GED 357: BASIC HYDROLOGY
Lectures:
10:00 – 11:55 @ A (Fri.)
Practicals:
9:00 – 9:55 (Fri.)
Credit Hrs:
2
Lecturer(s):
E. K. Appiah-Adjei / B. Ali
COURSE OUTLINE
WEEK
2-3
ACTIVITY
INTRODUCTION
 Definition; Hydrological Cycle; Drainage Patterns
 Practical Values of Hydrology and Hydrological Failures
3-5
CLIMATE
 Temperature and Humidity
 Precipitation - Formation; Types and Variation; Measurement; Data Analysis
EVAPORATION (+ EVAPOTRANSPIRATION)
 Factors affecting evaporation from free water surface
 Measurement and Estimation of Evaporation.
6-7
8-9
INFILTRATION
 Factors affecting infiltration; Methods of determining infiltration
 Storage and movement of infiltration
10-12
RUNOFF
 Sources and components; Factors affecting it volume and distribution
 Measurement, estimation and prediction of runoff
 Hydrograph Analysis
13-16
EXAMINATION
2
• LITERATURE
1. Lecture Notes
2. Engineering Hydrology by Wilson, E. M. (1983)
3. Hydrology in Practice by Shaw, E.M. (1988)
4. Principles of Hydrology by Ward & Robinson (1990)
5. Hydrology for Engineers by Linsley et. al (1982)
6. A Textbook of Hydrology by Reddy P.J.R. (2007)
7. Applied Hydrology by Chow et. al (1988)
8. Other relevant materials can be searched on the web?
• NOTES
• Grading {Exam (70%) + CA (30%) = 100%}
• CA (Mid-Semester Exam, Assignments, Tests, etc.)
• Attendance, Punctuality, etc.
3
INTRODUCTION
 HYDROLOGY
- It is the study of the origin, movement, distribution, and quality of
water on and over the surface of the earth
- It deals with all the water in and upon the earth (i.e., rain, surface
water, groundwater, etc.) and their usefulness to life
 Engineering hydrology deals with the very broad field of hydrology
pertinent to the design and operation of engineering projects for
the control and use of water
 Hydrological cycle describes the continuous movement of water
on, above and below the surface of the earth
- It forms the basis for the study of hydrology
- The cycle has no exact beginning or ending point but may be,
conveniently, assumed to start with evaporation from oceans
4
•
Infiltration is the movement (seepage) of water from land surface into the soil
•
Percolation is the downward movement of infiltrated water under gravity thru the
unsaturated zone to reach the water table
•
Interflow is the lateral movement of water in the vadose zone ( surface soil) during and
immediately after precipitation event into surface water
•
Baseflow is the seepage of water directly from the saturated zone (groundwater) to
surface water
•
Overland flow is the portion of precipitation that drains across the land surface to
streams (NB: Runoff; Surface Runoff)
5
Schematic Hydrological Cycle (After Fetter, 2001)
Atmosphere (Water Vapour)
Interflow
Ocean
(Seawater)
Rising magma in volcano (magmatic water)
Runoff
Evaporation
Precipitation
Surface Water
(Lakes, Ponds,
Streams and
Rivers)
Transpiration
Unsaturated Zone
(Soil Moisture)
Percolation
Evaporation
Infiltration
Precipitation
Evapotranspiration
Precipitation
Land Surface (Ice,
Snow, Depression
Storage, Trees, etc.)
Overland flow
Capillary rise
Zone of Saturation
(Groundwater)
Baseflow
Subsea Outflow
Lithosphere (Magmatic water)
6
• Things to note about the cycle:
1. The cycle may be short-circuited at several stages, e.g. precipitation
may be trapped by vegetation and re-vaporized back to the atmosphere;
2. There is no uniformity in the time a cycle is completed;
3. The intensity and frequency of the cycle depend on geography and
climate that varies according to latitude and season of year;
4. Man can exercise control on certain parts of the cycle, e.g. runoff can
be directed to a preferred storage place instead of it flowing naturally to a
stream or groundwater system; cloud seeding leading to artificial rainfall
5. The cycle emphasize four basic phases of interest to the
hydrologist, namely; precipitation, evapotranspiration, surface
stream flow, and groundwater
6. Cycle is evaluated quantitatively by the water balance equation:
CHANGE IN STORAGE = INFLOW – OUTFLOW
7
Estimates of the world’s water resource (UNESCO, 1971)
Volume (km3)
Parameter
Equivalent
Average Residence
Depth (m)
Time
1 370 x 106
2500
4000+ yrs
125 000
0.25
~ 10 yrs
Swamps
3 600
0.007
1 – 10 yrs
River Channels
1 700
0.003
~ 2 weeks
Moisture in Soil and Unsat. Zone
65 000
0.13
2 wks – 1 yr
4 x 106 to 60 x 106
8 – 120
2 wks – 10000 yrs
Ice Caps and Glaciers
30 x 106
60
10’s – 1000’s of yrs
Atmospheric Water
13 000
0.025
8 – 10 days
700
0.001
~ 1 week
Oceans and Seas
Freshwater Lakes and Reservoirs
Groundwater
Biological Water
Estimates of freshwater distribution
Soil Moisture
1.5%
Surface Water
3.5%
Groundwater
95.0%
8
Sample Questions
1. During 1996, the water budget terms for lake Bosumtwi included
precipitation of 43 cm/yr, evapotranspiration of 53 cm/yr, surface
water inflow of 1 cm/yr, surface outflow of 173 cm/yr, and
change in volume of -2 cm/yr. Determine the net groundwater
flow?
2. The annual averages rainfall for Weija catchment with an area of
120 km2 is 1745mm. If a statutory minimum discharge of 0.15m3/s
is maintained throughout the year in the river downstream and an
annual average evapotranspiration of 400mm is recorded,
estimate the volume of water available for supply. State and
explain the need for any assumption(s) made.
9
HYDROLOGY IN ENGINEERING
 The Engineer mainly uses hydrology in the design, building and
operation of hydraulic structures
– Enough hydrological data is needed; Engineer needs to analyse the
data; Predict the most likely quantities involved in extreme cases of
flood and drought (their frequency of occurrence) before applying it.
• The engineer applies the hydrological data in the:
1. Estimation of the water balance of a region/catchment
2. Design of dams for water supply, irrigation and power generation
(i.e., the reservoir capacity required to assure adequate water for
these purposes during droughts)
3. Design of bridges, culverts and general urban drainage system (i.e.,
flood flows to be expected at spillways, highway culverts, etc.)
4. Mitigation and prediction of floods, drought risk and landslides
10
 SOURCES OF HYDROLOGICAL DATA
1. Meteorological Services Department (MSD),
2. Ministry of Agriculture,
3. Ghana Water Company Limited (GWCL)
4. Civil Aviation Authority (CAA)
5. Volta River Authority (VRA)
6. Irrigation Development Authority (IDA)
7. Research Universities like Universities, CSIR, WRI, WRC, etc.
8. Others private organisations like CIDA, DANIDA, etc.
11
WEATHER AND HYDROLOGY
• Weather
- is the state of the atmosphere in terms of temperature, humidity,
precipitation and radiation within a day
- It is not the same everywhere each day; recorded daily and predicted
worldwide by meteorologist
 Hydrology of a region depends primarily on:
1. Climate
- It is the average weather in an area over a long time period
- It depends on the geographic position of the area, proportion of
land to water, and proximity to oceans and mountains
- Climatic factors of importance are the amount and distribution of
precipitation, humidity, temperature, and wind on evaporation and
snow melt
12
 Hydrology of a region depends primarily on (cont’d):
2. Topography
- It influence precipitation and occurrence of swamps, rivers, lakes
and rates of runoff
3. Geology
- It influence the formation of landforms (i.e., topography)
- It influence groundwater storage and movement in underlying
rocks of an area
13
Structure of the atmosphere (Shaw, 1996)
Atmosphere is a distinctive layer of air, water
vapour, and other layers of about 100 km
surrounding the earth. The layers are divided into
troposphere, stratosphere, mesosphere and
thermosphere
Troposphere is the most important layer to the
hydrologist because it contains about 75% of the
atmosphere’s weight and virtually all of it’s moisture
Height of the tropopause varies seasonally as a
result of changes in air temperature and pressure in
the atmosphere
Lapse rate is the decrease in temperature
with increasing altitude through the
troposphere (about 6.5 oC/km)
Rise in temperature b/n 20-50km is caused by a
layer of ozone, which absorbs short wave radiation
from the sun and release some of the energy as heat
14
Climatic parameters useful in hydrology include:
1. Temperature
• It is the measure of the hotness or coldness of a body or place
• It is measured with a thermometer in oC or K (SI units)
• Global temperature at the equator is warmer than at the poles
• Temperature varies in an irregular but characteristic way with
increasing altitude in the atmosphere
• Temperature profile define the atmosphere into layers known as
spheres -Troposphere, Stratosphere, Mesosphere and Thermosphere
• Daily temp varies from minimum around sunrise to maximum from
0.5 to 3 hrs after the sun has reached its zenith
• Mean daily temperature is average of daily max and min
temperatures
15
2. Atmospheric Pressure
- It’s defined as the weight of a column of air of unit cross-sectional
area from the level of measurement to the top of the atmosphere
- It is usually measured with a barometer in bars (1 bar = 100 kPa)
- It decreases with increasing altitude (higher you go lower it becomes)
3. Air density (ρ)
- may be estimated from the expression: ρ = p/RT,
where R (= 0.29 kJ/kgK) is dry air specific gas constant; and T is air temperature in
Kelvin.
- Air density also decreases with increasing altitude
16
Other Climatic Parameters – cont’d
4. Humidity
- It’s the amount of water vapour in the atmosphere; measured in mb
- Water vapour distribution on earth depends on temp; highest at
the equator and lower at poles
- Water vapour movement and phases determines the earth’s heat
and energy balance; Heat is absorbed and released upon evaporation
and condensation respectively.
5. Saturated Vapour Pressure (SVP)
- Air is said to be saturated when it contains the maximum amount of
water vapour it can hold at its prevailing temperature
- SVP is the pressure exerted by water vapour molecules on water
molecules in a closed system
17
- SVP’s relationship with air temperature is given by the SVP curve
SVP Curve of Water Vapour in Air
Saturation Vapour Pressure (mmHg)
35
30
25
es
R.H . 
20
ed
 100%
es
15
10
Y(T, e)
5
Td
0
-10
-5
0
5
10
15
20
25
30
35
Temperature (oC)
Dew Point, Td, is the temperature at which a mass of unsaturated air becomes
saturated when cooled at constant pressure
Saturation Deficit (SD) is the amount of water vapour that the air can hold at constant
temperature before becoming saturated; SD= es-
ed
18
- Saturated vapour pressure is also given by:
Td = dew point temperature [°C]; es = saturation vapor pressure [mb]
6. Relative Humidity
- It’s a measure of the degree of saturation of air
- measures of the relative amount of moisture in air to the amount
needed to saturate the air at the same temperature
- Given by: R.H. = е/еs x 100% (i.e. usually expressed in %)
19
Other Climatic Parameters – cont’d
7. Absolute Humidity
- It is equivalent to water vapour density
- It is the amount of water vapour contained in a given volume of air
- Vapour density is generally expressed as mass of water vapour per
unit volume of air at given temperature. Hence;
A.H. = mw /Va {gm-3}
8. Specific Humidity
- is the mass of water vapor contained in a unit of moist air
- Its relate the mass of water vapour mw (g) to mass of moist air (kg)
in a given volume; Given by the relation:
q = mw (g)/(mw + md) (kg); OR
= ρw/ρ {gkg-1}
20
Other Climatic Parameters – cont’d
8. Precipitable Water
- It’s the total amount of water vapour in a column of air expressed
as depth of liquid water in mm over the base area of the column
- It gives an estimate of the maximum possible rainfall under the
unreal assumption of total condensation
- It is given by:
where q is the average specific humidity, ∆p is the change in
pressure, and g is the acceleration due to gravity (9.81 m/s2)
21
Sample Question
• Measurement of pressure and specific humidity from a radiosonte
ascent are shown in Table below. Calculate the precipitable water in
a column of air up to the 600 level and rainfall (Assuming 68% of it
would fall as rain; Take g=9.81 m/s2).
Pressure (mb)
1006
920
800
Specific Humidity (g/kg) 14.00 13.40 10.20
740
9.40
700
7.20
660
6.60
600
5.60
500
4.00
22
Q1.
The relative humidity of air mass at a temperature of
24 oC is 64 %. Using the graph below, estimate the
moisture deficit and dew point.
35
Vapour Presure (mm Hg)
30
25
20
15
10
Q2.
5
The saturation deficit and dew point of a mass of air are
o
0 14.0 mmHg and 11 C, respectively. Calculate the
-15
-5
5
15
25
saturated vapour
pressure
and relative
humidity
of the35
Temperature ( oC)
air mass.
23
Class Test 1
1. During January 1996, the water budget terms for Lake
Victoria included precipitation of 19 cm, evaporation of
15 cm, surface water inflow of 1 mm, surface outflow
of 175 cm and change in lake storage of 1 mm.
Determine the net groundwater flow for January, 1996.
2. A mass of air at a temperature of 26 oC cools to dew
point at 11 oC. (i) What is the saturation deficit for this
air mass? (ii) Estimate the relative humidity of the air
mass.
24
PRECIPITATION
- is a product of water vapour condensation that falls on the
earth's surface
- is a major component of the hydrologic cycle, and the vital
source of fresh water for mankind and life on earth
- occurs when water vapour in atmosphere becomes greatly
saturated (via cooling and/or adding moisture), condenses and
falls down to the earth
- may be divided into two main categories:
1. Liquid e.g. rain, drizzle, dew, and condensation of fog droplets on
vegetation, etc.
2. Solid e.g. snow, snowflakes, hail, graupel, sleet, etc.
25
How is precipitation formed?
1. Rising water vapour (i.e. warm moist air)
- warm moist air may be forced to rise by contact with cold object,
mountain, etc.
2. Condensation
- Condensation nuclei (small particles in atm), provide a nuclei
around which saturated water vapour condenses
- Two types of condensation nuclei:
a) Hygroscopic (e.g. salt particles) – has high affinity for water
vapour and easily aids in condensation
b) Non-hygroscopic (dust, grit, ash, sooth) – needs some degree
of super saturation before attracting condensation
3. Coalescence
- water droplets fuse or collide to form larger droplets
- larger droplets overcome air resistance and fall as precipitation
26
Industry
Volcano
Fires
Sea Salt
Soil/Dust
27
Types of Precipitation
 The types of precipitation are related to the mechanisms of rising
of moist air mass, namely:
1. Convection:
- occurs when heated air on the land surface becomes less dense
and begins to rise
- warm air is forced to rise and cool to dew point
- leads to convective precipitation
28
2. Frontal Activity
- occurs when warm moist air mass comes in contact with a cold
object like the ground
- also occurs when a colder air mass intrude a slightly warm one
and causes it to rise, loose temperature and condense to fall
- leads to frontal precipitation (or continental rainfall)
29
3. Adiabatic expansion of rising air:
- results from mechanical lifting of moist horizontal air currents
over natural barriers such as mountain ranges
- warm air is forced to rise by an impeding mountain range
- leads to reduction in pressure causing lowering of temperature
without any transfer of heat
- this leads to formation of orographic precipitation
30
Measurement of Precipitation
 Precipitation is a primary input in most hydraulic projects (e.g.
used to estimate flood flows, infiltration, recharge, etc); hence its
accurate measurement is very necessary
 Generally, 3 basic rules must followed in measuring precipitation:
1. All measurements must be comparable and consistent
2. Standard instruments must be installed uniformly in
representative areas
3. Regular observational procedures must be adopted e.g., daily
weekly, or monthly
 Precipitation is usually measured at a point using standard
collectors of very simple construction e.g. rain gauge
- Snowfall is measured with a graduated stick or by determining the
equivalent amount of water in a unit of snow
- RADAR can also be used to estimate precipitation in atmosphere
31
Measurement of Precipitation – cont’d
 Two main types of rain gauges are available, namely: recording
and non-recording gauges
1. Non-Recording Gauges
- the amount of rainfall intercepted is measured by observer at
regular intervals
- made up of a collecting funnel (dia. = 5’’ and depth=4’’), inner
glass can and outer glass casing
- rain is led into the glass via funnel and then read later by observer
- In heavy falls, rain may overflow into the inner glass can, and in
very rare cases overflow into the outer casing of the gauge
-The outer and inner casings are designed not to allow evaporation
32
Measurement of Precipitation – cont’d
2. Recording Gauges
• automatically measure and record the amount and time of
rainfall
• Two basic types of recording gauges are available, namely:
- tilting siphon and tipping bucket types.
• Tilting Siphon:
- based on float gauge principle,
- rainfall is collected into a funnel and led into a float chamber,
- float move vertically as water rises in the chamber, and
- the movement is transmitted by means of a pulley and pen arm
to revolving chart that records the rainfall
33
Measurement of Precipitation – cont’d
• Tipping Bucket:
─funnel collects rain into one of the two compartments of the
tipping bucket system
─the rain is led into one compartment while emptying the other
─movement of the bucket is transmitted, mechanically or
electrically, to a moving strip chart for rainfall recording
34
Siting of Rain Gauges
• It is a professional job and must be decided with competence
• Amount measured by the gauge should be representative of the
surrounding area
• Should be sited in a area where conditions are permanent
• Best sited on level ground; avoid siting on steep slopes especially
towards wind direction
• The area must be properly drained to prevent flooding and possible
submergence of the gauge
• The gauge should not be over sheltered or over exposed:
- structures (of height, h) around the gauge should be at distance
not less than 2h from the gauge
- if in over exposed surroundings –e.g. Accra plains- build a turf wall
35
Wind shields…reduce catch errors
© UBC; Photo credits DeWalle & Levno
36
Analyses of Precipitation Data
• Point rainfall data may be analyzed in the form of chronological charts
or graphs e.g., moving average (or time series) curve, mass curve,
hyetograph, etc.
• In most hydrologic studies, it is important to know the areal
distribution of precipitation; this is usually the average rainfall depth
for the watershed or area under consideration
- the 3 standard ways of determining the areal precipitation are:
1. Arithmetic Mean
- calculates arithmetic mean of rain gauge measurements at an area
considering gauges located inside area under study only
- Mean Rainfall (R) = ∑ Ri / n
- suitable for area with even distribution of gauges and has no marked
topography
- not representative when used in mountainous areas
37
2. The Thiessen Method
- Rainfall amounts at individual stations are weighted by fractions of
the catchment area represented by gauges and then summed
- Rain gauge stations are used to divide the catchment area into
polygons by lines equidistant between pairs of adjacent gauges.
Mean Rainfall (R) = ∑ Ri ai/A,
where A = total area, ai = individual area, Pi = Precipitation at individual gauge station
- takes care of uneven distribution of rain gauges and allows for
areal weighing of precipitation data
- enables influential data outside a catchment to be incorporated in
estimating mean precipitation
- It is a widely used method
38
© UBC
39
3. Isohyetal Method
- Isohyets are contours (or lines) linking points of equal rainfall (or
precipitation rates)
- considered as one of the most accurate methods, although
subjective and depends on skill and a good knowledge of catchment’s
rainfall characteristics
- isohyets are drawn at chosen intervals across the catchment by
interpolating between the gauge measurements taking into account
the topography
- Mean Rainfall, R = ∑ airi/A
ai = inter isohyetal area, ri = average rainfall between isohyets, A = total catchment area.
- mainly used for analysing storm rainfalls since they are usually
localized over small areas
40
41
Characteristics of Precipitation
1. Intensity
- is the quantity of rain falling in a given time OR the rate at which rain
falls per unit time (e.g. mm/hr); may be represented by a hyetograph
- the greater the intensity, the shorter the duration of rainfall and vice
versa.
2. Duration
- is the time period during which rain falls OR the length of time over
which rain (or precipitation) occurs
3. Amount
- is the product of average intensity and duration. e.g., 1.5 mm/h * 6 h= 9 mm
4. Distribution
- is the relative occurrence of rainfall (or precipitation) in space or time
 Frequency – is the expectation that a given depth of rain will fall in a
given time; such an amount may be equaled or exceeded in a given
42
number of days or years
Missing Data
• is simply unrecorded data at a gauge station at a particular time
• Causes:
1. Sickness or death
2. Disaster
3. Laziness or forgetfulness
4. Instrumental failure 5. Drunkeness or forgetfulness
6. Industrial action
• Estimated by considering gauge readings around missing gauge by:
1. Interpolation from isohyetal map
2. Station Average Method:
where Pm = missing station value, Pj= precipitation at known stations around the missing gauge, N is
number of stations of known precipitation
3. Normal Ratio Method:
where PA(m) is the average precipitation at missing station; PA(j) is the average precipitation at the
43
known stations
Sample Question
One of the four monthly-read rain gauges on a
catchment area develops a fault in a month
when the other three gauges record 37, 43, and
51 mm respectively. If the average annual
precipitations amount of these three gauges are
726, 752 and 840 mm respectively and of the
broken gauge is 694 mm, estimate the missing
monthly precipitation at the latter.
44
EVAPORATION
• a physical process by which water is lost from a free wet surface
(e.g. roofs, soils, lakes, etc) as water vapour into the atmosphere.
• the net loss of water through the stomata in the leaves of plants
or vegetation covering soils is known as transpiration
• the combined effect of evaporation from wet surfaces and
transpiration through plants is known as evapotranspiration
• the process contribute to energy changes in the atmosphere and
serves as an important link between various phases of the cycle
• Its measurement and estimation is very important in scientific
studies and many water management problems
- e.g., design and operation of hydraulic structures
45
Factors affecting evaporation
1. Latent Heat (radiation) – energy is needed to break the bonds in
water molecules to bring them to the vapour state
2. Temperature of air and evaporating surface; increase in temp
increase evaporation rate.
3. Humidity – high humidity decrease ability of air to absorb more
water vapour and reduce evaporation rate
4. Wind – high and turbulent wind speed increase evaporation rate
5. Quality of water – the presence of solute reduces evaporation;
e.g. evaporation from sea water is less than that of pure water
6. Nature of evaporating surface – the larger the surface area and
roughness, the higher the amount (but lower the depth) of
46
evaporation
Factors affecting transpiration
1. Plant type – i.e. extent and efficiency of the roots in absorption of
moisture, stage of growth, leaf area, and stomata openings
2. Properties of the soil - i.e. water holding capacity; available water,
and depth of soil
3. Meteorological factors – includes solar radiation, temperature,
humidity, and wind speed
NB:
Generally, the factors that govern evaporation and transpiration
also govern evapotranspiration
47
Definition of Some Common Terms
1. Potential Evaporation: quantity of water that can be lost by a
pure water surface per unit time under existing atmospheric
conditions like wind, pressure, humidity and temperature
2. Potential Evapotranspiration: is the max amount of water capable
of being lost as water vapour in a given climate at an area when
there is no limitation on water supply
3. Actual Evapotranspiration: is the quantity of water vapour lost as
water vapour from an area under existing conditions
4. Bowen’s Ratio: is the ratio of loss of upward energy flux as
sensitive heat to energy used in evaporation at an evaporating
surface
5. Albedo: defined as ratio of reflected to incident electromagnetic
48
radiation; a measure of reflectivity of a surface.
Measurement of Evaporation
•
Evaporation is measured directly from a free water surface using:
1. Evaporation pans
- they are simple and inexpensive devices; observed regularly
- its values are usually too high, compared to actual measurement
from reservoir, hence a pan coefficient is applied to it value.
Eactual = Epan* K where K is pan coefficient
2. Atmometers
- also simple, inexpensive, and easy to operate; observed regularly
- 2 types available; Piche atmometer and Bellani atmometer
3. Lysimeters
- used to measure evapotranspiration
- Consist of buried tanks growing a crop; measure precipitation in
and drainage out; and/or weigh tank
49
Estimation of Evaporation
1. Water Budget Method
- consists of accounting for all waters entering and leaving a basin
- may be represented by: E = P + I ± U – O ± S
where U is underground flow, S is change in storage, I is inflow, O is outflow, E is
evapotranspiration and P is precipitation
- use standard procedures to estimate all the parameters involved
and determine E.
50
Evaporation Estimation –cont’d
2. Mass Transfer (Aerodynamic Method)
•
evaporation is driven by vapour pressure gradient and wind speed
•
Originates from Dalton (19th century): Eopen surface = f(u).(es – ea)
•
f(u) takes 2 forms: 1. f(u) = a(b + u),
2. f(u) = Nu
where u is wind speed; a, b, and N are empirical mass transfer coefficients
•
using the 1st form, Penman (1948) established that:
- Eo = a(b + u).(es – ed) → Eo = 0.35 (0.5 + u/100).(es – ed);
- and N is dependent on the height and units of air measurements
of the evaporating surface
•
Using the 2nd form, Harbeck (1962) determined that:
Eo = Nu. (es – ed) → Eo = 0.291 A-0.05 u (es – ed) mm/day
where A is reservoir surface area (m2), u is in m/s at height 2m, and es and ed are in mb
51
Evaporation Estimation –cont’d
3. Energy Budget Method
- similar to the water budget approach; it’s the most accurate method
- a heat balance following the principle of the conservation of energy is
evaluated from incoming, outgoing, and stored energy as:
Energy for open surface evaporation, QEo = Qs – Qrs – Ql – Qc ± Qg ± Qv
where Qs is short wave solar radiation, Qrs is reflected short wave radiation, Ql is long wave
radiation from the water body, Qc is sensible heat transfer to the air, Qg is the change in stored
energy, and Qv is energy transfer between water and bed
- The evaporation is estimated by: Eo = QEo/λ mms-1
where λ is the latent heat of vaporisation of water
- This approach involves a great deal of instrumentation; can’t be used
without the many data required
52
INFILTRATION
• is the movement of water from surface into the unsaturated zone
• the rain or snowfall first wets the surface b4 seeping into the soils
• It is a very critical stage in the hydrological cycle because it’s:
- determines how much water is stored in top soil for plant growth
- a major source of groundwater recharge (i.e. thru percolation)
- determines the proportion of water that ends up as runoff
53
• Infiltration amount and rates depends greatly on ability of the soil
to absorb falling precipitation
• Infiltration capacity – max rate at which water can enter a soil at
any given condition
- it varies from soil to soil; and
- also varies for the same soil type in dry and moist conditions
• For any soil under constant rainfall, infiltration rate decrease
exponentially and asymptotically to a near constant value fc (or
Ksat) of a soil is given by the relation (Horton, 1933):
ft = fc + (fo − fc)e − kt
ft is the infiltration capacity at time t; fc is the constant or equilibrium
infiltration capacity after the soil has been saturated; fo is the initial infiltration
capacity; k is a decay constant for a specific soil
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Factors affecting infiltration
1. Precipitation characteristics e.g. intensity, duration, size, etc
- if intensity < fc , all rainfall will infiltrate
- if intensity > fc , some of the water will lead to runoff
- large drops of rain may render soils impermeable
2. Soil type and characteristics
- surface soil pores, mineralogical composition and water content
largely control the infiltration rate
3. Vegetation cover
- root systems, organic debris and burrow animals serves as
preferential paths for infiltrating water
4. Land use practices – may either ease or hinder infiltration
5. Slope of land surface – steep slopes reduce infiltration and vice versa.
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Methods of determining infiltration
1. Water Budget Method
- Take stock of all water into an area and solve the hydrologic
mass balance equation with infiltration as unknown
2. Infiltrometers
- The commonest type is the double ring Infiltromemer
- used for measuring infiltration rate (in volume or depth/time)
- basically consists of an inner and outer ring inserted into ground,
filled with water and allowed to drain.
- the downward flow of the inner ring contributes to infiltration
while the outer ring drain leads to lateral flow
3. Horton formula is used when modeling infiltration process
ft = fc + (fo − fc)e − kt
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RUNOFF
• is a term used to describe the total flow from a basin collected at
its outlet in a stream (or river or some drain)
- Overland flow or surface runoff (component of runoff), is used to
describe water that flows on land surface after precipitation
- it’s a major component of the hydrological cycle
- can be expressed as volume/time or depth/time
• Watershed is a land area that produces runoff draining toward a
common point.
• Sources of runoff include:
1. Infiltration excess overland flow called Hortonian overland flow
2. Saturation excess overland flow ( or saturated overland flow)
3. Subsurface return flow ( or interflow)
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Factors affecting runoff
1. Meteorological factors affecting runoff:
- Type of precipitation (rain, snow, sleet, etc.)
- Rainfall intensity, amount, duration and distribution over basin
- evapotranspiration factors like temperature, relative humidity, etc
2. Physical geology and topography of land surface
- Soil type and characteristics
- Vegetation
- Slope of the drainage area
- Ponds, lakes, reservoirs, etc. in a basin, which prevent or delay runoff
from continuing downstream
3. Land use activities
- development and urbanization
- agricultural practices
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Effects of runoff
1. Erosion
- runoff transport significant amount of sediments from one area
to another or into streams, lakes, etc.
2. Environmental impacts
- runoff may transfer pollutants into surface water or groundwater
- excessive runoff may lead to flooding
- agricultural issues i.e. destroying tilled and bare soil farmland 59
Runoff Determination
1. Water budget approach; often incorporated into models
2. Velocity-Area (or open channel measurement) Method
- i.e. measuring flow rate or discharge of open channel which is a
common point for runoff collection from a catchment
- Discharge, Q = v.A; v is average velocity; A is csa of channel strip
- Q is estimated relative to stage
•
Average velocity of strip is determined by two basic methods; viz.
i. Velocity at 0.6y (y = depth of strip)
ii. Average of velocities at 0.2y and 0.8y
- the flow velocities are measured with a current meter
- velocity measurements are made at permanent gauge stations
- Stage is recorded at strips where velocities are measured
- velocities are measured under steady state in portions where
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flow is uniform
61
•
Two methods of determining discharge under the velocity-area
approach are:
i) Mean Section Method
ii) Mid-Section Method
62
Sample Question
1. Details of a gauging carried out by velocity-area method are show
below. Estimate the discharge using mean and mid-section methods
distance (m)
4
9
12
15
18
21
24
27
30
33
36
39
42
45
48
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depth (m)
0
1.1
1.7
2
2.1
2.1
1.9
1.8
1.8
1.6
1.3
1.4
1.3
1.6
1.5
0
Velocity (m/s)
0
0.3
0.4
0.4
0.4
0.3
0.3
0.3
0.3
0.3
0.3
0.2
0.2
0.1
0.1
0
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3. Dilution Gauging Method
- involves putting a tracer of known concentration in a river and
sampling the river at certain points to measure their concentration
-used when river is too shallow to make meaningful measurement
- an ideal method in small turbulent flowing streams with steep
gradients where current metering is not practicable
- there are two methods of applying the tracer, namely:
1. Constant Rate Method
- the discharge is given by:
2. Gulp Injection Method
- discharge is given by:
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• Conditions of Dilution Gauging
i.
Tracer should be completely mixed at point of measurement or
sampling
ii. Tracer should have high solubility
iii. Tracer should be non-toxic to aquatic life
iv. Background concentration of tracer should be low
v. Tracer should be stable (not reactive) in water
vi. Tracer should be capable of accurate quantitative analysis in very
dilute solutions
vii. Tracer should be cheap and readily available
•
Available tracers include Tritium isotope, Rhodamine B and
Bromine-82
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The data below relate to dilution gauging using the
constant rate injection method. Calculate the discharge of
the channel.
Injection solution flow rate
10 ml/s
Concentration of injection solution
200 g/l
Background concentration of chemical in river
20 µg/l
Concentration of chemical at sampling site.
80 µg/l
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4. Flow Rating Curve Method
- study the stage and discharge at a section over a long period
- gauging can be done with a staff gauge, crest gauges, autographic
recorders, and several other automatic gauges
- plot curve of discharge against stage readings
- use the curve to determine discharge when stage changes
provided upstream (or catchment) has not changed
• Aside the rating curve, the rating table and rating equation can also
be used to establish stage-discharge relation
• The rating equation is given as Q =aHb , a and b are constants that
can be obtained from measured data
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2. Hydrograph Analysis
• Hydrograph is a plot of discharge (runoff) against duration (time)
of a storm in a catchment area
• The components of the hydrograph are:
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• Some methods of hydrograph separation (i.e., separating baseflow
from runoff in a hydrograph) are:
1.
Use of empirical relationship N=0.827 A0.2
where N = days from peak to end, A = area of watershed in km2
i)
indicate the lowest discharge (A) on rising limb side before storm
ii)
extend A with a line to point B under the peak of the hydrograph
iii)
estimate N from the relation above and locate it away from the peak discharge
on the recession side (e.g., C)
iv)
Join the points A , B and C with a straight to obtain a separation for the
baseflow and runoff
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2. By determining a master depletion curve for a particular gauge
station and applying it to a given storm to determine baseflow
- The curve is derived from a continuous discharge record of
different stages of an area over years
- It’s the best method of determining baseflow but depends on
previous data, which may not be available
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DRAINAGE PATTERNS
• It’s the arrangement and disposition of streams which a drainage
system etches into the land surface and which may reflect the sum
total of factors influencing the number, size, and frequency of
streams in a particular area
•
1.
2.
3.
4.
5.
•
Stream patterns are influenced by:
Initial slope,
Lithology and lithological variations,
Structure (in its broadest sense),
Geological and geomorphological history of the area, and
Climate and rainfall regime of the area.
There are 3 main types of stream patterns viz. dendritic, trellis
(structures) and radial (hilly or mountainous areas).
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