
for a defined time period ahead (hour, day, week, month, year).
• Discuss the relationship of
Hydrology and Water Resource
Management.
while real-time operation means setting the operational variables
• Management is the process of administering and controlling
organizations and facilities used in the water sector.
• Water source protection involves the protection of surface water
sources (e.g. lakes, rivers, man made reservoirs) and
groundwater sources (e.g. spring protection, dug well protection,
and drilled well protection) to avoid water pollution (see also
pathogens and contaminants).
what is water resource management?
Water Resource Management (or WRM) is defined by the
One of the goals of water resource management is water
World Bank as the “process of planning, developing, and
security. It is not possible to ‘predict and plan’ a single path
managing water resources, in terms of both water quantity
to water security for rapidly growing and urbanizing global
and quality, across all water uses”. It includes the
populations. This is due to climatic and non-climatic
institutions, infrastructure, incentives, and information
uncertainties. To help strengthen water security, there is a
systems that support and guide water management.
need to build capacity, adaptability and resilience for the
According to the World Bank, water resources
future planning and management of water resources.
management seeks to harness the benefits of water by
According to the World Bank, achieving water security in
ensuring there is sufficient water of adequate quality for
the context of growing water scarcity, greater
drinking water and sanitation services, food production,
unpredictability, degrading water quality and aquatic
energy generation, inland water transport, and water-
ecosystems, and more frequent droughts and floods, will
based recreational, as well as sustaining healthy water-
require a more integrated and longer-term approach to
dependent ecosystems and protecting the aesthetic and
water management. This is in essence what water resource
spiritual values of lakes, rivers, and estuaries.
management is about: bringing together multiple
Water resource management also entails managing waterrelated risks, including floods, drought, and contamination.
The complexity of relationships between water and
households, economies, and ecosystems, requires
integrated management that accounts for the synergies
and tradeoffs of water's great number uses and values.
organisations, across different disciplines to plan for future
water usage holistically.
hydrology and water resource management
Water resource management includes consideration of
real-time operation and response, e.g., for flood
several disciplines of hydrology, including the global water
protection, to long-term time probabilistic series and
cycle, surface water and groundwater, water chemistry and
ensembles for planning, which consider changing natural
pollution and aquatic biology. This is according to S.J.
and anthropogenic drivers (land use, climate change).
Marshall in his Hydrology module in Earth Systems and
Since hydrology is a continuous process that is not divided
Environmental Sciences.
internally according to the needs of management, the
The balancing act involved in water management includes
a broad range of stakeholders and includes water policy
hydrological analysis must be geared to produce the
suitable information for the different management issues.
and legal experts. Hydrologists have essential input to
Hydrology is the backbone of Water Resources
t h e s e c o m p l e x a n d s o m e t i m e s c o n f ro n t at i o n a l
Development. Water Resources development deals
deliberations and negotiations. They also play a central
judicious usage of water resources which essentially can be
role in applied hydrology – engineering of major
assessed when we have adequate information of basin
waterworks to manage water. Water distribution systems
water surplus which again, essentially can be determined
have been a hallmark of civilization since Babylon, and the
through assessing precipitation, runoff,
modern stamp on this includes major hydroelectric dams
base flow in dominant drains (rivers)
and reservoirs, urban waterworks, and water treatment
(for assessing ground water reservoir). For this, one has to
facilities.
be adept in hydrology and adequate understanding of
Hydrological data of different types are required,
according to the management issue being addressed.
They range from short term now-casting/forecasting for
hydrological
with
overland flow,
and interceptions
cycle prevailing in the chosen catchment
where water resources has to be developed.