AIN SHAMS UNIVERSITY
FACULTY OF ENGINEERING
IRRIGATION & HYDRAULICS DEPT.
ENGINEERING HYDROLOGY
By
Dr. ASHRAF M. ELMOUSTAFA
2010/2011
TABLE OF CONTENTS
3...........................................................................................................................CHAPTER I
75......................................................................................................................CHAPTER IV
79........................................................................................................................CHAPTER V
80......................................................................................................................CHAPTER VI
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CHAPTER I
Introduction:
Hydrology is an earth science. It encompasses the occurrence,
distribution, movement, and properties of the waters of the earth and
their environmental relations. Hydrology has both applied and pure
science aspects. On the one hand, it is an important science that
studies how the water flows on the Earth. On the other hand,
understanding of fundamental hydrologic processes is necessary for
proper use and protection of water resources. Water is also an agent
for many other processes (weathering, transport of chemicals,
erosion, deposition).
Hydrology is the study of several physical processes;

Atmospheric processes: cloud condensation, precipitation.

Surface processes: snow accumulation, overland flow, river
flow, lake storage.

Subsurface processes: infiltration, soil-water storage,
groundwater flow.

Interfacial processes: evaporation, transpiration,
sedimentwater exchange.
Hydrologists are traditionally concerned with the supply of water for
domestic and agricultural use and the prevention of flood disasters.
However, their field of interest also includes hydropower generation,
navigation, water quality control, thermal pollution, recreation and
the protection and conservation of nature. In fact, any intervention in
the hydrological regime to fulfil the needs of the society belongs to
the domain of the hydrologist. This does not include the design of
structures (dams, sluices, weirs, etc.) for water management.
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Hydrologists contribute however, to a functional design (e.g. location
and height of the dam) by developing design criteria and to water
resources management by establishing the hydrological boundary
conditions to planning (inflow sequences, water resources
assessment). Both contributions require an analysis of the
hydrological phenomena, the collection of data, the development of
models and the calculation of frequencies of occurrence.
Application Fields:
Hydrological science has both pure and applied aspects.
Understanding the engineering hydrology science is essential for:
♦
Sustainable agriculture (foods for the growing population);
♦
Environmental protection and management;
♦
Water resources development and management;
♦
Prevention and control of natural disasters;
♦
Control problems of tidal rivers and estuaries;
♦
Soil erosion and sediment transport and deposition;
♦
Mitigation of the negative impacts of climatic change;
♦
Water supply; and
♦
Flood and drought control;
Traditional water management has focused on providing freshwater
resources to the needs of humans, livestock, commercial enterprises,
agriculture, mining, industry, and electric power.
Until 1950, pragmatic considerations dominated hydrology.
Theoretical approaches in hydrology have been increasingly
developed due to the development of digital computers since 1950.
The focus now is on how best to optimize the use of existing surfacewater projects and ground-water resources.
Challenge for the 21st century in hydrology will still be maintaining
water quantity and quality against to the increasing stress on water
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resources by the increasing world population, contamination, human
induced climate-hydrology change, and extreme events (flood and
drought).
Hydrology in Engineering
Hydrology is used in engineering mainly in connection with the
design and operation of hydraulic structures. What flood flows can be
expected over a spillway, at a highway culvert, or in an urban storm
drainage system? What reservoir capacity is required to assure
adequate water for irrigation or municipal water supply during
droughts'? What effect will reservoirs, levees, and other control works
exert on flood flows in a stream? What are reasonable boundaries for
the floodplain'? These are typical questions that the hydrologist is
expected to answer. The need to answer these questions has been
the principal incentive for the development of the techniques of
quantitative hydrology discussed in this text.
Large organizations such as National water research center as well as
water departments in universities of Egypt can maintain staffs of
hydrologic specialists to analyze their problems, but smaller offices
often have insufficient hydrologic work for full-time specialists. Hence,
many civil engineers are called upon for occasional hydrologic
studies. It is probable that these civil engineers deal with a larger
number of projects and a greater financial budget than the specialists
do. In any event, it seems that knowledge of the fundamentals of
hydrology is an essential part of the civil engineer’s training.
Hydrology deals with many topics. These topics may be classified in
two phases: data collection and methods of analysis. Chapters 2 to 5
deal with the basic data of hydrology. It is necessary to interpret
observed data and from this analysis to establish the systematic
pattern that governs these events. Without adequate historical data
for the particular problem area, the hydrologist is in a difficult
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position. Most countries have one or more agencies with
responsibility for data collection. It is important to know how these
data are collected and published, the limitations on their accuracy,
and the proper methods of interpretation and adjustment.
Typical hydrologic problems involve estimates of extremes rarely
observed in a small data sample, hydrologic characteristics at
locations where no data have been collected (such locations are
much more numerous than sites with data), or estimates of the
effects of human actions on the hydrologic characteristics of an area.
Generally, each hydrologic problem is unique in that it deals with a
distinct set of physical conditions within a specific river basin. Hence
quantitative conclusions of one analysis are often not directly
transferable to another problem. However, the general solution for
most problems can be developed from application of a few relatively
basic concepts.
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CHAPTER II
Hydrologic Cycle
Water, which is found everywhere on the earth, is one of the
most basic and commonly occurring substances. It is the only
substance on earth that exists naturally in the three basic forms of
matter, i.e., liquid, solid, and gas. The quantity of water varies from
place to place and from time to time. Although at any given moment
the vast majority of the earth's water is found in the world's oceans,
there is a constant interchange of water from the oceans to the
atmosphere to the land and back to the ocean. This interchange is
called the hydrologic cycle.
The next figure (2.1a and b) descries the hydrological cycle that
illustrate the movement of water in the earth atmosphere, surface
and below the ground. Some of these movements are caused by
external factors, i.e. the evaporation is caused by the solar energy,
while other is just naturally; i.e. infiltration and percolation.
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Figure 2.1a Hydrologic Cycle
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Figure 2.1b Hydrologic Cycle
The water in the oceans, seas, lakes and water bodies evaporates to
the atmosphere forced by the solar energy. When warm moist air is
lifted to the condensation level, precipitation in many forms such as
rain, hail, sleet, or snow forms and starts falling. Some of the water
evaporates as it is falling as it meets warm air currents and the rest
either reaches the ground or is intercepted by buildings, trees, and
other vegetation this portion is usually called the initial abstracted
portion of water.
The intercepted water evaporates directly back to the atmosphere
thus completing a part of the cycle. The remaining precipitation falls
to the ground's surface or onto the water bodies such as rivers, lakes,
ponds, and oceans.
The water that reaches the earth's surface either evaporates,
infiltrates into the root zone, or flows overland into puddles and
depressions in the ground or into swales and streams. The effect of
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infiltration is to increase the soil moisture. If the moisture content is
less than the field capacity of the soil, water returns to the
atmosphere through soil evaporation and by transpiration from plants
and trees. If the moisture content becomes greater than the field
capacity, the water percolates downward to become ground water.
Field capacity is the moisture held by the soil after all gravitational
drainage.
The part of precipitation that falls into puddles and depressions can
evaporate, infiltrate, or if it fills the depressions, the excess water
begins to flow overland until eventually it reaches natural
drainageways. Water held within the depressions is called depression
storage and is not available for overland flow or surface runoff.
Before flow can occur overland and in the natural and/or manmade
drainage systems, the flow path must be filled with water. This form
of storage, called detention storage, is temporary since most of this
water continues to run off after the rainfall ceases. The precipitation
that percolates down to ground water is maintained in the hydrologic
cycle as seepage into streams and lakes, as capillary movement back
into the root zone, or it is pumped from wells and discharged into
irrigation systems, sewers, or other drainageways. Water that
reaches streams and rivers may be detained in storage reservoirs
and lakes or it eventually reaches the oceans. Throughout this path,
water is continually evaporated back to the atmosphere, and the
hydrologic cycle is repeated.
World's surface water: precipitation, evaporation and runoff
The world's surface water is affected by different levels of precipitation,
evaporation and runoff in different regions. Figure 1.2 illustrates the different rates at
which these processes affect the major regions of the world, and the resulting uneven
distribution of freshwater. It shows the amount of precipitation in cubic kilometres for
each region, and the percentage of that amount which evaporates or becomes runoff.
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The text below the graphic discusses the uneven global availability of freshwater and its
implications (Peter H. Gleick, 'Water in Crisis', New York, Oxford University Press,
1993).
Figure 2.2 Relation between precipitation, runoff and evaporation of the World
Metrological Parameters Affecting Hydrologic Cycle
The hydrologic cycle, illustrated in figure 2.1, shows the
pathways where water travels as it circulates throughout global
systems by various processes. The visible components of this cycle
are precipitation, and runoff. However, other components, such as
evaporation, infiltration, transpiration, percolation, groundwater
recharge, interflow, and groundwater discharge, are equally
important. Table 1 summarizes the water distribution in hydrosphere.
Air temperature, pressure, humidity, wind, and solar Radiation are the
meteorological parameters that are used to study the hydrologic
processes: Precipitation, Runoff, Transpiration, and Evaporation.
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Air Temperature
It is measured by Thermometers located 1.25 m above the ground
and sheltered. Maximum temperature is measured by mercury
thermometer while minimum temperature is measured by alcohol
thermometer.
Mean daily Temp. = 1/2 [Max. day Temp. + Min. day Temp.]
Mean Monthly Temp. = 1/2 [Max. Month. Temp. + Min. Month. Temp.]
Mean annual Temp. = Average of the monthly means over the year
Normal (daily, month.) Temp. = average of (daily, month.) over 30
years
Humidity
Air has the property of absorbing water vapor (moisture) and is
measured by the Psychrometer. At each level of absorption, there is a
certain level of vapor pressure ev. For every temperature, there is a
maximum value of vapor pressure called saturated vapor pressure at
which air cannot absorb moisture any more. Equation (1) gives the
values of es corresponding to each Temperature.
es = 611 exp (17.27 T / 237.3 + T)
(1)
where, es in Pa and T in degree Celsius.
Relative Humidty (Rh) defines the air's capacity of absorbing moisture
and can be expressed as a percentage:
Rh = (ev / es) X 100
Where ev
and
es
(2)
are measured in units of
100 Pa]
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mbar [1 mbar =
or of 1 mm Hg [1 mm Hg = 1.33 mbar]
Dew point (Td): is the temperature at which ev reaches es for the
same conditions and water vapor starts to condense. Dew point can
be computed using equation (1) if T is replaced by Td and the normal
vapor pressure is considered as saturated one.
Wind
Wind speed, W, and its direction are measured by the anemometer and wind
vane respectively. Wind speed is measured by units of Knot or mph, where, 1 Knot =
1.852 km/h and 1 mph = 1.61 km/h
The relationship between wind speed and elevation is expressed by the power law
profile equation
W/W0 = (Z / Z0)k
(3)
where, k is Von Karman coefficient and it depends on the nature of surface and its value
varies between 0.1 and 0.6.
Solar radiation
Solar radiation is the source of energy on the earth and it is measured by units of
Watt/m2 and KJ/m2. It is also measured by the radiometer in micro-meter (10-6 m) and
the important term is the net radiation, Rn, that is used in some methods of estimating
evapotranspiration.
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Figure 2.3 Solar Radiation
Climate of Egypt
Figure 2.4 shows the different locations of metrological stations in Egypt. These
stations are mainly used to give detailed measurements of different climate data
(temperature, precipitation, pressure, humidity, and wind speed).
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Sakha
Alexandria
Baltim
Mansoura
Port Said
Gemmeiza
Ismailia
Bilbeis
Tahrir
Giza
Helwan
Beni Suef
Minya
Mallawi
Bahariya
Asyut
Sohag
Shandaweel
Kharga
Dakhla
Kom Ombo
Aswan
Figure 2.4 Location of meteorological stations in Egypt
The FAO organization classified the world according to the precipitation rates to many
classes. According to this classification, Egypt falls within the arid region of North
Africa with an average annual precipitation in few mm, Figure 2.5.
Figure 2.5 Climate Zones of Egypt, FAO Classification
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Hydrologic Cycle Main Elements
Water budget
The water volume in the globe is considered to be constant but changes from a
phase to another and this relation is known as the water budget which states that the
change in the storage within a certain domain is equal to the summation of the inflow,
outflow, underground flow, evaporation and precipitation.
The water budget is accounting of the volume of flow rate of water in all possible
locations. Since the density is constant it may be interpreted as a mass balance. One has
focus interest on a region and determines how the quantity of water in the region can be
changed taking several forms.
Inputs - Outputs ± accumulation = 0
The water budget equation for any domain (area or place) can be written in its simplest
form as follows;
S = P + I ± U - O -E
(4)
Where,
I is the Inflow to the domain, E is the Evaporation, O is the Outflow from the domain,
U is the Underground flow from or into the domain, P is the Precipitation, and S is the
Storage change
Evaporation
The transformation of water from liquid to gas phases as it moves from the
ground or bodies of water into the overlying atmosphere. The source of energy for
evaporation is primarily solar radiation. Evaporation often implicitly includes
transpiration from plants, though together they are specifically referred to as Total
annual evapotranspiration amounts to approximately 505,000 km3 (121,000 cu mi) of
water, 434,000 km3 (104,000 cu mi) of which evaporates from the oceans.
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Figure 2.6 Evaporation Process
Evapotranspiration
Evapotranspiration (ET) is a term used to describe the sum
of evaporation and plant transpiration from the Earth's land surface toatmosphere.
Evaporation accounts for the movement of water to the air from sources such as
the soil, canopy interception, andwaterbodies. Transpiration accounts for the movement
of water within a plant and the subsequent loss of water as vapor through stomatain
its leaves. Evapotranspiration is an important part of the water cycle. An element (such
as a tree) that contributes to evapotranspiration can be called an evapotranspirator.
Potential evapotranspiration (PET) is a representation of the environmental demand for
evapotranspiration and represents the evapotranspiration rate of a short green crop,
completely shading the ground, of uniform height and with adequate water status in the
soil profile. It is a reflection of the energy available to evaporate water, and of
the wind available to transport the water vapour from the ground up into the
lower atmosphere. Evapotranspiration is said to equal potential evapotranspiration
when there is ample water.
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.
Figure 2.7 Evapotanspiration Process
Evaporation and transpiration (which involves evaporation within plant stomata) are
collectively termed evapotranspiration. Evaporation is caused when water is exposed to
air and the liquid molecules turn into water vapor which rises up and forms clouds.
Evaporation
Theory
For molecules of a liquid to evaporate, they must be located near the surface, be
moving in the proper direction, and have sufficient kinetic energy to overcome liquid18/80
phase intermolecular forces. Only a small proportion of the molecules meet these
criteria, so the rate of evaporation is limited. Since the kinetic energy of a molecule is
proportional to its temperature, evaporation proceeds more quickly at higher
temperatures. As the faster-moving molecules escape, the remaining molecules have
lower average kinetic energy, and the temperature of the liquid thus decreases. This
phenomenon is also called evaporative cooling.
This is why evaporating sweat cools the human body. Evaporation also tends to proceed
more quickly with higher flow rates between the gaseous and liquid phase and in
liquids with higher vapor pressure. For example, laundry on a clothes line will dry (by
evaporation) more rapidly on a windy day than on a still day. Three key parts to
evaporation are heat, humidity and air movement.
On a molecular level, there is no strict boundary between the liquid state and the vapor
state. Instead, there is a Knudsen layer, where the phase is undetermined. Because this
layer is only a few molecules thick, at a macroscopic scale a clear phase transition
interface can be seen.
Evaporative equilibrium
If evaporation takes place in a closed vessel, the escaping molecules accumulate
as a vapor above the liquid. Many of the molecules return to the liquid, with returning
molecules becoming more frequent as the density and pressure of the vapor increases.
When the process of escape and return reaches an equilibrium, the vapor is said to be
"saturated," and no further change in either vapor pressure and density or liquid
temperature will occur. For a system consisting of vapor and liquid of a pure substance,
this equilibrium state is directly related to the vapor pressure of the substance, as given
by the Clausius-Clapeyron relation:
where P1, P2 are the vapor pressures at temperatures T1, T2 respectively, ΔHvap is the
enthalpy of vaporization, and R is the universal gas constant. The rate of evaporation in
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an open system is related to the vapor pressure found in a closed system. If a liquid is
heated, when the vapor pressure reaches the ambient pressure the liquid will boil.
The ability for a molecule of a liquid to evaporate is largely based on the amount of
kinetic energy an individual particle may possess. Even at lower temperatures,
individual molecules of a liquid can evaporate if they have more than the minimum
amount of kinetic energy required for vaporization.
But vaporization is not only the process of a change of state from liquid to gas but it is
also a change of state from a solid to gas. This process is also known as sublimation but
can also be known as vaporization.
Factors influencing the rate of evaporation
A.
Pressure
In an area of less pressure, evaporation happens faster because there is less exertion on
the surface keeping the molecules from launching themselves.
B.
Surface area
A substance which has a larger surface area will evaporate faster as there are more
surface molecules which are able to escape.
C.
Temperature
If the substance is hotter, then evaporation will be faster.
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Figure 2.8 Vapor pressure of water vs. temperature. 760 Torr = 1 atm.
D.
Density
The higher the density, the slower a liquid evaporates. The stronger the forces keeping
the molecules together in the liquid state, the more energy one must get to escape.
E. Wind speed:
The higher the wind speed, the more evaporation
F. Temperature:
The higher the temperature, the more evaporation
G. Humidity:
The lower the humidity, the more evaporation
Evaporation Estimation Methods
2.5.7.1. Evaporation from open water
All emperical formulae were determined for free water surface conditions
(potential Evaporation). Emperical formulae are applicable only for the conditions
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under which they were driven. Monthly evaporation from lakes or reservoirs can be
computed using the emperical formula developed by Meyer
E = C (es - ev) . (1 + 0.1 W25)
(5)
Where;
E = evaporation in inches/month
es = sat. vapor pressure in inches of Hg
ev = actual vapor pressure in inches of Hg
W25 = average wind speed in mph at a height of 25 ft
C = coefficient
= 11 for lakes and reservoirs
= 15 for shallow ponds
2.5.7.2. Estimating evapotranspiration
Evapotranspiration, ET , is the process of water loss from land, water surfaces
and vegetation. The majority of the ET estimating methods were developed to predict ET
from a well-watered short green crop (typically alfalfa or grass). The SCS Blaney-
Criddle method is a method that is widely used throughout the world to estimate the
seasonal actual ET.
The amount of ET is related to how much energy is available for vaporizing water. The
energy is provided by solar radiation, but measuring solar radiation requires
instrumentation not available at most field sites. Blaney and Criddle assumed that mean
monthly air temperature and monthly percentage of annual daytime hours could be used
instead of solar radiation to provide an estimate of the energy received by the crop.
They defined a monthly consumptive use factor, (ET), as
ET = 4.57 K P (T + 17.8)
(6)
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Where, ET in cm, K is the ET crop coefficient, P is the mean monthly percentage of
annual daytime hours, and T is the mean monthly air temp. in oC.
Another formula widly used for evapotranspiration calculation is the FAO Penman
Monteith equation. However, this equation needs many input parameters.
0.408 ∆(Qn − G ) + γg (T2 )U 2 (es − ea )
∆ + γ (1 + 0.34 U 2 )
ETo =
Where:
ETo = Evapo-transpiration rate in mm/day
∆
=
es
4098es
(Ta + 237.3) 2
= saturated vapor pressure at mean air temperature in m.bar
=
Ta
Qn
6.108e
 17.27 Ta 


 T + 237.3 
 a

= mean Daily Temp
= (1-α)Rs – (RIo – RIi)
α
= the albedo constant = 0.23
Rs
= the incoming short wave radiation = (0.25+0.5
n
= actual duration of sun rise in hours.
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n
)Ra
N
N
= mean daily duration of maximum possible sunshine hours
from tables
Ra
= Extra Terrestrial Radiation in equivalent evaporation
mm/day
RIo
= the out going long wave radiation
RIi
= the incoming long wave radiation
Rnl = (RIo – RIi) =
2x10-9(Ta+273)4 (0.34-0.044
G
ea
)(0.1+0.6
= soil heat flux = 0.057 (MTi – MTi-1)
MTi = mean air temperature of month i
MTi-1 = mean air temperature of the previous month
γ
= 0.665 x 10-3 Pa
Pa
 293 − 0.0065 Z 
= atmospheric pressure in K.Pa = 101.3

293


Z
= elevation above sea level
900
g(T2) = T + 273
2
T2 = Ta = mean daily air temperature at 2m hight
U2
= Wind speed at 2 m height (m/sec)
ea
= air vapor pressure in m.bar = es x RH
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5.25
n
)
N
RH
= relative humidity
2.5.7.3. Direct Measuring
A. Evaporation pan
An evaporation pan is used to hold water during observations for the
determination of the quantity of evaporation at a given location. Such pans are of
varying sizes and shapes, the most commonly used being circular or square. The best
known of the pans are the "Class A" evaporation pan and the "Sunken Colorado Pan".
In Europe, India and South Africa, a Symon's Pan (or sometimes Symon's Tank) is
used. Often the evaporation pans are automated with water level sensors and a small
weather station is located nearby.
An Evaporation pan is a measurement that combines or integrates the effects of several
climate elements: temperature, humidity, solar radiation, and wind. Evaporation is
greatest on hot, windy, dry days; and is greatly reduced when air is cool, calm, and
humid. Pan evaporation measurements enable farmers and ranchers to understand how
much water their crops will need.
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A variety of evaporation pans are used throughout the world. There are formulas for
converting from one type of pan to another.
Figure 2.9 Class A evaporation pan
B.
Hook gauge evaporimeter
It is a precision instrument used to measure changes in water levels due to evaporation.
The device consists of a sharp hook suspended from a micrometer cylinder, with the
body of the device having arms which rest on the rim of a still well. The still well
serves to isolate the device from any ripples that might be present in the sample being
measured, while allowing the water level to equalize. The measurement is taken by
turning the knob to lower the hook through the surface of the water until capillary
action causes a small depression to form around the tip of the hook. The knob is then
turned slowly until the depression "pops," with the measurement showing on the
micrometer scale. Evaporation rate is determined by a sequence of measurements over a
set time interval.
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Figure 2.10 Hawk gauge evaporimeter
Using Standard Evaporation Pan: Squared (British Pan) or Circular (American Pan), the
potential evaporation is measured. Pan reading should be correlated by Pan Coefficient
Cp that depends on the pan dimensions, type and sitting.
Actual Evaporation Rate (Ea) = Cp . [Potential Evaportaion Rate (Ep)]
Where Cp is the pan correlation coeficient < 1.0
Infiltration
Infiltration is the process of water entering the soil. The rate of infiltration is the
maximum velocity at which water enters the soil surface. When the soil is in good
condition or has good soil health, it has stable structure and continuous pores to the
surface. This allows water from rainfall to enter unimpeded throughout a rainfall event.
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A low rate of infiltration is often produced by surface seals resulting from weakened
structure and clogged or discontinuous pores.
Infiltration rate in soil science is a measure of the rate at which soil is able to absorb
rainfall or irrigation. It is measured in inches per hour or millimeters per hour. The rate
decreases as the soil becomes saturated. If the precipitation rate exceeds the infiltration
rate, runoff will usually occur unless there is some physical barrier. It is related to the
saturated hydraulic conductivity of the near-surface soil. The rate of infiltration can be
measured using an infiltrometer.
We should differentiate between percolation and infiltration Percolation is the process
by which water moves through soil because of gravity. It should be mentioned that the
main reason of studying Infiltration is determining the runoff in the rain fall-runoff
relation.
The rate and quantity of water which infiltrates is a function of soil type, soil moisture,
soil permeability, ground cover, drainage condition, depth of water table i.e. water
characteristics and intensity and volume of precipitation.
Infiltration is the downward movement of water from the land surface into the soil
profile. Some water that infiltrates will remain in the shallow soil layer, where it will
gradually move vertically and horizontally through the soil and subsurface material.
Eventually, it might enter a stream by seepage into the stream bank. Some of the water
may continue to move deeper (percolate), recharging the local groundwater aquifer. A
dry soil has a defined capacity for infiltrating water. The capacity can be expressed as a
depth of water that can be infiltrated per unit time, such as inches per hour. soil has a
defined capacity for infiltrating water. The capacity can be expressed as a depth of
water that can be infiltrated per unit time, such as inches per hour.
Soil can be a excellent temporary storage medium for water, depending on the type and
condition of the soil. Proper management of the soil can help maximize infiltration and
capture as much water as allowed by a specific soil type.
If water infiltration is restricted or blocked, water does not enter the soil and it either
pond on the surface or runs off the land. Thus, less water is stored in the soil profile for
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use by plants. Runoff can carry soil particles and surface applied fertilizers and
pesticides off the field. These materials can end up in streams and lakes or in other
places where they are not wanted. Soils that have reduced infiltration have an increase
in the overall amount of runoff water. This excess water can contribute to local and
regional flooding of streams and rivers or results in accelerated soil erosion of fields or
stream banks.
2.5.8.1. Definitions

Infiltration. The downward entry of water into the immediate surface of soil or
other materials.

Infiltration capacity. The maximum rate at which water can infiltrate into a
soil under a given set of conditions.

Infiltration rate. The rate at which water penetrates the surface of the soil,
expressed in cm/hr, mm/hr, or inches/hr. The rate of infiltration is limited by the
capacity of the soil and the rate at which water is applied to the surface. This is a
volume flux of water flowing into the profile per unit of soil surface area
(expressed as velocity).

Percolation. Vertical and lateral movement of water through the soil by gravity.
2.5.8.2. Water movement during infiltration
As precipitation infiltrates into the subsurface soil, it generally forms an
unsaturated (vadose) zone and a saturated (phreatic) zone. In the unsaturated zone, the
voids (spaces between grains of gravel, sand, silt, clay, and cracks within rocks) contain
both air and water. Although a lot of water can be present in the unsaturated zone, this
water cannot be pumped by wells because it is held too tightly by capillary forces. The
upper part of the unsaturated zone is the soil-water zone. The soil zone is cris- crossed
by roots, openings left by decayed roots, and animal and worm burrows, which allow
the precipitation to infiltrate into the soil zone. Water in the soil is used by plants in life
functions and leaf transpiration, but it also can evaporate directly to the atmosphere.
Below the unsaturated zone is a saturated zone where water completely fills the voids
between rock and soil particles.
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Water movement in the vadose zone is generally conceptualized as occurring in the
three stages of infiltration, redistribution, and drainage or deep percolation, as
illustrated in Figure 2.11. As described above, infiltration is defined as the initial
process of water entering the soil resulting from application at the soil surface.
Capillary forces or matric (negative pressure) potentials are dominant during this phase.
Redistribution occurs in the next stage where the infiltrated water is redistributed within
the soil profile after water application to the soil surface stops. During redistribution,
both capillary and gravitational effects are important. Simultaneous drainage and
wetting takes place during this stage. Evapotranspiration takes place concurrently
during the redistribution stage, and will impact the amount of water available for deeper
penetration within the soil profile. The final stage of water movement is termed deep
percolation or recharge, which occurs when the wetting front reaches the water table.
The term "infiltration" is typically used as a single terminology to describe all three
stages of water movement through the vadose zone. The terms, "water flux,"
"infiltration rate," and "rate of water movement" are also used interchangeably.
Figure 2.11 Infiltration process
2.5.8.3. Principles governing the infiltration process
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Infiltration is governed by three main factors perception, gravity and capillary action.
While smaller pores offer greater resistance to gravity, very small pores pull water
through capillary action in addition to and even against the force of gravity.
A-
rate and duration of water application
If rainfall supplies water at a rate that is greater than the infiltration capacity,
water will infiltrate at the capacity rate, with the excess either being ponded, moved as
surface runoff, or evaporated. If rainfall supplies water at a rate less than the infiltration
capacity, all of the incoming water volume will infiltrate. In both cases, as water
infiltrates into the soil, the capacity to infiltrate more water decreases and approaches a
minimum capacity. When the supply rate is equal to or greater than the capacity to
infiltrate, the minimum capacity will be approached more quickly than when the supply
rate is much less than the infiltration capacity.
If water is ponded over the soil surface, the rate of infiltration exceeds the soil
infiltration capacity. If water is applied slowly, the infiltration rate may be slower than
the soil infiltration capacity. If a high rainfall intensity was very high the infiltration
rate decreases much faster than if it was a slow intensity, figure 2.12.
Figure 2.12 Effect of Rainfall intensity on Infiltration rate
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Generally, soil-water infiltration has a high rate in the beginning, decreases rapidly, and
then slowly decreases until it approaches a constant rate. As shown in Figure 2.13, the
infiltration rate will eventually become steady and approach the value of the saturated
hydraulic conductivity.
Figure 2.13 Infiltration rate
B- soil characteristics
Infiltration is governed by two forces: gravity and capillary action. While smaller pores
offer greater resistance to gravity, very small pores pull water through capillary action
in addition to and even against the force of gravity.
capillary action is affected by soil characteristic such as :
• Texture: The type of soil (sandy, silty, clayey) can control the rate of infiltration. For
example, a sandy surface soil normally has a higher infiltration rate than a clayey
surface soil. A soil survey is a recorded map of soil types on the landscape.
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Figure 2.14 Effect of soil type on Infiltration rate
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Figure 2.15 Effect of soil type on Accumulated Infiltration
• Crust: Soils that have many large surface connected pores have higher intake rates
than soils that have few such pores. A crust on the soil surface can seal the pores and
restrict the entry of water into the soil.
• Compaction: A compacted zone (plowpan) or an impervious layer close to the
surface restricts the entry of water into the soil and tends to result in ponding on the
surface.
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Figure 2.16 Effect of Compaction in a sandy soil on Infiltration rate
• Water Content: The content or amount of water in the soil affects the infiltration rate
of the soil. The infiltration rate is generally higher when the soil is initially dry and
decreases as the soil becomes wet. Pores and cracks are open in a dry soil, and many of
them are filled in by water or swelled shut when the soil becomes wet. As they become
wet, the infiltration rate slows to the rate of permeability of the most restrictive layer.
Figure 2.17 Effect of initial water content on Infiltration rate
• Aggregation and Structure: Soils that have stable strong aggregates as granular or
blocky soil structure have a higher infiltration rate than soils that have weak, massive,
or platelike structure. Soils that have a smaller structural size have higher infiltration
rates than soils that have a larger structural size.
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• Ground water table level: the far the ground water table level from ground surface
the increase in infiltration rate.
• Organic Matter: An increased amount of plant material, dead or alive, generally
assists the process of infiltration. Organic matter increases the entry of water by
protecting the soil aggregates from breaking down during the impact of raindrops.
Particles broken from aggregates can clog pores and seal the surface and decrease
infiltration during a rainfall event.
• slope of the land: The slope of the land can also indirectly impact the infiltration
rate. A steep slope will result in runoff, which will impact the amount of time the water
will be available for infiltration. In contrast, gentle slopes will have less of an impact
on the infiltration process due to decreased runoff.
• vegetation cover: vegetation cover tends to increase infiltration by retarding surface
flow, allowing time for water infiltration. Plant roots may also increase infiltration by
increasing the hydraulic conductivity of the soil surface through the creation of
additional pore space.
2.5.8.4.
Methods of measuring Infiltration
A.
Lab Measurements (Rain fall simulator)
A device (sprinklers) that simulate the rainfall with certain intensity, rate and time by
placing the soil with whatever conditions I like (slope, soil characteristics, etc.) taking
in consideration all the boundary conditions for the artificial water shed developed and
by measuring the volume at the predefined outlet the infiltration will be the difference
between the volume out from the sprinklers and the volume measured from the outlet.
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Figure 2.18a Rainfall Simulator
Figure 2.18b Rainfall Simulator
B.
Field Measurements (Infiltrometers)
There are three main problems related to the use of infiltrometers:
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1. The pounding of the infiltrometer into the ground deforms the soil causing cracks and
increasing the measured infiltration capacity.
2. Natural rainfall reaches terminal velocity. Also natural droplet sizes differ with
different types of storms. Pouring water from a measuring cup however loses this
momentum and variance.
3. With single ring infiltrometers, water spreads laterally as well as vertically and the
analysis is more difficult.
i. Single ring infiltrometer
The single ring involves driving a ring into the soil and supplying water in the ring
either at constant head or falling head condition. Constant head refers to condition
where the amount of water in the ring is always held constant. Because infiltration
capacity is the maximum infiltration rate, and if infiltration rate exceeds the infiltration
capacity, runoff will be the consequence, therefore maintaining constant head means the
rate of water supplied corresponds to the infiltration capacity. The supplying of water is
done with a Mariotte's bottle. Falling head refers to condition where water is supplied in
the ring, and the water is allowed to drop with time. The operator records how much
water goes into the soil for a given time period. The rate of which water goes into the
soil is related to the soil's hydraulic conductivity.
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Figure 2.19 Single Ring Infiltrometer
ii. Double ring infiltrometer
Double ring infiltrometer requires two rings: an inner and outer ring. The purpose is to
create a one dimensional flow of water from the inner ring, as the analysis of data is
simplified. If water is flowing in one-dimension at steady state condition, and a unit
gradient is present in the underlying soil, the infiltration rate is approximately equal to
the saturated hydraulic conductivity. An inner ring is driven into the ground, and a
second bigger ring around that to help control the flow of water through the first ring.
Water is supplied either with a constant or falling head condition, and the operator
records how much water infiltrates from the inner ring into the soil over a given time
period.
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Figure 2.20 Double Ring Infiltrometer
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iii.
Other types of Infiltrometers
Figure 2.21 Infiltrometers
2.5.8.5. Methods of calculations of Infiltration
Infiltration can be measured by calculation of infiltration rate. The infiltration
rate (ƒ), expressed in inches per hour or centimeters per hour, is the rate at which water
enters the soil at the surface. If water is ponded on the surface, the infiltration occurs at
the potential infiltration rate. If the rate of supply of water at the surface, for example
by rainfall, is less than the potential infiltration rate then the actual infiltration rate will
also be less than the potential rate. Most infiltration equations describe the potential
rate. The cumulative infiltration F is the accumulated depth of water infiltrated during a
given time period and is equal to the integral of the infiltration rate over that period:
F (t) =
where τ is a dummy variable of time in the integration. Conversely, the infiltration rate
is the time derivative of the cumulative infiltration:
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A. Horton’s Equation
Several rainfall-runoff generating processes have been recognized over the
years (Dunne, 1978; Freeze, 1980; Beven, 1989). The transformation of precipitation
into surface runoff is controlled by the independent interaction of many spatially
variable processes. Horton runoff (Horton, 1933) and Dunne runoff (Dunne and Black,
1970) are perhaps the two most important conceptual models for surface runoff.
Horton runoff is considered the excess of precipitation intensity over soil infiltration
rate at a point (Freeze, 1974).:
f(t) = f c + ( f 0 - f c ) e-kt
where f(t) is the infiltration at time t (cm/hr), f0 is the initial infiltration rate (cm/hr), fc
is the constant infiltration rate (cm/hr), and k is a decay constant.
Figure 2.22 Infiltration rate Curve and Hyetograph
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B. Philip’s Equation
Philip (1957) solved Richards equation under less restrictive conditions by
relating conductivity and diffusivity to the soil moisture content. The cumulative
infiltration F can expressed
F = S t 1/2 + Kt
where S is sorptivity. The infiltration rate at time t can be obtained by differentiating
the above equation
1
f(t) = S t -1/2 + K
2
C. Green-Ampt method
Green and Ampt (1911) developed approximate solutions of Richards equation
for infiltration calculation. The Green-Ampt method of infiltration estimation accounts
for many variables that other methods, such as Darcy's law, do not. It is a function of
the soil suction head, porosity, hydraulic conductivity and time. Once integrated, one
can easily choose to solve for either volume of infiltration or instantaneous infiltration
rate.
1, if rainfall intensity (i) is <= Ks, then f = i
2. If I > Ks, then f = I until F = its = Fs
Fs=
[( θ s - θ i )ψ f ]
[1 - i/ K s ]
M dψ f
=
[1 - i/ K s ]
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where θs is the saturated moisture content, θi is the initial moisture content, ψf is tension
or suction, and Md is the initial moisture deficit.
M dψ f
f = K s [1 ]
F
for i > K s
3. Following surface saturation, in the case of i = Ks, f=i.
Using this model one can find the volume easily by solving for F(t). However the
variable being solved for is in the equation itself so when solving for this one must set
the variable in question to converge on zero, or another appropriate constant. A good
first guess for F is Kt. The only note on using this formula is that one must assume that
h0, the water head or the depth of ponded water above the surface, is negligible. Using
the infiltration volume from this equation one may then substitute F into the
corresponding infiltration rate equation below to find the instantaneous infiltration rate
at the time, t, F was measured.
Figure 2.23 Wetting front Movement "Green-Ampt method"
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Interception
A portion of the rainfall is intercepted by plant foliage, buildings, and other
objects. This water is not available for runoff. Interception typically removes about 0.55
mm during a single storm event. Values as high as 1.5 mm have been reported. The
amount of precipitation captured by vegetation and trees is determined by comparing
the precipitation in gages beneath the vegetation with that recorded nearby under the
open sky.
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CHAPTER III
Precipitation process
The word precipitation in chemistry refers to material falling out of suspension.
The same definition can be applied when studying weather and from meteorology refers
to all forms of liquid or solid water particles that form in the atmosphere and then fall to
the earth's surface.
Different shapes of precipitation
Precipitation occurs in various forms.

Rain is precipitation that is in the liquid state when it reaches the earth.
Form of precipitation in which separate drops of water fall to the Earth's surface
from clouds. The drops are formed by the accumulation of fine droplets that
condense from water vapor in the air. The condensation is usually brought about
by rising and subsequent cooling of air. Sometimes rain will show up on the
RADAR but there is no rain reaching the ground this phenomenon is called
virga.

Drizzle is liquid precipitation that reaches the surface in the form of
drops that are less than 0.5 millimeters in diameter

Snow is frozen water in a crystalline state. Snow occurs when the layer
of the atmosphere from the surface of the earth through the cloud is entirely
below freezing. The precipitation falls from the cloud as snow and does not
melt at all while falling to the ground.

Hail is frozen water in a 'massive' state. Hail is a product of very intense
thunderstorms. Hail is rarely seen when the surface air temperature is below
freezing. It forms as a byproduct of strong updrafts that exist in thunderstorms.
The cumulonimbus clouds that are associated with thunderstorms can grow to
heights where the temperature is below freezing. Drops of water will rise up
with the upward directed wind as they collide with other droplets and grow
larger. This will eventually result in the droplet freezing into a hailstone.
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
Sleet is melted snow that is an intermixture of rain and snow. Sleet is
nothing more than frozen raindrops. Sleet occurs when there is a warm layer of
air above a relatively deep sub-freezing layer at the surface. The layer above
freezing will allow for liquid precipitation but as the drops hit the cold layer,
they will freeze and hit the ground as frozen water droplets. Sleet usually
doesn't last long and mainly occurs ahead of warm fronts during winter months
Of course, precipitation that falls to earth in the frozen state cannot become part of the
runoff process until melting occurs. Much of the precipitation that falls in mountainous
areas and in the northerly latitudes falls in the frozen form and is stored as snow pack or
ice until warmer temperatures prevail.
Precipitation is the primary input parameter of the hydrologic cycle.
Condensation may be attributed to one or more of the following
causes
A. Dynamic or adiabatic cooling
B. Mixing of air masses having different temperatures
C. Contact cooling with the Earth
D. Cooling by radiation
Figure 3.1 Convergent lifting
The most important cause of condensation is dynamic cooling. Condensation rates
associated with other causes are usually small and these rarely produce appreciable
precipitation. Dew, frost, and fog are condensation forms commonly associated with
radiational and contact cooling.
The average annual precipitation on certain locations is a function of
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a. latitude (high in latitudes of rising air and low in latitudes of
descending air).
b. elevation (precipitation increases with elevation).
c. distance from moisture sources.
d. position within the continental land mass.
e. prevailing wind direction.
f. relation to mountain ranges (more rain on windward sides than
leeward sides).
g. relative temperatures of land and bordering oceans.
Types of storms
Precipitation can be classified by the origin of the lifting motion that causes the
precipitation. Each type is characterized by different spatial and temporal rainfall
regimens. The three major types of storms are classified as convective storms,
orographic storms, and cyclonic storms. A fourth type of storm is often added, the
hurricane or tropical cyclone, although it is a special case of the cyclonic storm.
Convective
Precipitation from convective storms results as warm moist air rises from lower
elevations into cooler overlying air. Heating of air at the interface with the ground, the
heated air expands with a result of reduction of weight and the air will rise. The
characteristic form of convective precipitation is the summer thunderstorm. The surface
of the earth is warmed considerably by mid- to late afternoon of a summer day, the
surface imparting its heat to the adjacent air. The warmed air begins rising through the
overlying air, and if proper moisture content conditions are met (condensation level),
large quantities of moisture will be condensed from the rapidly rising, rapidly cooling
air. The rapid condensation may often result in huge quantities of rain from a single
thunderstorm spawned by convective action, and very large rainfall rates and depths are
quite common beneath slowly moving thunderstorms. A summer thunderstorm is the
typical convective storm. Convective storms are important in highway design due to
their intensity.
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Figure 3.2 Convectional lifting
Orographic
Orographic precipitation results as air is forced to rise over a fixed-position
geographic feature such as a range of mountains. It is due to mechanical lifting of moist
air masses over natural barriers such as mountains. The characteristic precipitation
patterns of the Pacific coastal states are the result of significant orographic influences.
Mountain slopes that face the wind (windward) are much wetter than the opposite
(leeward) slopes. In some areas, the west-facing slopes may receive upwards of 2500
mm (100 in) of precipitation annually, while the east-facing slopes, only a short
distance away over the crest of the mountains, receive on the order of 500 mm (20 in)
of precipitation annually.
Figure 3.3 Orographic Storms, Christopherson,2000
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Cyclonic
Cyclonic precipitation is caused by the rising or lifting of air as it converges on
an area of low pressure. Air moves from areas of higher pressure toward areas of lower
pressure. In the middle latitudes, cyclonic storms generally move from west to east and
have both cold and warm air associated with them. These mid-latitude cyclones are
sometimes called extra-tropical cyclones or continental storms.
Continental storms occur at the boundaries of air of significantly different temperatures.
A disturbance in the boundary between the two air parcels can grow, appearing as a
wave as it travels from west to east along the boundary.
Generally, on a weather map, the cyclonic storm will appear, with two boundaries or
fronts developed. One has warm air being pushed into an area of cool air, while the
other has cool air pushed into an area of warmer air. This type of air movement is called
a front; where warm air is the aggressor, it is a warm front, and where cold air is the
aggressor, it is a cold front.
The precipitation associated with a cold front is usually heavy and covers a relatively
small area, whereas the precipitation associated with a warm front is more passive,
smaller in quantity, but covers a much larger area. Tornadoes and other violent weather
phenomena are associated with cold fronts.
Frontal lifting
The existence of an area with low pressure causes surrounding air to move into
the depression, displacing low pressure air upwards, which may then be cooled to dew
point. If cold air is replaced by warm air (warm front) the frontal zone is usually large
and the rainfall of low intensity and long duration. A cold front shows a much steeper
slope of the interface of warm and cold air usually resulting in rainfall of shorter
duration and higher intensity (see figure 3.4). Some depressions are died-out cyclones.
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Figure 3.4a Cold front Lifting, Christopherson,2000
Figure 3.4b Warm front Lifting, Christopherson,2000
Hurricanes and Typhoons
Hurricanes, typhoons, or tropical cyclones develop over tropical oceans that
have a surface water temperature greater than 29°C (84°F). A hurricane has no trailing
fronts, as the air is uniformly warm since the ocean surface from which it was spawned
is uniformly warm. Hurricanes can drop tremendous amounts of moisture on an area in
a relatively short time. Rainfall amounts of 350 to 500 mm (14 to 20 in) in less than 24
hours are common in well-developed hurricanes, where winds are often sustained in
excess of 120 km/h (75 mi/h).
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Measuring Rainfall
Measuring precipitation covers rain, hail, snow, rime, hoar frost and fog, and is
traditionally measured using various types of rain gages such as the non-recording
cylindrical container type or the recording weighing type, float type and tipping-bucket
type.
Most rain gauges generally measure the precipitation in millimeters. The level of
rainfall is sometimes reported as inches or centimeters.
Rain gauge amounts are read either manually or by AWS (Automatic Weather Station).
The frequency of readings will depend on the requirements of the collection agency.
Some countries will supplement the paid weather observer with a network of volunteers
to obtain precipitation data (and other types of weather) for sparsely populated areas. In
most cases the precipitation is not retained, however some stations do submit rainfall
(and snowfall) for testing, which is done to obtain levels of pollutants.
Rain gauges have their limitations. Attempting to collect rain data in a hurricane can be
nearly impossible and unreliable (even if the equipment survives) due to wind extremes.
Also, rain gauges only indicate rainfall in a localized area. For virtually any gauge,
drops will stick to the sides or funnel of the collecting device, such that amounts are
very slightly underestimated, and those of .01 inches or .25 mm may be recorded as a
trace.
Another problem encountered is when the temperature is close to or below freezing.
Rain may fall on the funnel and freeze or snow may collect in the gauge and not permit
any subsequent rain to pass through. Rain gauges, like most meteorological
instruments, should be placed far enough away from structures and trees to ensure that
any effects caused are minimized.
The requirements for gauge construction are:
1. The rim of the collector should have a sharp edge.
2. The area of the aperture should be known with an accuracy of 0.5 %.
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3. Desig is such that rain is prevented from splashing in or out.
4. The reservoir should be constructed so as to avoid evaporation.
5. In some climates the collector should be deep enough to store one day's
snowfall.
Wind turbulence affects the catch of rainfall. Experiments in the Netherlands using a
400 cm2 raingauge have shown that at a height of 40 cm the catch is 3-7 % less than at
ground level and as much as 4-16 % at a height of 150 cm. Tests have shown that rain
gauges installed on the roof of a building may catch substantially less rainfall as a result
of turbulence (10-20%).
Wind is probably the most important factor in rain-gauge accuracy. Updrafts resulting
from air moving up and round the instrument reduce the rainfall catch. Figure 2.5
shows the effect of wind speed on the catch according to Larson & Peck (1974). To
reduce the effects of wind, raingauges can be provided with windshields. Moreover,
obstacles should be kept far from the rainguage (distance at least twice the height of
such an object) and the height of the gauge should be minimised (e.g. ground-level
raingauge with screen to prevent splashing).
History
The first known records of rainfalls were kept by the Ancient Greeks about 500
B.C. This was followed 100 years later by people in India using bowls to record the
rainfall. The readings from these were correlated against expected growth, and used as a
basis for land taxes. In the Arthashastra, used for example in Magadha, precise
standards were set as to grain production. Each of the state storehouses were equipped
with a standardised rain gauge to classify land for taxation purposes.
Some sources state that the Cheugugi of Korea was the world's first gauge, while other
sources say that Jang Yeong Sil developed or refined an existing gauge. In 1662
AD, Christopher Wren created the first tipping-bucket rain gauge in Britain.
In modern ageThe first rain gauges were installed in 1991 as a joint effort between
MSD and the United Geological Survey (USGS). The rain gauge information was to be
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used for MSD studies and USGS research. In 1997, MSD took over sole responsibility
of the rain gauge network. These data logger rain gauges were non-telemetered and
required MSD personnel to download the information that was stored within the rain
gauge. Though labor intensive, these rain gauges work extremely well and remain in
operation today.
In 1997, eleven telemetry-equipped rain gauges were installed. The primary purpose of
these rain gauges was to provide real-time data for emergency response support. The
majority of these rain gauges were installed at MSD facilities located throughout
Jefferson County. For the purposes of emergency response support, these rain gauges
performed adequately. However, with the implementation of the Real Time Control
(RTC) project, these telemetered rain gauges did not meet the requirements of the RTC.
Their geographic distribution and the telemetry system used at the time were deemed
insufficient to provide the needed information in a timely manner. In order to meet the
goals of the RTC project and to provide even better emergency response support, the
telemetered rain gauge system needed to be updated.
In Spring of 2003, fifteen new telemetry-equipped rain gauges were installed
throughout Jefferson County. This updated rain gauge system serves two primary
functions – to calibrate weather service NEXRAD radar with rain gauge data, and to
provide rainfall predictions at least two hours in advance. This information will be
utilized by both MSD’s RTC project and for emergency response preparation. The new
rain gauge network also provides a better geographical coverage of Jefferson County.
Standard Rainfall gauges
Generally, standard gauges measure precipitation at or near the ground, and are
observed at least once a day. The sizes of the gauges are made big enough to collect
more than the average one-day or maximum 1-2 hour precipitation which differs
according to various climatic conditions.
The standard gauges are also commonly used to measure both rain and snow, and the
latter affects fundamentally the form and dimensions of a particular national gauge
(snow gauges are bigger). Thus, in countries with negligible snowfall but much rain or
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where different gauges are used for rain and snow (e.g., Canada), the height of the
gauge orifice varies between zero and more than 1 meter above the ground.
Figure 3.5 Standard rainfall gauge
Automated Rain Gauge
There are electronic rain gauges that measures rain fall, and are also self
emptying and frost proof. The basic idea is the rain collector’s measuring spoon being
automatically tipped and emptied when the pre-adjusted water weight has been reached.
These instruments use a thin nozzle to produce single uniform droplets corresponding
to a fixed volume of water. Each droplet is detected by an optical system giving a single
pulse output that is counted. The measurement resolution can be < 0.001 mm with an
upper limit of the RI range of about 50 mm/h. The single droplet resolution of approx.
10 mm³ results in a high temporal resolution.
Higher RI (within the allowed measurement range) can be measured instantaneously
and directly by a frequency measurement of the output pulses. Because of the thin
nozzle for the droplet formation, field operation needs great attention and service.
Therefore, these systems are mainly used for research purposes.
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Figure 3.6 Tipping bucket rainfall gauge
Another type is a weighing bucket that moves a pen downward with the rainfall
accumulating in the collecting bucket. The pen draws a line on a graph paper folded
around a rotating cylinder. The resulting curve is the mass curve of the rainfall during
the recorded rainfall events.
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Figure 3.7 Weighing bucket rainfall gauge
Optical Rainfall gauges
These have a row of collection funnels. In an enclosed space below each is
a laser diode and a phototransistor detector. When enough water is collected to make a
single drop, it drips from the bottom, falling into the laser beam path. The sensor is set
at right angles to the laser so that enough light is scattered to be detected as a sudden
flash of light. The flashes can be translated to amount of water and the rate of flashing
can represent the time scale.
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High Precision Single-Unit Rain Gauge
The bucket, 4" in diameter, measures each rain drop, displays it on the digital display
with 3/8" numerals, and then empties itself. Simply place it outside on a hard, level
surface and watch it record rainfall up to 99.999 inches. Convenient one touch reset
button lets you keep annual, monthly, or storm-by-storm totals. The unit has no moving
parts, gold-plated sensors for reliability, and is not damaged by freezing conditions.
Weather Radar
Radar is an object detection system that uses electromagnetic waves to identify
the range, altitude, direction, or speed of both moving and fixed objects such as aircraft,
ships, motor vehicles, weather formations, and terrain. The term RADAR was coined in
1941 as an acronym for RAdio Detection And Ranging. The term has since entered the
English language as a standard word, radar, losing the capitalization. Radar was
originally called RDF (Radio Direction Finder, now used as a totally different device)
in the United Kingdom, in order to preserve the secrecy of its ranging capability.
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The radar dish, or antenna, transmits pulses of radio waves or microwaves
which bounce off any object in their path. The object returns a tiny part of the wave's
energy to a dish or antenna which is usually located at the same site as the transmitter.
The time it takes for the reflected waves to return to the dish enables a computer to
calculate how far away the object is, its radial velocity and other characteristics.
:A radars components are
A transmitter that generates the radio signal with an oscillator such as a
klystron or a magnetron and controls its duration by a modulator.
•
•
A waveguide that links the transmitter and the antenna.
A duplexer that serves as a switch between the antenna and the
transmitter or the receiver for the signal when the antenna is used in both
situations.
•
A receiver. Knowing the shape of the desired received signal (a pulse),
an optimal receiver can be designed using a matched filter.
•
An electronic section that controls all those devices and the antenna to
perform the radar scan ordered by a software.
•
•
A link to end users.
A weather radar is a type of radar used to locate precipitation, calculate its
motion, estimate its type (rain, snow, hail, etc.), and forecast its future position and
intensity. Weather radars are mostly doppler radars, capable of detecting the motion of
rain droplets in addition to intensity of the precipitation. Both types of data can be
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analyzed to determine the structure of storms and their potential to cause severe
weather.
The Precipitation Radar was the first spaceborne instrument designed to provide
three-dimensional maps of storm structure. These measurements yield invaluable
information on the intensity and distribution of the rain, on the rain type, on the storm
depth and on the height at which the snow melts into rain. The estimates of the heat
released into the atmosphere at different heights based on these measurements can be
used to improve models of the global atmospheric circulation.
The Precipitation Radar has a horizontal resolution at the ground of about 3.1
miles (five kilometers) and a swath width of 154 miles (247 kilometers). One of its
most important features is its ability to provide vertical profiles of the rain and snow
from the surface up to a height of about 12 miles (20 kilometers). The Precipitation
Radar is able to detect fairly light rain rates down to about .027 inches (0.7 millimeters)
per hour. At intense rain rates, where the attenuation effects can be strong, new methods
of data processing have been developed that help correct for this effect. The
Precipitation Radar is able to separate out rain echoes for vertical sample sizes of about
820 feet (250 meters) when looking straight down. It carries out all these measurements
while using only 224 watts of electric power—the power of just a few household light
bulbs. The Precipitation Radar was built by the National Space Development Agency
(JAXA) of Japan as part of its contribution to the joint US/Japan Tropical Rainfall
Measuring Mission (TRMM)
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Weather Satellite
A weather satellite is a type of satellite that is primarily used to monitor the
weather and climate of the Earth. These meteorological satellites, however, see more
than clouds and cloud systems. City lights, fires, effects of pollution, auroras, sand and
dust storms, snow cover, ice mapping, boundaries of ocean currents, energy flows, etc.,
are other types of environmental information collected using weather satellites. Visiblelight images from weather satellites during local daylight hours are easy to interpret
even by the average person; clouds, cloud systems such as fronts and tropical storms,
lakes, forests, mountains, snow ice, fires, and pollution such as smoke, smog, dust and
haze are readily apparent. Even wind can be determined by cloud patterns, alignments
and movement from successive photos.
Disdrometer
A disdrometer is an instrument used to measure the drop size distribution and
velocity of falling hydrometeors. Some disdrometers can distinguish between rain,
graupel, and hail. The uses for disdrometers are numerous. They can be used for traffic
control, scientific examination, airport observation systems, and hydrology. The latest
disdrometers employ microwave or laser technologies. 2D video disdrometers can be
used to analyze individual snowflakes.
Other types of modern rain gauges are data loggers, Infrared recorders, Wire less and
data logging rain gauges.
Figure 3.8 Data Logging rainfall gauge
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Weather Logger (Weather station)
Developed for measuring and recording in
remote areas, Portable and operates as a
completely self contained system. It
includes Instrument shelter and needs no
power and depend only on solar/battery
supply. The record rate could be user
selectable from once a minute to once an
hour. It has a RAM of a suitable size for
storing data that can be downloaded.
The Weather station should be able to record;
Wind Speed, for range between 0 – 200 km/s with a high
accuracy.


Wind Direction, range 0 – 360 degree with high resolution

Temperature, recommended range (-40o – 65o)C

Relative Humidity,

Dew Point,

Barometric Pressure,

Solar Radiation,

Rainfall depth, unlimited collection of rainfall

Rainfall intensity.
Rain gages Network establishment
Catch area of a rain gauge is very small compared to the aerial extent of a storm.
Hence to get a representative picture of a storm over the entire drainage basin, the
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number of rain gauges should be as large as possible (drainage area/rain gauge should
be small). The rain gauge network should consist of adequate number of rain gauges
evenly distributed all over the drainage basin. However, the number of rain gauges is
many a time restricted by economic considerations as well as topography, accessibility
etc. Desired density would also depend on the purpose. 10% of rain gauge stations
should be equipped with self recording rain gauges.
Aim of design rain gauges networks
Establish a rain gauge network with an optimum density of rain gauges from
which a reasonably accurate information about storms can be obtained.
Recommendations on rain gauge density
 Flat regions of temperate, Meditteranean and tropical zones
•
•
Ideal : 1 station for 600-900 sq .km.
Acceptable : 1 station for 900-3000 sq .km.
 Mountainous regions of temperate, Meditteranean and tropical zones
•
•
Ideal : 1 station for 100-250 sq .km.
Acceptable : 1 station for 250-1000 sq .km.
 Arid and polar zones
•
Ideal : 1 station for 1500-10000 sq .km. Depending on the feasibility.
Adequacy of rain gauge stations
The optimum number of rain gauge stations that should exist in order that the
mean rainfall can be estimated with an assigned percentage of error is given by
If the value of ε is small, the number of rain gauge stations required will be
more.
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If there are m rain gauge stations in the catchment (existing), each recording rainfall
values
, in a known time, the coefficient of variation
is
given by
=Standard deviation
Index of Wetness
• Index of Wetness =
It gives an idea of the wetness of that year and hence is a measure of the deficiency of
rainfall. A 60% index of wetness means a deficiency of 40%.
• Deficiency ~ 30-45% – Large
• Deficiency ~ 45-60% – Serious
• Deficiency ~ >60% – Disastrous
Interpretation of Rainfall data (data Screening)
In order to avoid erroneous conclusions it is important to give the proper
interpretation to precipitation data, which often cannot be accepted at face value. For
example, a mean annual precipitation value for a station may have little meaning if the
gage site has been changed significantly during the period for which the average is
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computed. Also, there are several ways of computing average precipitation over an
area, each of which may give a different answer.
The precipitation data screening objective is: the identification of unreliable or
spurious data resulting from instrument problems such as power outages, obstruction of
the gage by debris, and clock synchronization problems. The result of the precipitation
data screening procedures will be to either reject spurious data or correct data where
appropriate.
Data screening tools and Criteria
Since screening the precipitation data requires careful examination of large time
series files, graphical tools are required to facilitate this process. The primary screening
tool is a standard report of precipitation data which allow for the comparison of
precipitation data collected from several raingage stations. Wherever possible, standard
criteria are used to assess the precipitation data. However, in many cases screening the
precipitation data requires application of subjective criteria. In general, data which are
obviously spurious are removed from the record. In addition, significant timing shifts
resulting from unreliable data logger clocks are identified and a time shift correction
factor is estimated.
Estimating of missing data
Many precipitation stations have short breaks in their records because of
absence of the observer or because of instrumental failures. It is often necessary to
estimate this missing record. In the procedure used by the U.S. Environmental Data
Service, precipitation amounts are estimated from observations at three stations as close
to and as evenly spaced around the station with the missing record as possible. If the
normal annual precipitation at each of the index stations is within 10% of that for the
station with the missing record, a simple arithmetic average of the precipitation at the
index stations provides the estimated amount.
If the normal annual precipitation at any of the index stations differs from that at
the station in question by more than 10%, the normal-ratio method is used. In this
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method, the amounts at the index stations are weighted by the ratios of the normalannual-precipitation values. That is, precipitation Px at station X is:
PX = 1/3 ((NX/NA) PA + (NX/NB) PB + (NX/NC) PC)
in which N is the normal annual precipitation.
Multiple linear regression will yield an equation of the form
PX = a + bAPA + bBPB + bCPC
Where a should be near zero and the b’s will approximate the three coefficients
of the original Equation divided by 3. The advantage of the regression qpproach is that
it allows for some weighting of the stations and adjusts, to some extent, for departures
from the normal ratio assumption of the Equation. If a large amount of data must be
estimated, a random error term tSy should be added to the equation. In this term t a
normal random number with mean of zero and standard deviation of 1, and Sy is the
standard error of estimate of PX. Inclusion of the term recognizes the departures from
the regression, and maintains the standard deviation of the estimated values of PX near
the observed standard deviation.
Estimates of missing precipitation data are generally most reliable for general
type storms over flat terrain or over relatively smooth windward mountain slope. Sever
and spotty convective activity and rugged terrain lessen the reliability. Estimates for
long intervals (month or year) are more reliable than those for short intervals such as (a
day).
Double-Mass Analysis
Changes in gage location, exposure, instrumentation, or observational procedure
may cause a relative change in the precipitation catch. Frequently these changes are not
disclosed in the published records. Current U.S. Environmental Data Service practice
calls for new station identification whenever the gage location is changed by as much as
8 km and/or 30 m in elevation
Double-mass analysis tests the consistency of the record at a station rent
accumulated values of mean precipitation for a group of surrounding stations. In the
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following Figure, for example, a change in slope about 1961 indicates a change in the
precipitation regime at Dillon, Colo. A change due to metrological causes would not
cause a change in slope, as all base stations would be similarly affected. The station
history for Dillon discloses a change in gage location in June 1961. To make the record
prior to 1961 comparable with that for the more recent location, it should be adjusted by
ratio of the slopes of the two segments of the double-mass curve (0.74/1.19). The
consistency of the record for each of the base stations should be tested, and those
showing inconsistent records should be dropped before other stations are tested
adjusted.
Considerable caution should be exercised in applying the double-mass
technique. The plotted points always deviate about a mean line, and changes in slope
should be accepted only when marked or substantiated by other evidence.
Estimation of missing Data
In the area of water resources planning and management, complete data sets are
required on many variables such as rainfall, stream flow, evapotranspiration and
temperature. Unfortunately, records of hydrological processes are usually short and
often have missing observations. The existence of data gaps might be attributed to a
number of factors such as interruption of measurements because of equipment failure,
effects of extreme natural phenomena such as hurricanes or human-induced factors such
as wars, mishandling of observed records by field personnel, or accidental loss of data
files in the computer system
The solutions described here are not perfect. Because the causes of missing or
erroneous data are infinitely various. In this report we will review the methods of
determining the missing data of a rain gauge.
A.
Neighboring stations
Missing data can be estimated using data of neighboring station, this method is divided
to three types:1- Nearest Neighbor by distance (ND): selecting the closest gauge with data.
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2- Nearest Neighbor by correlation (NC): selecting the neighboring gauge that
has the highest correlation with the gauge to be patched.
3- Inverse Distance Weighted (IDW): using multiple neighboring gauges
weighted by distance.
Pc = ∑P1dci-k
dci-k∑
.Pc is the rainfall for the gauge to be patched
.Pi is a neighbor gauge
.dci is the distance between the gauges
.K is a weight known as friction distance that ranges from 1.0 – 6.0
Reference is (Patching and Disaccumulation of Rainfall Data for Hydrological)
(Modeling) Department of Mathematics, The Australian National University, Canberra
B.
The Normal ratio method:
Normal ratio method (NRM) is used when the normal annual precipitation at any of the
index station differs from that of the interpolation station by more than 10%. In this
method, the annual precipove itation values P1,P2,P3,….Pm at neighboring m stations
1,2,3,…..M respectively, it is required to find the missing annual precipitation Px at a
station X not included in the above M stations. Further, the normal annual precipitations
N1, N2, ……., N .. at each of the above (M+1) stations including station X are known.
If the normal annual precipitation at various stations are within about 10% of the
normal annual precipitation at station X, then a simple arithmetic average procedure is
followed to estimate Px. Thus
[Px= 1/M[P1+P2+……………….Pm
C.
Double-mass curve technique:
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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 n (usually 5 to 10) base stations in the neighborhood of the problem station
X is selected.
Annual (or monthly mean) rainfall data of 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).
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 slope is normally taken as
significant only where it persists for more than five years.
Calculating Average Rainfall
The arithmetic-mean method
The arithmetic-mean method is the simplest method of determining areal
average rainfall. It involves averaging the rainfall depths recorded at a number of gages.
This method is satisfactory if the gages are uniformly distributed over the area and the
individual gage measurements do not vary greatly about the mean.
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The Thiessen method
If some gages are considered more representative of the area in question than
others, then relative weights may be assigned to the gages in computing the areal
average. The Thiessen method assumes that at any point in the watershed the rainfall is
the same as that at the nearest gage so the depth recorded at a given gage is applied out
to a distance halfway to the next station in any direction.
The relative weights for each gage are determined from the corresponding areas
of application in a Thiessen polygon network, the boundaries of the polygons being
formed by the perpendicular bisectors of the lines joining adjacent gages[Fig. 3.4.3(6)].
If there are / gages, and the area within the watershed assigned to each is Aj9 and Pj is
the rainfall recorded at the 7th gage, the areal average precipitation for the watershed is
where the watershed area
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The Thiessen method is generally more accurate than the arithmetic mean
method, but it is inflexible, because a new Thiessen network must be constructed each
time there is a change in the gage network, such as when data is missing from one of
the gages. Also, the Thiessen method does not directly account for orographic
influences on rainfall.
The Isohyetal method
The isohyetal method overcomes some of these difficulties by constructing
isohyets, using observed depths at rain gages and interpolation between adjacent gages
[Fig. 3.4.3(c)]. Where there is a dense network of raingages, isohyetal maps can be
constructed using computer programs for automated contouring.
Once the isohyetal map is constructed, the area Aj between each pair of
isohyets, within the watershed, is measured and multiplied by the average Pj of the
rainfall depths of the two boundary isohyets to compute the areal average precipitation
by Eq. (3.4.1). The isohyetal method is flexible, and knowledge of the storm pattern can
influence the drawing of the isohyets, but a fairly dense network of gages is needed to
correctly construct the isohyetal map from a complex storm. Other methods of
weighting rain gage records have been proposed, such as the reciprocal-distancesquared method in which the influence of the rainfall at a gaged point on the
computation of rainfall at an ungaged point is inversely proportional to the distance
between the two points (Wei and McGuinness, 1973).
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Singh and Chowdhury (1986) studied the various methods for calculating areal
average precipitation, including the ones described here, and concluded that all the
methods give comparable results, especially when the time period is long; that is, the
different methods vary more from one to another when applied to daily rainfall data
than when applied to annual data.
Characteristics of rainfall events
Intensity (mm/hour)
Intensity is defined as the time rate of rainfall depth and is commonly given in
the units of millimeters per hour (inches per hour). All precipitation is measured as the
vertical depth of water (or water equivalent in the case of snow) that would accumulate
on a flat level surface if all the precipitation remained where it fell. A variety of rain
gauges have been devised to measure precipitation. All first-order weather stations use
gauges that provide nearly continuous records of accumulated rainfall with time. These
data are typically reported in either tabular form or as cumulative mass rainfall curves.
The Rainfall variation during a storm is expressed as a hypetograph.
In any given storm, the instantaneous intensity is the slope of the mass rainfall
curve at a particular time. For hydrologic analysis, it is desirable to divide the storm
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into convenient time increments and to determine the average intensity over each of the
selected periods. Intensity is the most important rainfall characteristic in various
engineering designs because it is directly related to the peak flow such as highway,
bridge, and flood control.
A hyetograph is also used to describe the variation of the storm with time. The
temporal distribution of the storm affects the shape of the direct runoff hydrograph. For
example, for a storm having an average rainfall intensity of 2 mm/hr will not produce
much
direct
runoff
and
most
of
rainfall
will
enter
the
subsurface.
Rainfall duration
The duration of the storm is directly related to the volume of surface runoff and
ground-water recharge. High intensities are generally associated with short duration
storms. Large water volumes are generally associated with long duration storms.
Frequency
Hydrologic systems are sometimes impacted by extreme events, such as severe
storms, floods, and droughts. The magnitude of an extreme event is inversely related to
its frequency of occurrence, very severe events occurring less frequently than more
moderate events. The objective of frequency analysis of hydrologic data is to relate the
magnitude of extreme events to their frequency of occurrence through the use of
probability distributions.
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Spatial distribution
Storm location, areal extent, and storm movement are usually determined by the
origin of the storm. For instance, cold fronts produce localized fast-moving storms.
Warm fronts give origin to slow-moving widespread precipitation.
A storm taking place far from the basin outlet would produce longer hydrographs and
lower peaks than if the same storm occurred near the outlet. A localized storm would
likely produce smaller peaks and s shorter hydrograph than if the same storm covered
the whole watershed. A storm moving away from the outlet will produce an earlier and
smaller peak than if the storm moves towards the outlet.
In most circumstances, it is assumed that rainfall is uniform over the entire watershed
for the duration of the storm !!!.
Frequency Analysis of Rainfall Data
The frequency of occurrence of a storm of given magnitude and duration is
important to establish a measure of risk. For a given storm duration, the probability that
an event of certain magnitude has of being equaled or exceeded in any one year is
termed the probability of exceedance. Frequency can be represented by the return
period, which is the average number of years between events of a given magnitude or
greater. The return period is related to the probability of exceedance by
T =
R
1
P
where T is the return period and P is the probability of exceedance.
In highway and other designs, frequencies are needed to select appropriate
rainfall values that will result in design stream flows. A storm of a given frequency
does not generally produce a peak discharge of the same frequency. However, these
frequencies are commonly assumed to be the same, especially if models are used to
estimate runoff from precipitation.
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CHAPTER IV
4.1.
Hydro morphology
a. Drainage Area:
Drainage area of a watershed is considered the most important
parameter of it. The drainage area reflects the volume of water the can
be generated from rainfall and that is available for runoff. The area of
watershed is defined by watershed delineation that can be done
manually or using computer programs.
b. Watershed Length:
Watershed length is defined as the distance measured along the principal
flow path of a watershed from the watershed outlet to the basin divide.
Watershed length affects the value of travel time and consequently the
runoff hydrograph at the watershed outlet. The longer the watershed
length the more time it takes for water to be conveyed from the
beginning of the watershed to the outlet. Consequently, if all other
factors are the same, a watershed with a longer length will have a slower
response to a given precipitation input than a watershed with a shorter
channel length. Longer watershed lengths result in lower peak
discharges and longer hydrographs, Figure 1e.
c. Watershed Slope:
Watershed slope is the rate of change of elevation along the principal
flow path of a watershed. Steep slopes tend to result in rapid responses
to local rainfall excess and consequently higher peak discharges. The
runoff is quickly removed from the watershed, so the hydrograph is
short with a high peak, Figure 1a.
The total volume of runoff is also affected by slope. If the slope is very
flat, the rainfall excess will not be removed as rapidly. The process of
infiltration will have more time affect the rainfall excess, thereby
resulting in a reduction of total volume.
d. Watershed Shape:
The watershed shape is not directly used in hydrological design
methods; however, it affects the way runoff will bunch up at the outlet.
For example, a circular watershed will cause runoff from various parts
of the watershed to reach the outlet at the same time which results in
high value of peak runoff discharge, while an elliptical watershed of the
same area that has outlet at one end of its major axes will have a lower
value of peak discharge.
e. Land Cover and Use:
Land cover affects the amount of rainfall that will be intercepted by land
and deducted from rainfall precipitation before runoff is generated.
Different hydrological analysis and design methods use kind of index to
represent the land cover parameter.
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The rational method uses a runoff coefficient (C) to represent the
percentage of rainfall that will be transformed into runoff. Typical
values of this runoff coefficient are used depending on the type of land
cover and land use. For example, a value of 0.90 can be used for
impervious surfaces like building roofs and asphalt roads, while a value
of 0.15 can be used for lawns and open parks. A set of “C” values is also
available for lumped areas depending on land use. For example, a value
of 0.75 can be typically used for commercial districts, while a value of
0.30 is used for low density residential districts.
The Soil Conservation Service (SCS) uses the runoff curve number
(CN) to represent the effect of land cover. Typical values of CN are
available and can be defined based on the soil type, land cover,
hydrological condition, and antecedent moisture content.
f. Hydraulic Roughness:
Hydraulic roughness is a composite of the physical characteristics which
influence the flow of water across the surface, whether natural or
channelized. It affects both the time response of a drainage channel and
the channel storage characteristics. Hydraulic roughness has a marked
effect on the characteristics of the runoff resulting from a given storm.
The peak rate of discharge is inversely proportional to hydraulic
roughness, i.e., the lower the roughness, the higher the peak discharge.
Roughness affects the runoff hydrograph in a manner opposite of slope.
The lower the roughness, the more peaked and shorter in time the
resulting hydrograph will be for a given storm, Figure 1b.
The total volume of runoff is virtually independent of hydraulic
roughness. An indirect relationship does exist in that higher roughnesses
slow the watershed response and allow some of the abstraction
processes more time to affect the runoff. Roughness also has an
influence on the frequency of discharges of certain magnitudes by
affecting the response time of the watershed to precipitation events of
specified frequencies.
g. Drainage Density
Drainage density can be defined as the ratio between the number/total
length of well defined drainage channels and the total drainage area in a
given watershed.
Drainage density has a strong influence on both the spatial and temporal
response of a watershed to a given precipitation event. If a watershed is
well covered by a pattern of interconnected drainage channels, and the
over-land flow time is relatively short, the watershed will respond more
rapidly than if it were sparsely drained and flow time was relatively
long. The mean velocity of water is normally lower for overland flow
than it is for flow in a well defined natural channel. High drainage
density increases the response of a watershed leading to higher peak
discharges and shorter hydrographs for a given precipitation event,
Figure 1d.
h. Antecedent Moisture Conditions
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Antecedent moisture conditions, which are the soil moisture conditions
of the watershed at the beginning of a storm, affect the volume of runoff
generated by a particular storm event. Runoff volumes are related
directly to antecedent moistures. The lower the moisture in the ground at
the beginning of precipitation, the lower will be the runoff; conversely,
the higher the moisture content of the soil, the higher the runoff
attributable to a particular storm.
Figure 1: Effects of Watershed Geomorphologic Characteristics on the Flood
Hydrograph
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4.2.
Time of Concentration
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CHAPTER V
5.1.
Hydrograph
5.2.
Base Flow Separation
5.3.
Direct Runoff Hydrograph
5.4.
Unit Hydrograph
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CHAPTER VI
6.1.
Flood Routing
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