17CV73 - Hydrology and Irrigation Engineering

17CV73 – HYDROLOGY AND IRRIGATION ENGINEERING

Module 1 Hydrology & Precipitation

Introduction, Importance of hydrology, Global and Indian water availability, practical
application of hydrology, Hydrologic cycle (Horton’s representation) and engineering
representation.
Precipitation: Definition, forms of precipitation, types of precipitation, measurement of
precipitation (Simon’s gauge & Siphon gauge only), optimum number of rain gauge
stations, consistency of rainfall data (double mass curve method), computation of mean
rainfall, estimation of missing data, presentation of precipitation data, moving average
curve, mass curve, rainfall hyetographs.
Lecture 1& 2
Hydrology
Introduction
drology means the science of water. It is science that deals with the occurrence, circulation
Hydrology
and distribution of water of the earth and earth’s atmosphere. It is concerned with the water in
streams and lakes, rainfall and snowfall, snow and ice on the land and water occurring below the
earth’s surface in the pores of the soil and rocks. Hydrology is a very broad subject of an inter
inter-
disciplinary nature drawing support from allied sciences, such as meteorology, geology,
statistics, chemistry, physics and fl
fluid mechanics.
Hydrology is basically an applied science.
1. Scientific hydrology – the study which is concerned chiefly with academic aspects.
2. Engineering or applied hydrology – a study concerned with engineering applications.
In a general sense engineering hydrology deals with (i) estimation of water resources, (ii) the
study of processes such as precipitation, runoff, evaporation and their interaction and (iii) the
study of problems such as floods and droughts, and strategies to combat them.
Lecture 3
Global
Globa and Indian Water availability
Global water budget
Water is the most important natural resource and is vital for all life on earth. The well being and
the development of our society is dependent on the availability of water. This most precious

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resource is sometimes scarce sometimes abundant and is always very unevenly distributed, both
in space and time. Table 1 indicates the approximate distribution of world’s water resources
among separate elements of the hydrology.
Table 1 Distribution of World’s Water Resources
Location Water volume in km3 Percentage of total water
Fresh water lakes 125000 0.009
Saline lakes 104200 0.008
Stream channels 1200 0.0001
Ground water (< 0.8 km deep) 4168200 0.31
Ground water (> 0.8 km deep) 4168200 0.31
Soil moisture etc., 66700 0.005
Ice caps and glaciers 29177300 2.15
Atmosphere 12900 0.001
Oceans 1321310000 97.2
8
Approx totals 13.6 * 10 100

The oceans which cover 71% of the surface are of earth and which have an average depth of 3.8
km hold as large as 97% of the earth’s water, while 2% is frozen in ice caps. The deep
groundwater accounts for 0.31%. This 99.31% of water is of no use to man. The remaining 0.69
% which is of the order of 4.374 *106 km3 represents the fresh water resources with which the
man has to deal. Surprisingly, at any given instant of time rivers and lakes hold only 3% of fresh
water (or 0.0093% of total water) and atmosphere holds only 0.3% of fresh water (0.00093% of
total water).
a estimated to be equal and each being of the
The global annual precipitation and evaporation are
order of 100 cm. Therefore the global average annual precipitation volume (based on surface are
of 510 *106km2 of the earth whose mean radius is 6370 km) works out to 510000 km3. The
average moisture available in the atmosphere at any time is only 12900 km3. This implies that
the entire atmospheric moisture must be replaced approximately 40 times every year. Interpreted
in another way, the average residence period of the moisture in the atmosphere is only slightly
more than 9 days.

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India’s water Budget

The basic source of water for India is the rainfall over the most part of the country and snowfall
in the northern region. While the rainfall is recorded by more than 3000 rain gauges set up by
Indian Meteorological Department and the state Governments, the snowfall record is scanty. The
rainfall varies from place to place and from year to year.
The average annual rainfall for the country is about 119.4 cm and with the country’s
geographical area of 3.28 * 106 km2 it is equivalent to 3916 km3 and together with snowfall
which is not yet fully assessed the annual water resources may be approximated to 4000 km3.
Out of this, 700 km3 is lost to atmosphere,
atmosp 2150 km3 is soaked into the ground and the balance
1150 km3 becomes the direct surface runoff to the streams.
According to Irrigation Commission of India (1972), the total annual surface water flow in the
country is about 1800 km3. This includes 200 km3 of runoff brought in by the streams and rivers
from the catchments lying outside the country, 450 km3 of groundwater runoff carried by the
rainy periods and 1150 km3 of direct surface runoff from precipitation ( out
streams during non-rainy
of which 100 km3 is the contribution by the snowfall).
Out of the total surface water of 1800 km3, about 150 km3 is stored in various reservoirs and
tanks, another 150 km3 is utilised through diversion works and direct pumping and the remaining
1500 km3 goes to the sea and some adjoining countries. Even after full irrigation development in
the country, the use of water through diversions and direct pumping is expected to increase to
only 450 km3 and the balance 1050 km3 would continue to flow to the sea and outside the
country.
Out of 2150 km3 of infiltrated water only 500 km3 percolates deeply and becomes groundwater
while the remaining 1650 km3 is retained as soil moisture and goes back to the atmosphere as
evaporation and transpiration.
During floods the water surface in the streams is higher than the adjoining water
water-table along
most of its course and therefore some river water seeps into the adjoining area and adds to the
groundwater. Such contributions to groundwater during floods have been estimated to be of the
order of 50 km3. Similarly the additions from the irrigation systems to the groundwater are
estimated to be 120 km3. Thus the total groundwater available comes to 670 km3. This on full
development of water resources is likely to increase to 850 km3.

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Lecture 4
Practical application of hydrology
1. Design of hydraulic structures
2. Municipal and industrial water supply
3. Irrigation
4. Hydropower
5. Flood control
6. Navigation
7. Erosion and sediment control
8. Pollution abatement
Lecture 5
Hydrologic cycle
Water occurs on the earth in all its three states, viz, liquid, solid and gaseous, and in various
degrees of motion. Evaporation water from water bodies, such as oceans, and lakes, formation &
movement of cloud, rain and
d snow fall, stream flow & GW movement are some example of the
dynamic aspects of water. The various aspects of water related to the earth can be explained in
terms of a cycle known as the hydrologic cycle.

A convinient starting point to describe the cycle

cycle is in the oceans. Water in the oceans evaporate
due to heat energy provided by solar radiation. The water vapour moves upwards and forms
clouds. While much of the clouds condense and fall back to the oceans as rain, a part of the
clouds is driven to the
he land areas by winds. There they condense and precipitate onto the land

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mass as rain, snow, hail, sleet etc. A part of the precipitation may evaporate back to the
atmosphere even while falling. Another part may be intercepted by vegetation, structures and
other such surface modifications from which it may be either evaporated back to atmosphere or
move down to the ground surface.

The portion of the water that reaches the ground enters the earth’s surface through infiltration,
enhance the moisture content of the soil and reach the groundwater body. Vegetation sends a
portion of the water from under the ground surface back to the atmosphere through the process of
transpiration. The precipitation reaching the ground surface after meeting the needs of
infiltration and evaporation moves down the natural slope over the surface and throu
through a
network of gullies, streams and rivers to reach the ocean. The groundwater may come to the
surface through springs and other outlets after spending a considerable longer time than the
precipitation
surface flow. The portion of the p recipitation which by a variety of paths above and below the
surface of the earth reaches the stream channel is called runoff. Once it enters a stream channel,
runoff becomes stream flow.

The main components of the hydrologic cycle can be broadly classifie

classifiedd as transpiration (flow
components and storage components as below:

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Lecture 6
Precipitation
Precipitation is any form of solid or liquid water that falls from the atmosphere to the earth’s
surface.. Rainfall, drizzle, hail and snow are examples of precipitation. In India, rain is the
most common form of precipitation.

For precipitation to form: (i) Atmosphere must have moisture, (ii) Sufficient nuclei present to
aid condensation, (iii) Weather condition must be good for condensation and (iv) Product of
condensation must reach the earth.
Under proper weather conditions, the water vapour condenses over nuclei to form tiny water
droplets of sizes less than 0.1 mm in diameter. The nuclei are usually salt particles or
products of combustion and are normally available in plenty. Wind speed facilities the
movement of clouds while its turbulence retains the water droplets in suspension. Water
droplets in a cloud are somewhat similar to the particles in a colloidal suspension.
Precipitation results when water droplets come together and coalesce to form larger drops
that can drop down. A considerable part of this precipitation gets evaporated back to the
atmosphere. The net precipitation at a place and its form depend upon a number of
meteorological factors, such as the weather elements like wind, temperature, humidity and
pressure in the volume region enclosing the clouds and the ground surface at the given place.
Forms of precipitation

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Rain
It is the term used to describe the precipitation in the form of water drops of sizes larger than
0.5mm. It is the principal form of precipitation in India. Maximum size of a rain drop is
about 6mm.

Type Intensity

1. Light rain trace to 2.5mm/h

2.Moderate rain 2.5mm/h to 7.5mm/h

3.Heavy rain > 7.5mm/h

Snow It consists of ice crystals which usually combine to form flakes. Density varies from 0.06
to 0.15 g/cm3. Avg.density = 0.1 g/cm3. In India, snow occurs only in the Himalayan regions.

Drizzle A fine sprinkle of numerous water droplets of size less than 0.5mm & intensity less than
1mm/h. Drops are so small that they appear to float in air.

Glaze When rain or drizzle comes in contact with cold ground at around 0◦C, the water drops
freeze to form an ice coating called glaze or freezing rain.

Sleet It is frozen rain drops of transparent grains which form when rain falls through air at
subfreezing temperature. In Britain, Sleet denotes precipitation of snow and rain simultaneously.

Hail It is a showery precipitation in the form of irregular pellets or lumps of ice of size more
than 8mm.It occur in violent thunderstorms in which vertical currents are very strong.

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Types of precipitation
1) Frontal precipitation
A front is the interface between two distinct air masses. Under certain favourable condition when
a warm air mass and cold air mass meet, the warmer air mass is lifted over the colder one with
the formation of a front. The ascending warmer air cools adiabatically with the consequent
formation of clouds and precipitation.

2) Convective precipitation
In this type of precipitation a packet of air which is warmer
er than the surrounding air due to
localized heating rises because of its lesser density. Air from cooler surroundings flows to take
up its place thus setting up a convective cell. The warm air continues to rise, undergoes cooling
and results in precipitation. Depending upon the moisture, thermal and other conditions light
showers to thunderstorms can be expected in convective precipitation. Usually the areal extent of
such rain is small, being limited to a diameter of about 10 km.

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3) Orographic precipitation
The moist air masses may get lifted-up
lifted up to higher altitudes due to the presence of mountain
barriers and consequently undergo cooling, condensation and precipitation. Such a preci
precipitation
is known as Orographic precipitation. Thus in mountain ranges, the windward slopes have heavy
precipitation and leeward slopes light rainfall.

4) Cyclonic precipitation

A cyclone is a large low pressure region with circular wind motion. The pressure gradient exists
towards the centre of the cyclone. Two types of cyclones are recognized: tropical cyclones and
extratropical cyclones.
The source areas of the tropical cyclones
cyclones are the tropical seas of the low latitudes. The formation
of the tropical cyclones is believed to be due to the proper combination of (i) intense sunshine,
(ii) high humidity and temperature, (iii) low surface friction, (iv) deflection force of the earth’s
rotation
otation and (v) favourable lapse rate. The tropical cyclones are relatively smaller in area with
their diameter ranging from 80 km to 500 km. They develop into violent storms. The centre of

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the cyclonic storm

orm is called the eye of the storm. The energy of the tropical cyclones is rapidly
dissipated when they move over the large land areas because of the increased friction and lack of
the moisture supply and therefore they die soon. The tropical cyclones are called hurricanes in
North America and typhoons in Japan and simple cyclones in India.
Extratropical cyclones are mainly frontal in nature. They tend to develop wherever air masses of
general
contrasting properties converge. As seen from the description of the ge neral circulation, such
areas exist in the middle latitudes (polar from) where the polar and tropical air masses meet. The
principal energy source of these extratropical cyclones lies in the temperature and density
contrasts between the two conflicting air masses. The pressure gradients developed from these
contrasts are generally sufficient to produce wind speeds of 30 to 80 km/h and as the two masses
become mixed, the storm dies. The extratropical cyclones are less violent than the tropical
cyclones.
Lecture 7
Measurement of Precipitation
Precipitation is expressed in terms of the depth to which rainfall water would stand on an area if
all the rain were collected on it. Thus 1 cm of rainfall over a catchment area of 1 km2 represents
a volume of water equal to 104 m3. The precipitation is collected and measured in a raingauge.
The raingauge is also variously known as hyetometer, ombrometer or pluviometer.
There are certain difficulties which come in the way of accurate measurement of precipitation.
They are: (i) any object used as a precipitation gauge must be above the ground surface in order
to avoid the splash from the surroundings, flooding and accidents. As such it cannot measure the
amount of precipitation exactly at the level of ground surface since the object extending above
the surface of earth can create eddy currents which may affect the amount of catch. (ii) The
measurement is never subjected to check by repetition and seldom by duplication. (iii) In urban
localities it is difficult
ifficult to locate the sites to be free from the effects of wind and at the same time
clear of obstructions. (iv) The sample is unbelievable small.

Non-Recording Raingauges - Symon’s Raingauge

Standard non-recording
recording raingauge prescribed by the IMD is the Symon’s gauge. The gauge
consists of a funnel with a sharp edged rim of 127 mm diameter, a cylindrical body, a receiver
with a narrow neck and handle and a splayed base which is fixed in the ground. The receiver

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should have a narrow neck and should be sufficiently protected from radiation to minimize the
loss of water from the receiver by evaporation. To prevent rain from splashing in and out, the
vertical wall of the sharp edged rim is made deep enough and the slope of funnel steep enough.
The rain falling into the funnel is collected in the receiver kept inside the body and is measured
by means of a special measure glass which is graduated in mm.

Symon’s Raingauge
The receiver has a capacity of 175 mm of rain. In regions of heavy rainfall, raingauges with
receivers of 375 mm or 1000 mm capacity may be used. The measure glass has a capacity of 25
mm and can be read to nearest 0.1 mm. the gauge is fixed on a masonry or concrete foundation
of size 60 cm x 60 cm x 60 cm which is sunk into the ground. Into this foundation the base of the
gauge is cemented as shown in figure so that the rim of the gauge is exactly 30 cm above the
ground level. The top of the gauge
ge should be perfectly horizontal.
At the routine time of observation the funnel is removed, the receiver is taken out and the rain
water collected in the receiver is carefully poured into the measure glass and read without any
parallax error. When the rainfall
nfall exceeds 25 mm the measure glass will be used as many times as
required. The measured rainfall in the 24 hours ending with 8.30 A.M. is recorded as the rainfall
of the day on which 8.30 A.M. observation is taken.
In regions of heavy rainfall, if it is suspected that the receiver may not hold the entire rainfall of
the day the measurements must be done more frequently with the last measurement being taken
at 8.30 A.M. the sum of all the readings taken in the last 24 hours is recorded as the rainfall of
the day.

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Recording Type Raingauge

recording raingauge give the amount of rainfall only. They cannot provide the information
Non-recording
regarding when exactly the rain commenced, when the rain ended, what is the intensity of
rainfall and how the intensity of rainfall varies within the duration of the storm. In order to
record the beginning and end of the rain and to measure the intensity of rainfall, a continuous
record of rainfall with time is required. For this
t purpose we have to use the recording raingauges.
Recording raingauges usually work by having a clock-driven
clock driven drum carrying a graph on which a
pen records the cumulative depth of rainfall continuously.

Syphon Gauge
It uses the siphon mechanism to empty the rainwater collected in the float chamber. This is
adapted by I.M.D. The details of construction of this type of raingauge are shown in figure.
Rainwater entering the gauge at the top is led into the float chamber through a funnel and filter.
The purposee of the filter is to prevent dust and other particles from entering the float chamber
which may hinder the siphon mechanism.

The float chamber consists of a float with a vertical stem protruding outside, to the top of which
a pen is mounted. This pen rests on a chart secured around a clock driven drum. There is a small
compartment by the side of the float chamber which is connected to the float chamber through a
small opening at the bottom. This is called siphon chamber which houses a small vertical ppipe
with bottom end open and the top end almost touching the top of the chamber. During the storm
the rainwater collected in the float chamber raises the water surface in it and along with the water

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surface the float also rises enabling the pen to make a trace of cumulative depth of rainfall on the
chart.
When the float chamber is completely filled with water, the pen reaches the top of the chart. At
this instant the siphoning occurs automatically through
through the pipe in the siphon chamber, the float
chamber is emptied and the pen is brought to zero on the chart again. As the rain continues the
pen rises again from the zero of the chart. The complete siphoning should be over in less than 15
seconds of time.
e. This gauge cannot record precipitation in the form other than rain unless some
sort of heating device is provided inside the gauge. The float may be damaged if the rainfalls
catch freezes.

place during the storm is shown in the

A chart from a float type raingauge with siphoning taking place
figure. This chart indicated that the gauge has siphoned once at 1:30 h of the next day, for
example 10 + 4 = 14 mm. if the rainfall is of large intensity, the siphoning may occur more than
once during the period of the chart.
hart.

Selection of Raingauge Station

The amount of rainfall collected by a raingauge depends on its exposure conditions and therefore
great care must be exercised in selecting a suitable site for its location. According to Indian
Standards the following precautions
recautions must be strictly observed while selecting a site for a
raingauge.
1. The gauge shall be placed on a level ground, not upon a slope or a terrace and never on a
wall or roof
2. On no account the raingauge shall be placed on a slope such that the ground falls away
steeply in the direction of prevailing wind.

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3. The distance of the raingauge from any object shall not be less than twice the height of
the object above the rim of the gauge.
4. Great care shall be taken at mountain and coast stations so that the gauges are not unduly
exposed to the sweep of the wind. A belt of trees or a wall on the side of the prevailing
wind at a distance exceeding twice its height shall form an efficient shelter.
5. In hills where it is difficult to find a level space, the site for the gauge shall be chosen
where it is best shielded from high winds and where the wind does not cause eddies.
6. A fence is so located that distance of the fence is not less than twice its height.

Lecture 8
Adequacy of Raingauges
The optimal number of stations that should exist to have an assigned percentage of error in the
estimation of mean rainfall is obtained by statistical analysis as

where N= optimal number of stations, ε= allowable degree of error in the estimate of the
mean rainfall and Cv = coefficient of variation of the rainfall values at the existing m stations
rainfall values P1,
(in per cent). If there are m stations in the catchment each recording rai
P2,...,Pi,...Pm in a known time, the coefficient of variation Cv is calculated as:

In calculating N it is usual to take ε = 10%. It is seen that if the value of ε is small, the number of
raingauge stations will be more. According to WMO recommendations, at least 10% of the total
raingauges should be of self-recording
recording type.

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Adjustment of Missing Data by Double Mass Curve Method

If the conditions
ons relevant to the recording of a raingauge station have undergone a significant
change during the period of record, inconsistency would arise in the rainfall data of that station.
This in consistency would be felt from the time the significant change took
took place.
Some of the common causes for inconsistency of record are:
(i) Shifting of a raingauge station to a new location,
(ii) The neighborhood of the station undergoing a marked change,
(iii) Change in the ecosystem due to calamities, such as forest fires, landslides, and
(iv) Occurrence of observational error from a certain date.
The checking for inconsistency of a record is done by the double mass curve technique. This
technique is based on the principle that when each recorded data comes from the same parent
population, they are consistent.

A group of 5 to 10 base stations in the neighborhood of the problem station X is selected. The
data of the annual rainfall of the station X and also the average rainfall of the group of base
stations covering a long period is arranged in the reverse chronological order.
The accumulated precipitation of the station X (∑P
( x) and the accumulated values of the average
∑Pav) are calculated starting from the latest record.
of the group of base stations (∑P
Values of ∑Px are plotted against ∑Pav for various consecutive time periods.
A decided break in the slope of the resulting plot indicates a change in the precipitation regime
of station X.
The precipitation values at station X beyond the period of change of regime is corrected by using
the relation
Pcx = Px /

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Lecture 9
Methods of Computing Average Rainfall
Rain gauge represent only point sample of the areal distribution of storm. But for hydrological
analysis requires rain fall over an area, such as over a catchment. So it is required to convert
point rainfall values at various stations into an average value over a catchment. To convert the
point rainfall values at various stations into an average value over a catchment the following
three methods are in use: (i) Arithmetical mean method, (ii) Thiesson polygon method, and (iii)
Isohyetal method.
(i) Arithmetic Mean Method

When the rainfall measured at various stations in a catchment show little variation, the average
precipitation
ion over the catchment area is taken as the arithmetic mean of the station values. Let P1,
P2, P3, P3,….. Pi ........... Pn are the rain fall values in a given period in N stations within a catchment.
Then mean precipitation,

In practice, this method is used very rarely.

(ii) Thiesson polygon method
This method is first proposed by Thiessen in 1911. In this method representative area for each
rain gauge is considered. The rainfall recorded at each station is given a weightage on the basis
of an area closest to the station.

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These areas are found out using a method consisting of the following three steps:
1. Joining the rain gauge station locations by straight lines to form triangles
2. Perpendicular bisectors for each of the sides of the triangle are drawn. These bisectors
form a polygon around each station. These bounding polygon are called Thiesson polygons
3. Calculate the area enclosed around each rain gauge station bounded by the polygon edges
(and the catchment boundary, wherever appropriate) with a planimeter to find the area of
influence corresponding to the rain gauge.

For the given example, the “weighted”

“weighted” average rainfall over the catchment is determined as,

(iii) Isohyetal Method

This is considered as one of the most accurate method. The method requires the plotting of
isohyets and calculating the areas enclosed either between the isohyets or between an iisohyet
and the catchment boundary. Isohyet is a contour, joining points of equal rainfall in the
basin. In the Isohyetal method, the catchment area is drawn to scale and the raingauge staions
are marked. The procedure is similar to the drawing of elevationn contours based on spot
levels. The area between two adjacent isohyets are then determined with a planimeter.

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Interpolation of Missing Data

The record at many raingauge stations may consist of short breaks due to several reasons such as
the absence of the observer, instrumental failures etc. It is better to estimate these missing
records and fill the gaps rather than to leave them.

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(i) Arithmetic Average Method

If the normal annual precipitations at the adjacent stations are within 10% of the normal rainfall
of the station under consideration, then the missing rainfall data may be es
estimated
timated as a simple
arithmetic average of the rainfalls at the adjacent gauges. If the missing precipitation at station X
is Px, and P1, P2, …….,Pm are the rainfalls at the m surrounding raingauges,

1
Px = (P1+P2+……+Pm)

(ii) Normal Ratio Method

If the normal annual rainfalls at the surrounding gauges differ from the normal annual rainfall of
the station in question by more than 10%, the normal ratio method is preferred. In this method
the rainfall values at the surrounding stations are weighted by
by the ratio of the normal annual
rainfalls.
Let P4 is the precipitation at the missing location, N1, N2, N3 and N4 are the normal annual
precipitation of the four stations and P1, P2 and P3 are the rainfalls recorded at the three stations 1,
2 and 3 respectively.

Lecture 10
Presentation of rainfall data
Hyetograph
A hyetograph is a plot of the intensity of rainfall against the time interval. The hyetograph is
derived from the mass curve and is usually represented as a bar chart.

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The area under a hyetograph represents the total precipitation received in the period.
Mass Curve of Rainfall
It is a plot of accumulated precipitation against time, plotted in chronological order. Records of
float type, weighing bucket type etc raingauges are of this form. It gives information on duration
and magnitude of a storm. Intensity at various time intervals in a storm = slope of the curve. It
can be prepared for non-recording
recording raingauges also if the approximate start and end of a storm are
known. Intensity at various time intervals in a storm = slope of the curve. It can be prepared for
non-recording raingauges also if the approximate start and end of a storm are known.

Moving average method

Moving average is a technique for smoothening out the high frequency fluctuations of a time
series and to enable the trend, if any, to be noticed. The graphical representation of rainfall in any
of the above three methods may not show any trend or cyclic pattern present in the data. The
moving average curve smoothens out the extreme variations and indicate the trend or cyclic
pattern, if any,, more clearly. It is also known as the moving average curve. The procedure to
construct the moving average curve is as follows. The moving average curve is constructed with
a moving period of m year where m is generally taken to be 3 or 5 years. Let x1, x2, xn be

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the sequence of given annual rainfall in the chronological order. Let yi denote the ordinate of the
moving average curve for the ith year. Then for m = 3, yi is computed from
1 + 2 + 3
2 =
3
2+ 3+ 4
3 =
3
:
:
−1 + + +1
=
3
:
:
−2 + −1 +
−1 =
3

The computed value of y correspond in time to the middle value of the x’s being averaged and
therefore it is convenient to use odd values of m. A moving average of m applied to a sequence
of n values yields a sequence of (n-2k)
(n values, where k = (m-2)/2. For any general m, the y terms
can be expressed as
+
1
= ∑ ; = + 1, + 2, … … ( − )
=−

Although it is possible to use moving averages with any m, it is necessary that m be small
compared to n. The moving average technique can be applied to other hydrological parameters as
well such as temperature, wind speed etc. A 3-year
3 year moving average curve superimposed over the
original sequence.
data. However,
Generally no persistent regular cycles can be expected in the annual rainfall data
annual or seasonal cycles may be noticed when the moving average curve is constructed for
monthly rainfall data.
Lecture 11
Numerical problem to draw mass rainfall curve, to estimate rainfall intensity
Lecture 12
Numerical problem to calculate mean
m rainfall, to estimate missing data

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Lecture 13
Module 2 Losses
Evaporation: Introduction, process, factors affecting evaporation, measurement (IS class A
pan). Estimation using empirical formulae (Meyer’s and Rohwer’s equation), reservoir
evaporation and control. Evapo
Evapo-transpiration:
transpiration: Introduction, factors affecting, consumptive
use, AET, PET, measurement, estimation (Blaney criddle method) Infiltration:
ion,
Introduction, factors affecting infiltration capacity, measurement (double ring
infiltrometer), Horton’s equation of infiltration, infiltration indices.

Evaporation
Introduction
Evaporation is the process in which a liquid changes to the gaseous state at the free surface,
below the boiling point through the transfer of heat energy.

Factors Affecting Evaporation

The rate of evaporation is dependent on (i) the vapour pressures at the water surface and air
above, (ii) air and water temperatures, (iii) wind speed, (iv) atmospheric pressure, (v) quality of
water, and (vi) size of the water body.
(i) Vapour Pressure
The rate of evaporation is proportional to the difference between the saturation vapour pressure
at the water temperature, ew and the actual vapour pressure in the air, ea.
Thus EL=C (ew-ea), Daltons law of evaporation.
Where, EL = rate of evaporation (mm/day)
C = a constant
ew & ea are in mm of mercury

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Evaporation continues till ew = ea

If ew > ea , Condensation takes place
(ii) Temperature
The rate of evaporation increases with an increase in the water temperature. Regarding air
temperature high correlation between evaporation rate and air temperature does not exist. Thus
mperature it is possible to have evaporation to different degrees in
for the same mean monthly temperature
a lake in different months.
(iii) Wind
Wind aids in removing the evaporated water vapour from the zone of evaporation and
consequently creates greater scope for evaporation. However, if the wind velocity is large
enough to remove all the evaporated water vapour, any further increase in wind velocity does not
influence the evaporation. Thus the rate of evaporation increases with the wind speed up to a
critical speed beyond which any furth
further increase in the wind speed has no influence on the
evaporation rate. This critical wind-speed
wind value is a function of the size of the water surface. For
large water bodies high-speed
speed turbulent winds are needed to cause maximum rate of evaporation.
(iv) Atmospheric
heric Pressure
Other factors remaining same, a decrease in the barometric pressure, as in high altitudes,
increases evaporation.
(v) Soluble Salts
When a solute is dissolved in water, the vapour pressure of the solution is less than that of pure
water and hencee causes reduction in the rate of evaporation. The percent reduction in evaporation
approximately corresponds to the percentage increase in the specific gravity. Thus, for example,
under identical conditions evaporation from sea water is about 2-3%
2 an that from fresh
less than
water.
(vi) Heat Storage in Water Bodies
Deep water bodies have more heat storage than shallow ones. A deep lake may store radiation
energy received in summer and release it in winter causing less evaporation in summer and more
evaporation in winter compared to a shallow lake exposed to a similar situation. However, the
effect of heat storage is essentially to change the seasonal evaporation rates and the annual
evaporation rate is seldom affected.

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Lecture 14
Measurement
Estimation of evaporation is of utmost importance in many hydrologic problems associated with
planning and operation of reservoirs and irrigation systems. In arid zones, this estimation is
particularly important to conserve the scarce water resources.
The amount of water evaporated from a water surface is estimated by the following methods:
(i) Using Evaporimeter Data,
 Class A Evaporation Pan
 ISI Standard pan (Modified class A Pan)
(ii) Empirical Evaporation Equations,
 Meyer’s Equation
 Rohwer’s equation
(iii) Analytical methods.

Evaporimeter
Evaporimeters are water-containing
containing pan which are exposed to the atmosphere and the loss of
water by evaporation measured in them at regular intervals.
Class A Pan

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It is a standard pan of 1210 mm diameter and 255 mm depth used by the US Weather Bureau and
is known as Class A Land Pan. The depth of water is maintained between 18 cm and 20 cm. the
pan is normally made of unpainted galvanized iron sheet. Monel metal is used where corrosion is
a problem. The pan is placed on a wooden platform of 15 cm height above the ground to allow
free circulation of air below the pan. Evaporation measurements are made by measuring the
depth of water with a hook gauge in a stilling well.
ISI Standard Pan

This pan evaporimeter specified by IS:5973, and known as the modified Class A Pan consists of
a pan of diameter 1220 mm and depth 255 mm. The pan is made of copper sheet 0.9mm thick,
tinned inside and painted white outside. The pan is placed on a square wooden pla
platform of width
1225mm and height 100mm above ground level to allow free air circulation below the pan. A
fixed point gauge indicates the level of water. Water is added to or removed from the pan to
maintain the water level at a fixed mark using a calibratedd cylindrical measure. The top of the
pan is covered with a hexagonal wire net of GI to protect water in the pan from birds. Presence
of the wire mesh makes the temperature of water more uniform during the day and night.
Evaporation from this pan is about 14% lower as compared to that from an unscreened pan.
Pan Coefficient(Cp)
Evaporation pans are not exact models of large reservoirs
Their major drawbacks are the following:

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1. They differ from reservoirs in the heat storage capacity and heat transfer characteristics
from the sides and the bottom. Hence evaporation from a pan depends to some extent on
its size (Evaporation from a pan of about 3m dia is almost the same as that from a large
lake whereas that from a pan of about 1m dia is about 20% in excess of this).
2. The height of the rim in an evaporation pan affects wind action over the water surface in
the pan. Also it casts a shadow of varying size on the water surface.
3. The heat transfer characteristics of
o the pan material is different form that of a reservoir.
Hence evaporation measured from a pan has to be corrected to get the evaporation from a large
lake under identical climatic and exposure conditions.
Lake Evaporation = Pan Coefficient x Pan Evaporation
Values of Pan Co-efficient
S.No. Types of pan Average value Range

1. Class A Land Pan 0.70 0.60 - 0.8

2. ISI Pan (modified Class A) 0.80 1.10
0.65 -1.10
3. Colorado Sunken Pan 0.78 0.75 - 0.86
4. USGS Floating Pan 0.80 O.70 - 0.82

Lecture 15
Empirical Methods
Most of the available empirical equations for estimating lake evaporation are a Dalton type
equation of the general form.

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(i) Meyer’s Formula

(ii) Rohwer’s Formula

Accounts for the effect of pressure in addition to the wind speed effect

Wind Velocity
In the lower part of the atmosphere, up to a height of about 500m above the ground level, wind
velocity follows the one-seventh
seventh power law as,

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Lecture 16
Methods to reduce Evaporation losses
1. Reduction of Surface area
2. Mechanical Covers
3. Chemical Films
(i) Reduction of Surface Area
The volume of water lost by evaporation is directly proportional to the surface area of the water
body. Hence the reduction of surface area wherever feasible reduces evaporation losses.
Measures like having deep reservoirs in place of wider ones and elimination
elimination of shallow areas can
be considered under this category.
(ii) Mechanical Covers
Permanent roofs over the reservoir, temporary roofs and floating roofs such as rafts and light
light-
weight floating particles can be adopted wherever feasible. Obviously these meas
measures are limited
to very small water bodies such as ponds, etc.
(iii) Chemical Films
This method consists of applying a thin chemical film on the water surface to reduce
evaporation. Currently this is the only feasible method available for reduction of evaporation of
reservoirs up to moderate size. Cetyl alcohol (hexadecanol) and Stearyl alcohol (octadecanol)
These forms monomolecular layers on a water surface. These layers act as evaporation inhibitors
by preventing the water molecules to escape past them.

Features of thin films

The film is strong and flexible and does not break easily due to wave action. If punctured due to
the impact of raindrops or by birds, insects, etc., the film closes back soon after. It is pervious to
oxygen and carbon dioxide; the water quality is therefore not affected by its presence. It is
colorless, odourless and nontoxic.

Cetyl alcohol
Most suitable chemical for use as an evaporation inhibitor. It is a white, waxy, crysta
crystalline solid
and is available as lumps, flakes or powder. It can be applied to the water surface in the form of
powder, emulsion or solution in mineral turpentine. Roughly about 3.5 N/hectare/day of cetyl

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alcohol is needed for effective action. The chemical is periodically replenished to make up the
losses due to oxidation, wind sweep of the layer to the shore and its removal by birds and insects.
film pressure of 4 x 10-2 N/m is
Evaporation reduction can be achieved to a maximum if a film
maintained.
Advantages
Controlled experiments with evaporation pans have indicated an evaporation reduction of about
60% through use of cetyl alcohol. Under field conditions, the reported values of evaporation
20 30% can be achieved easily in
reduction range from 20 to 50%. It appears that a reduction of 20-30%
small size lakes (< 1000 hectares) through the use of these monomolecular layers. The adver
adverse
effect of heavy wind appears to be the only major impediment affecting the efficiency of these
chemical films.
Lecture 17
Evapo-transpiration
Transpiration
Transpiration is the evaporation of water from plants.
Environmental factors that affect the rate of transpiration
1. Light
speeds up transpiration by
Plants transpire more rapidly in the light than in the dark. Light also speeds
warming the leaf.
2. Temperature
Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as
the temperature rises. At 30°C, a leaf may transpire three times as fast as it does at 20°C.
3. Humidity
When the
he surrounding air is dry, diffusion of water out of the leaf goes on more rapidly.
4. Wind
When there is no breeze, the air surrounding a leaf becomes increasingly humid thus reducing
the rate of transpiration. When a breeze is present, the humid air is carried away and replaced by
drier air.
5. Soil water
A plant cannot continue to transpire rapidly if its water loss is not made up by replacement from
the soil. The volume of water lost in transpiration can be very high. It has been estimated that

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over the growing season, one acre of corn (maize) plants may transpire 400,000 gallons (1.5
million liters) of water. As liquid water, this would cover the field with a lake 15 inches (38 cm)
deep. An acre of forest probably does even better.
Evapo-transpiration
transpiration (consumptive use)

Lecture 18
Potential Evapotranspiration (PET)
If sufficient moisture is always available to completely meet the needs of vegetation fully
covering the area, resulting Evapotranspiration is called Potential Evapotranspiration.
Actual Evapotranspiration (AET)
The real Evapotranspiration occurring in a specific situation is called Actual Evapotranspiration .
If water supply is adequate, soil moisture = Field capacity, then AET = PET. If water supply is
less than PET, AET/PET < 1

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Factors Affecting Evapotranspiration

Measurement of Evapo-transpiration
transpiration

1. Tank and Lysimeter methods

2. Field experimental plots

3. Soil moisture studies

4. Integration method

5. Inflow and outflow studies for large area

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Tank and Lysimeter methods

Lecture 19 & 20

Estimation

Blaney Criddle Method

Empirical formula based on data from arid western United states. This formula assumes that the
PET is related to hours of sunshine and temperature

The Potential Evapotranspiration in a crop growing season is given by,

ET = 2.54KF & F= Σ PhTf /100

Where, ET = PET in a crop season in cm

K=an empirical coefficient, depends on the type of the crop & stage of growth

F= sum of monthly consumptive use factors for the period

Ph = monthly percent of annual day-time

day time hours, depends on the latitude of the place

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Tf = mean monthly temperature

Lecture 21

Infiltration

Infiltration is the flow of water into the ground through the soil surface. The distribution of soil
moisture within the soil profile during the infiltration
infiltration process is illustrated in the figure. When
water is applied at the surface of a soil, four moisture zones in the soil as indicated in the figure
can be identified.

Zone 1: At the top, a thin layer of saturated zone is created.

Zone 2: Beneath zone

ne 1, there is a transition zone.

Infiltration Capacity ( fp ) :

The maximum rate at which a given soil at a given time can absorb water is defined as the
infiltration capacity ( fp ) .The actual rate of infiltration f can be expressed as :

f = fp when i ≥ fp and f = i when i ≤ fp

Where i= intensity of rain fall

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Infiltration capacity of a soil is high at beginning of a storm & has an exponential decay as the
time elapses.

Factors Affecting Infiltration

1. Characteristics of the soil

2. Condition of the soil surface

3. Vegetative cover

4. Current moisture content

5. Soil temperature

6. Precipitation

7. Slope of the land

8. Fluid Characteristics

Characteristics of Soil

Type of soil, i.e. sand, silt clay, its texture,structure,permeability & under drainage are the
important characteristics under this category. Loose, permeable, sandy soil will have a larger
infiltration capacity than a tight, clayey soil. Soil with good u
under
nder drainage will have higher
infiltration capacity. Transmission capacity determines the overall infiltration rate of the layered
soil. Dry soil absorbs more water.

Surface of Entry

At the soil surface, the impact of rain drops causes the fines in the soil
soil to be displaced and these
turn can clog the pore spaces in the upper layers of the soil which affects the infiltration rate.
Thus surface covered with grass & other vegetation reduces the above said effect & pronounced
influence on the infiltration rate.

Precipitation

The greatest factor controlling infiltration is the amount and characteristics (intensity, duration,
etc.) of precipitation that falls as rain or snow. Precipitation that infiltrates into the ground often
seeps into streambeds over an ext extended
ended period of time, thus a stream will often continue to flow
when it hasn't rained for a long time and where there is no direct runoff from recent precipitation.

Land cover

Some land covers have a great impact on infiltration and rainfall runoff. Veget ation can slow the
Vegetation
movement of runoff, allowing more time for it to seep into the ground. Impervious surfaces,

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such as parking lots, roads, and developments, act as a "fast lane" for rainfall - right into storm
drains that drain directly into streams. Agriculture and the tillage of land also changes the
infiltration patterns of a landscape. Water that, in natural conditions, infiltrated directly into soil
now runs off into streams.

Slope of the land

Water falling on steeply-sloped

sloped land runs off more quickly and infiltrates less than water falling
on flat land.

Fluid characteristics

More impurities present – Turbidity. The turbidity of the water, especially clay and colloid
content block the fine poress and reduce infiltration capacity. Temperature of the water affects the
viscosity by which infiltration rate also affected. Contamination of water by dissolved salts
affects the soil structure & in turn affects the infiltration rate

Lecture 22

Measurement of Infiltration

1. Flooding type infiltrometer.

a. Double ring Infiltrometer
2. Measurement of subsidence of free water in a large basin or pond.
3. Rain fall simulator
4. Hydrograph analysis

Double Ring Infiltrometer

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Lecture 23

Horton’s equation of Infiltration

Horton gives infiltration capacity as a function of time as:

Where:

Fp = infiltration rate into soil, (mm/hr)

Fc = final steady state infiltration capacity occurring at t=tc (mm/hr)

Fo = initial infiltration capacity (mm/hr)

t = time from beginning of storm, sec

k = Horton's decay coefficient which depends upon soil characteristics & vegetation cover.
(1/sec)

Lecture 24

Infiltration Index

In hydrological
logical calculations involving floods it is convenient to use a constant value of
infiltration rate for the duration of the storm. The defined average infiltration rate is called
Infiltration Index.

Two types of indices are in common use

φ- index

W- index

φ- index

This is defined as the rate of infiltration above which the rainfall volume equals runoff volume.

If the rain fall intensity is less than φ,then the infiltration rate is equal to the rain fall intensity. If
the rain fall intensity is larger than φ, the difference b/w the rainfall & infiltration in an interval
of time represents the run off volume.

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W- index

This is the average infiltration rate during the time when the rainfall
rainfall intensity exceeds the
infiltration rate.

Where, R = total storm precipitation (cm)

R= Total storm runoff (cm)

Ia = Initial losses (cm)

te = duration of the rain fall excess, i.e. the total time in which the rain fall intensity is greater
than W (in hours)

W = defined average rate of infiltration (cm)

Lecture 25
Module 3
Runoff: Definition, Concept of Catchment, factors affecting runoff, rainfall – runoff
relationship using regression analysis.
Hydrograph:Definition, components of hydrograph, base flow separation, unit
hydrograph, assumption, application and limitations, derivation from simple storm
hydrographs, S curve and its computations, conversion of UH of different durations.

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Runoff
Definition
Runoff means the draining or flowing off of precipitation from a catchment area through a
surface channel. It thus represents the output from the catchment in a given unit of time.
Concept of catchment
Consider a catchment area receiving precipitation. For a given precipitation, the
evapotranspiration, initial loss, infiltration and detention storage requirements will have to be
first satisfied before the commencement of runoff. When these are satisfied, the excess
precipitation moves over the land surfaces to reach smaller channels. This portion of the runoff is
called overland flow and involves building up of storage over the surface and draining off of the
same.
Flows from several small channels join b igger channels and flows from these in turn combine to
bigger
form a larger stream, and so on, till the flow reaches the catchment outlet. The flow in this mode,
where it travels all the time over the surface as overland flow and through the channels as open
open-
channel
nnel flow and reaches the catchment outlet is called surface runoff.
A part of the precipitation that infilters moves laterally through upper crusts of the soil and
returns to the surface at some location away from the point of entry into the soil. This co
component
of runoff is known variously as interflow, through flow, storm seepage, subsurface storm flow or
quick return flow. The amount of interflow depends on the geological conditions of the
catchment. A fairly pervious soil overlaying a hard impermeable surface is conductive to large
interflows. Depending upon the time delay between the infiltration and the outflow, the interflow
is sometimes classified into prompt interflow.
Another route for the infiltered water is to undergo deep percolation and reach the groundwater
storage in the soil. The groundwater follows a complicated and long path of travel and ultimately
reaches the surface. The time lag, i.e. the difference in time between the entry into the soil and
outflows from it is very large, being of th
thee order of months and years. This part of runoff is
called groundwater runoff or groundwater flow. Ground water flow provides the dry
dry-weather
flow in perennial streams.

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Based on the time delay between the precipitation and the runoff, the runoff is classified into two
categories as
1. Direct runoff, and 2. Base flow
Direct Runoff
It is that part of the runoff which enters the stream immediately after the rainfall. It incl
includes
surface runoff, prompt interflow and rainfall on the surface of the stream. In the case of snow-
melt, the resulting flow entering the stream is also a direct runoff. Sometimes terms such as
direct storm runoff and storm runoff are used to designate direct runoff.
Base flow
The delayed flow that reaches a stream essentially as groundwater flow is called base flow.
Many times delayed interflow is also included under this category. In the annual hydrograph of a
perennial stream the base flow is easily recognized as the slowly decreasing flow of the stream in
rainless periods.
Natural flow
Runoff representing the response of the catchment to precipitation reflects the integrated effects
of a wide range of catchment, climate and rainfall characteristics. True runoff is therefore stream
flow in its natural condition, i.e. without human intervention. Such a stream flow unaffected by
works of man, such as reservoirs and diversion structures on a stream, is called natural flow or
virgin flow.
When there exists storage or diversion works on a stream, the flow on the downstream channel is
affected by the operational and hydraulic characterises of these structures and hence does not
represent the true runoff, unless corrected for the diversion of flow and return fflow.
low.

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Hydrograph
A plot of discharge in a stream plotted against time chronologically is called a hydrograph.
Depending upon the unit of time involved,
 Annual hydrograph showing the variation of daily or weekly or 10 daily mean flows over
a year
 Monthly hydrographs showing the variation of daily mean flows over a month.
 Seasonal hydrographs depicting the variation of the discharge in particular season such as
the monsoon season or dry season.
 Flood hydrograph or hydrographs due to a storm representing stream flow due to a storm
over a catchment.
Water year
In annual runoff studies it is advantageous to consider a water year beginning from the time
evapotranspiration losses. In India, June 1st is the
when the precipitation exceeds the average evapotranspiration
beginning of a water year which ends on May 31st of the following calendar year. In a water year
a complete cycle of climatic changes is expected and hence the water budget will have the least
amount of carryover.
Runoff characteristics of streams
A study of the annual hydrographs of streams enables one to classify streams into three classes as
(i) perennial, (ii) intermittent, and (iii) ephemeral.
A perennial stream is one which always carries some flow. There is considerable amount of
groundwater flow throughout the year. Even during the dry seasons the water table will be above
the bed of the stream.

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An intermittent stream has limited contribution from the groundwater. During the wet season the
water table is above the stream bed and there is a contribution of the base flow to the stream
flow. However, during dry seasons the water table drops to a level lower than that of the stream
bed and the stream dries up. Excepting for an occasional storm which can produce a short-
duration flow, the stream remains dry for the most part of the dry months.
An ephemeral stream is one which does not have any base-flow
base flow contribution. The annual
hydrograph of such a river shows series of short
short-duration
duration spikes marking flash flows in response
to storms. The stream becomes dry soon after the end of the storm flow. Typically en ephemera
ephemeral
stream does not have any well defined, channel. Most of the rivers in arid zones are of the
ephemeral kind.
Runoff volume
Yield
The total quantity of surface water that can be expected in a given period from a stream at the
outlet of its catchment is known
wn as yield of the catchment in that period.
Lecture 26
Factors affecting runoff
The runoff from a drainage basin is influenced by various factors which may be put under two
groups, namely the climatic factors and the physiographic factors.

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The climatic factors include

(i) Type of precipitation
(ii) Intensity of rainfall
(iii) Duration of rainfall
(iv) Areal distribution of rainfall
(v) Direction of storm movement
(vi) Antecedent precipitation
(vii) Other climatic factors that effect evaporation and transpiration.
The physiographic factors are
(i) Land use
(ii) Type of soil
(iii) Area of the basin
(iv) Shape of the basin
(v) Elevation
(vi) Slope
(vii) Orientation
(viii) Type of drainage network
(ix) Indirect drainage
(x) Artificial drainage
Rainfall-Runoff
Runoff relationship using regression analysis
Rainfall-Runoff Correlation
The relationship between rainfall in a period and the corresponding runoff is quite complex and
is influenced by a host of factors relating to the catchment and climate. Further, there is the
problem of paucity of data which forces one to adopt simple correlations for adequate estimation
of runoff. One of the most common methods is to correlate seasonal or annual measured runoff
values (R) with corresponding rainfall (P) values. A commonly adopted method is to fit a linear
regression line between R and P and to accept the result if the correlation is nearer unity. The
equation of the straight-line
line regres
regression between runoff R and rainfall P is
R = aP + b
And the values of the coefficient a and b are given by

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In which N = number of observation sets R and P. The coefficient of correlation r can be

calculated as

The value of r lies between 0 and 1 as R can have only positive correlation with P. The value of
0.6<r<1.0 indicates good correlation.
Lecture 27
Hydrograph
Definition
A hydrograph is a plot of the variation of discharge with respect
respect to time (it can also be the
variation of stage or other water property with respect to time). Discharge is the volume of water
flowing past a location per unit time

Components of Hydrograph Hydrograph Components

D Rain
Rainfall
MA = base flow recession

AB = rising limb
P
BC = crest segment
B C CD = falling limb

DN = base flow recession

Discharge in m3/s
Points B and C = inflection points

Peak Flood Direct Runoff

M A D
Base Flow N

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Consider a concentrated storm producing a fairly uniform rainfall of duration, D over a

catchment. After the initial losses and infiltration losses are met, the rainfall excess reaches the
stream through overland and channel flows. In the process of translation a certain amount of
storage is built up in the overland and channel-flow
channel flow phases. This storage gradually depletes after
the cessation of the rainfall. Thus there is a time lag between the occurrence of rainfall in the
basin and the time when that water passes the gauging station at the basin outlet. The runoff
measured at the stream-gauging
gauging station will give a typical hydrograph as shown in figure.

The duration of the rainfall is also marked in this figure to indicate the time lag in the rainfall and
runoff. The hydrograph of this kind which results due to an isolated storm is typically single
peaked skew distribution of discharge and is known variously as storm hydrograph, flood
hydrograph
raph or simply hydrograph.

It has three characteristic regions: (i) the rising limb AB, joining point A, the starting point of the
rising curve and point B, the point of inflection, (ii) the crest segment BC between the two points
of inflection with a peak P in between, (iii) the falling limb or depletion curve CD starting from
the second point of inflection C.

Rising Limb
The rising limb of a hydrograph, also known as concentration curve represents the increase in
discharge due to the gradual building up of storage in channels and over the catchment surface.
The initial losses and high infiltration losses during the early period of a storm cause the
discharge to rise rather slowly in the initial periods. As the storm continues, more and more flow
from distant parts reach the basin outlet. Simultaneously the infiltration losses also decrease with
time. Thus under a uniform storm over the catchment, the runoff increases rapidly with time.

Crest Segment
The crest segment is one of the most important parts of a hydrograph as it contains the peak
flow. The peak flow occurs when the runoff from various parts of the catchment simultaneously
contribute amounts to achieve the maximum amount of flow at the basin outlet. Estimation of the

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peak flow and its occurrence, being important in flood-flow

flood flow studies are dealt with in detain
elsewhere in this book.
Recession Limb
The recession limb, which extends from the point of inflection at the end of the crest segment
(point C) to the commencement of the natural groundwater flow (point D), represents the
withdrawal of water from the storage built up in the basin during the earlier phases of the
hydrograph. The starting point of the recession limb, i.e. the point of inflection represents the
condition of maximum storage.
Lecture 28
Base flow separation
Method of base flow separation

Method 1 – straight line method

In this method the separation of the base flow is achieved by joining with a straight line the
beginning of the surface runoff to a point on the recession limb representing the end of the direct
runoff. In figure point A represents the beginning of the direct runoff and it is usually easy to
identify in view of the sharp change in the runoff rate at that point. Point B, marking the end of
the direct runoff is rather difficult to locate exactly.
An empirical equation for the time interval N (days) from the peak to the point B is
N = 0.83 A 0.2
Where A = drainage area in km2 and N is in days.
Points A and B are joined by a straight line ttoo demarcate to the base flow and surface runoff.

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A
C
B

Method 2
In this method the base flow curve existing prior to the commencement of the surface runoff is
extended till it intersects the ordinate drawn at the peak (point C). This point is joined to point B
by a straight line. Segment AC and CB demarcate the base flow and surface runoff. This is
base
probably the most widely used base-flow separation procedure.

Method 3
In this method the base flow recession curve after the depletion of the flood water is extended
backwards till it intersects the ordinate at the point of inflection (line EF). Point A and F are
joined by an arbitrary smooth curve. This method of base-flow
b flow separation is realistic in situations
where the groundwater contributions are significant and reach the stream quickly.
The surface runoff hydrograph obtained after the base-flow
base flow separation is also known as direct
runoff hydrograph (DRH).

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Effective Rainfall (ER)

Effective rainfall (also known as Excess rainfall) (ER) is that part of the rainfall that becomes
direct runoff at the outlet of the watershed. It is thus the total rainfall in a given
given duration from
which abstractions such as infiltration and initial losses are subtracted.
Lecture 29
Unit hydrograph
Definition
The unit hydrograph of a drainage basin is defined as a hydrograph of direct runoff resulting
from 1 cm of effective rainfall applied uniformly over the basin area at a uniform rate during a
specified period of time.

Assumptions
The unit hydrograph is originally named the unit graph.
1. The effective rainfall is uniformly distributed within its duration
2. The effective rainfall is uniformly distributed throughout the whole area of its basin
3. The base periods of the direct runoff hydrographs produced by effective rainfall of same
duration are also same.
4. The ordinates of the direct runoff hydrographs of a common base period (or the direct
runoff hydrographs produced by effective rainfalls of same duration) are directly
proportional to the total volume of direct runoff represented by the respective
hydrographs
5. For a given drainage basin the hydrograph of runoff due to a given peri
period of rainfall
reflects the unchanging characteristics of the basin
Assumptions 1 and 2 are incorporated in the definition of unit hydrograph itself. Assumption (d)
is known as the principle of linearity or principle of superposition. Assumption (c) and (e
(e) imply
that the shape of the runoff hydrograph remains same irrespective of at whatever time it may
occur as long as the duration of rainfall producing it is same. This is known as the principle of
time invariance.
Two basic assumption constitute the foundations for the unit hydrograph theory
They are (i) the time invariance and (ii) the linear response

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Time Invariance
This first basic assumption is that the direct runoff response to a given effective rainfall in a
invariant. This implies that the DRH for a given ER in a catchment is always
catchment is time-invariant.
the same irrespective of when it occurs.
Linear Response
The direct runoff response to the rainfall excess is assumed to be linear. This is the most
important assumption of the unit hydrograph theory. Linear response means that if an input x1 (t)
causes an output y1 (t) and an input x2 (t) causes y2 (t), then an input x1 (t) + x2 (t) gives and
output y1 (t) + y2 (t).
ear response in a unit hydrograph enables the method of superposition to
The assumption of linear
be used to derive DRHs. Accordingly, if two rainfall excess on D-h duration each occur
consecutively, their combined effect is obtained by superposing the respective DRHs with due
care being taken to account for the proper sequence of events.

Limitations
1. Since uniform intensity over long durations is less likely the storms selected for unit
hydrograph analysis should be of short duration.
2. Since uniform aerial distribution of rainfall over large areas is likely, the unit
hydrographs can be applied only to drainage basins with small areas. The unit
hydrographs are best suited to areas not more than 5000 km2 although they have been
applied to fairly large areas with varying degree of success. The unit hydrograph is also
not suitable for areas less than 200 hectares. Basins with odd shapes, particularly those
which are long and narrow, will commonly have very uneven rainfall distribution and
hence unit hydrographs are not well adapted to such basins.
3. The base period of the direct runoff hydrograph should depend on the intensity of rainfall
also, since channel storage is more in intense storms elongating the base period.
4. The principle of linearity is not valid strictly.
5. According to principle of time invariance, the direct runoff hydrograph from a drainage
basin due to a given pattern of effective rainfall, at whatever time it may occur, is
invariable. It is known that basin
bas characteristics change with seasons, man
man-made

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adjustments etc. these changes may not produce significant effect on the direct runoff in
many cases.
6. The unit hydrograph method cannot be applied when a major portion of storm
precipitation is in the form of snow.
Lecture 30 & 31
Problem
1. Given below are the ordinates of a 6-h
6 h hydrograph for a catchment. Calculate the
ordinates of the DRH due to a rainfall excess of 3.5 cm occurring in 6 h.
Time (h) 0 3 6 9 12 15 18 24 30 36 42 48 54 60 69
UH ordinate 0 25 50 85 125 160 185 160 110 60 36 25 16 8 0
(m3/s)

Time (h) Ordinate of 6 h UH Ordinate of 3.5 cm DRH (m3/s)

(m3/s) (6h UH * 3.5)
0 0 0
3 25 87.5
6 50 175.0
9 85 297.5
12 125 437.5
15 160 560.0
18 185 647.5
24 160 560.0
30 110 380.0
36 60 210.0
42 36 126.0
48 25 87.5
54 16 56.0
60 8 28.0
69 0 0

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2. Following are the ordinates of a storm hydrograph of a river draining a catchment

area of 423 km2 due to a 6 h isolated storm. Derive the ordinates of a 6 h
hydrograph for the catchment.
Time from start -6 0 6 12 18 24 30 36 42 48
of storm (h)
Discharge (m3/s) 10 10 30 87.5 115.5 102.5 85.0 71.0 59.0 47.5

Time from start 54 60 66 72 78 84 90 96 102

of storm (h)
Discharge (m3/s) 39.0 31.5 26.0 21.5 17.5 15.0 12.5 12.0 12.0

A straight line joining the beginning and end of direct runoff is taken as the divide line for base
base-
flow separation. The ordinates of DRH are obtained by subtracting the base flow from the
ordinates of the storm hydrograph.
Volume of DRH = 60 * 60 * 6 * (sum of DRH ordinates)
= 60 * 60 * 6 * 587 = 12.68 Mm3
Drainage area = 423 Mm3 = 423 Mm3
Runoff depth = ER depth = 12.68/423 = 0.03 m = 3 cm.
6 h unit hydrograph.
The ordinates of DRH are divided by 3 to obtain the ordinates of 6-h
Time Ordinate of flood Base Ordinate of Ordinate of 6-h
3
(h) hydrograph flow DRH (m /s) unit hydrograph
3 3
(m /s) (m /s)
-6 10.0 10.0 0 0
0 10.0 10.0 0 0
6 30.0 10.0 20.0 6.7
12 87.5 10.5 77.0 25.7
18 111.5 10.5 101.0 33.7
24 102.5 10.5 101.0 33.7
30 85.0 11.0 74.0 24.7

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36 71.0 11.0 60.0 20.0

42 59.0 11.0 48.0 16.0
48 47.5 11.5 36.0 12.0
54 39.0 11.5 27.5 9.2
60 31.5 11.5 20.0 6.7
66 26.0 12.0 14.0 4.7
72 21.5 12.0 9.5 3.2
78 17.5 12.0 5.5 1.8
84 15.0 12.5 2.5 0
90 12.5 12.5 0
96 12.0 12.0 0
102 12.0 12.0 0

Lecture 32 to 34
3. From the given 4 h unit hydrograph derive the 8 h and 12 h unit hydrograph.
Time (h) 0 2 4 6 8 10 12 14 16 18
Ordinate of 4h 0 12.52 21.32 23.54 17.84 14.79 12.18 10.04 8.26 6.51
UH (m3/s)

Time (h) 20 22 24 26 28 30 32 34
Ordinate of 4h 4.98 3.95 3.05 2.26 1.60 1.07 0.53 0
UH (m3/s)

Computation of 8 h UH from 4 h UH

Time Ordinates of 4h Lagged Combined Ordinates of 8 h

(h) UH (m3/s) by 4 h hydrograph UH (m3/s)
(m3/s) (m3/s)
(1) (2) (3) (4) = (2)+(3) (5) = (4)/2
0 0 - 0 0
2 12.52 - 12.52 6.26

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4 21.32 0 21.32 10.66

6 23.54 12.52 36.06 18.03
8 17.84 21.32 39.16 19.58
10 14.79 23.54 38.33 19.17
12 12.18 17.84 30.02 15.01
14 10.04 14.79 24.83 12.42
16 8.26 12.18 20.44 10.22
18 6.51 10.04 16.59 8.30
20 4.98 8.26 13.24 6.62
22 3.95 6.51 10.46 5.23
24 3.05 4.98 8.03 4.02
26 2.26 6.95 6.21 3.11
28 1.60 6.05 4.65 2.33
30 1.07 2.26 3.33 1.67
32 0.53 1.60 2.13 1.07
34 0 1.07 1.07 0.54
36 0.53 0.53 0.27
38 0 0 0

Computation of 12 h UH from 4 h UH

Time Ordinates of 4h Lagged Lagged by Combined Ordinates of 8 h

3 3
(h) UH (m /s) by 4 h 8 h (m /s) hydrograph UH (m3/s)
(m3/s) (m3/s)
(1) (2) (3) (4) (5) = (2)+(3)+(4) (6) = (5)/3
0 0 - - 0 0
2 12.52 - - 12.52 4.17
4 21.32 0 - 21.32 7.11
6 23.54 12.52 - 36.06 12.02
8 17.84 21.32 0 39.16 13.05
10 14.79 23.54 12.52 50.85 16.95

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12 12.18 17.84 21.32 51.34 17.11

14 10.04 14.79 23.54 48.37 16.12
16 8.26 12.18 17.84 38.28 12.76
18 6.51 10.04 14.79 31.34 10.45
20 4.98 8.26 12.18 25.42 8.47
22 3.95 6.51 10.04 20.50 6.83
24 3.05 4.98 8.26 16.29 5.43
26 2.26 6.95 6.51 12.72 4.24
28 1.60 6.05 4.98 9.63 3.21
30 1.07 2.26 3.95 7.28 2.43
32 0.53 1.60 3.05 5.18 1.73
34 0 1.07 2.26 3.33 1.11
36 0.53 1.60 2.13 0.71
38 0 1.07 1.07 0.36
40 0.53 0.53 0.18
42 0 0 0

4. A storm produced rainfall intensities of 0.75, 2.25 and 1.25 cm/h on the drainage
basin in successive time periods of 4 h each. Assuming a base flow of 10 m3/s and a
index of 2.5 mm/h compute total runoff hydrograph resulting from this storm.
φ-index

φ-index
index of 2.5 mm/h = 0.25 cm/h

Depth of rainfall in the first 4 h period = (0.75 – 0.25) * 4 = 2 cm

Depth of rainfall in the first 4 h period = (2.25 – 0.25) * 4 = 8 cm

Depth of rainfall in the first 4 h period = (1.25 – 0.25) * 4 = 4 cm

Time Ordinates of Direct runoff in (m3/s) from effective rainfalls Base Total runoff
(h) 4h UH during 3 successive 4 h periods flow (m3/s)
(m3/s) 2 cm 8 cm 4 cm Combined (m3/s)
hydrograph (m3/s)
(1) (2) (3) (4) (5) (6) = (3) + (4) +(5) (7) (8) = (6) +(7)

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0 0 0 - 0 10 10
2 12.52 25.04 - - 25.04 10 35.04
4 21.32 42.64 0 - 42.64 10 52.64
6 23.54 47.08 100.16 - 147.24 10 157.24
8 17.84 35.68 170.56 0 206.24 10 216.24
10 14.79 29.58 188.30 50.08 267.98 10 277.98
12 12.18 24.36 142.72 85.28 252.36 10 262.36
14 10.04 20.08 118.32 94.16 232.56 10 242.56
16 8.26 16.52 97.44 71.36 185.32 10 195.32
18 6.51 13.02 80.32 59.16 152.50 10 162.50
20 4.98 9.96 66.08 48.72 124.76 10 134.76
22 3.95 7.90 52.08 40.16 100.14 10 110.14
24 3.05 6.10 39.84 33.04 78.98 10 88.98
26 2.26 4.52 31.60 26.04 62.16 10 72.16
28 1.60 3.20 24.40 19.92 47.52 10 57.52
30 1.07 2.14 18.08 15.80 36.02 10 46.02
32 0.53 1.06 12.80 12.20 26.06 10 36.06
34 0 0 8.56 9.04 17.60 10 27.60
36 4.24 6.40 10.64 10 20.64
38 0 4.28 4.28 10 14.28
40 2.12 2.12 10 12.12
42 0 0 0 10 10

Exercise problems

1. Given below are the ordinates of observed runoff produced by a storm whose
duration is believed to be 3 hours on a drainage basin of area 665.3 km2. Assuming a
constant base flow of 60 m3/s derive the ordinates of 3 hour UH.

Time Flow m3/s 3hUH

(Answer)
I day 3 am 60 0
6 am 60 0
9 am 600 54

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12 noon 950 89
3 pm 800 74
6 pm 700 64
9 pm 610 55
12 midnight 530 47
II day 3 am 460 40
6 am 400 34
9 am 350 29
12 noon 310 25
3 pm 270 21
6 pm 240 18
9 pm 210 15
12 midnight 190 13
III day 3 am 170 11
6 am 150 9
9 am 130 7
12 noon 110 5
3 pm 90 3
6 pm 80 2
9 pm 70 1
12 midnight 60 0
2. Derive 9 h UH for the drainage basin given in the previous p roblem, using t he 3 h
UH derived the re.

Time (h) 0 3 6 9 12 15 18 21 24 27
Ordinate of 3h UH (m3/s) 0 18 47.7 72.3 75.7 64.3 55.3 47.3 40.0 34.3
Time (h) 30 33 36 39 42 45 48 51 54 57
3
Ordinate of 3h UH (m /s) 29.3 25 21.3 18 15.3 13 11 9 7 5
Time (h) 60 63 66 69 72 Answer
Ordinate of 3h UH (m3/s) 3.3 2 1 0.3 0

3. The direct runof f hydr ograph resulting from a 5. 0 c m of eff ective r ainfall of 6 h
duration is given below. Determine th e ordinates of the 6 h unit hydrograph.

Time (h) 0 6 12 18 24 30 36 42 48 54
Direct runoff (m3/s) 0 25 175 320 360 310 230 165 105 60
6h UH (m3/s) (answer) 0 5 35 64 72 62 46 66 21 12
Time (h) 60 66 72
Ordinate of 3h UH (m3/s) 30 10 0

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6h UH (m3/s) (answer) 6 2 0

Lecture 35
Module 4

Irrigation: Definition, need for irrigation, benefits and ill effects of irrigation, systems of
irrigation: Surface and ground water, flow irrigation, Lift irrigation, Bandhara irrigation.

Water Requirement of Crops: Duty, delta and base period, relationship

relationship between them,
factors affecting duty of water crops and crop seasons of India, Irrigation efficiencies.
Frequency of irrigation.

Introduction

Irrigation may be defined as the process of supplying water to land by artificial means for the
purpose of cultivation.
tivation. Ordinarily water is supplied to land by nature through rain but generally it
is not enough for the proper growth of the plants. The basic objective of irrigation is to
supplement the natural supply of water to land so as to obtain an optimum yiel yield from the crop
grown on the land.

Need for Irrigation

If the normal rainfall at any place is adequate to meet the total water requirements of the crops
grown and the time interval of the rainfall is such that water is available whenever the plants
need it, then irrigation is not required. Such ideal conditions exist only for some small regions of
the world where the normal rainfall is sufficient to fulfill the water requirements of the crops
grown. For most of the other regions of the world crop production
productio is not possible without the
help of irrigation.

The necessity for irrigation is summarized below:

(i) Deficient Rainfall: When rainfall is less than 100 cm, irrigation water is essentially
required. Rainfall deficiency vis-à-vis
vis irrigation requirement by crops can be briefed
as follows:

Rainfall (cm) Irrigation Requirement

100 rainfall needs to be supplemented by irrigation

100 – 50 Rainfall is insufficient. Irrigation is essential

50 – 25 Irrigation is essentially required

Less than 25 No crop can be grown without irrigation

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(ii) Non-Uniformity of rainfall: Where rainfall is sufficient but is not uniform,

concentrated as it usually is in monsoon months, there is acute requirement of
irrigation in other periods.
(iii) Augmentation of crop yields: New high yielding varieties of crops have higher water
requirement for giving higher yields. Sugarcane and rice have higher requirement of
water.
(iv) Exacting water requirement: The high yielding varieties of crops have more exacting
requirement of water.
(v) Cash crops cultivation: Cash crops require higher and assured supply of water with
frequent watering for maturity.

Benefits of Irrigation

1. Increase in food production

2. Protection from famine.
3. Cultivation of cash crops
4. Elimination of mixed cropping
5. Addition to the wealth of the country
6. Increase in prosperity of people
7. Generation of hydro-Electric
Electric power
8. Domestic and Industrial water supply
9. Inland navigation
10. Improvements of communication
11. Canal plantations
12. Improvement in the ground water storage
13. Aid in Civilisation
14. General development of the country

Ill-effects of Irrigation

1. Breeding places for mosquitoes

2. Water-logging
Lecture 36

System of Irrigation

Systems of irrigation are broadly classified as follows:

(i) Flow irrigation or gravity irrigation

(ii) Lift irrigation or pumped irrigation

Flow irrigation. Is that of irrigation in which the supply of irrigation water available is at such a
level that it is conveyed on to the land by the gravity flow.

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Flow irrigation may further be divided into two classes: (i) Perennial irrigation system (ii)
Inundation or flood irrigation system.

Perennial irrigation: In perennial irrigation system, the water required for irrigation is supplied in
accordance with the crop requirements throughout the crop period. Therefore, some storage head
works, such as dams and storage weirs or barrages are required to store the excess water during
floods and release it to the crops as and when it is required.

Inundation irrigation: Inundation irrigation is carried out out by deep flooding and thorough
saturation of the land to be cultivated which is then drained off prior to the planting of the crop.

Depending upon the source from which the water is drawn, flow irrigation can be further
subdivided into three types:

(i) Direct
ct irrigation ( River canal irrigation) : Diversion scheme
(ii) Storage irrigation (Reservoir or tank irrigation): Storage scheme
(iii) Combined storage and diversion scheme.

1. Direct Irrigation or River Canal Irrigation

In this direct irrigation system, water is directly diverted to the canal without attempting to store
the water. For such a system, a low diversion weir or diversion barrage is constructed across the
river. This raises the water level in the river and thus diverts the water to the canal taking off
upstream as shown in figure.
Generally, a direct irrigation scheme is of a smaller magnitude, since there are no rigid controls
over the supplies. One or two main canals may take off directly from the river. Cross drainage
works are constructed wherever natural drains or distributary streams cross the canals. In a
bigger scheme, there may be branch canal taking off from the main canal.

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2. Storage Irrigation or Tank irrigation

In storage irrigation system, a solid barrier, such a barrier, such as a dam or a storage weir is
constructed across the river and water is stored in the reservoir or
or lake so formed.
Depending upon the water
requirements of crops, or the
hydroelectric power generation
and upon the flow of water in the
river, the volume of storage
required is decided. From the
contour plan of the basin at the
site of construction, th
the elevation
storage curve for the reservoir is
known. The height of the dam is
then decided from this curve,
corresponding to the storage
storage-
volume required. In India, most of
the irrigation schemes fall under
this category.
Storage irrigation scheme is
comparatively
aratively of a bigger
magnitude, and involves much
more expenditure than a direct
irrigation scheme. One or more main canals take off from the reservoir. Due to the formation of
reservoir, some land property may be submerged to the upstream of the dam. A network of canal
system conveys water to the agricultural fields, through various regulatory works. Cross Cross-
drainage works such as aqueducts, siphon aqueducts, super passages and canal siphons are
constructed wherever natural drains cross the canals.

3. Combined system (storage cum diversion scheme)

We have seen that in the storage irrigation system, water is stored in the reservoir since the river
is not perennial, while in the direct irrigation system, the river is perennial and hence the water is
diverted from the river to the canal. Sometimes a combined scheme is adopted in which the
water is first stored in the reservoir formed at the upstream side of the dam, and this water is used
for water power generation. The discharge from the power house is fed back into the river, to the
downstream side of the dam. Thus, sufficient quantity of flow is again available in the river. At
suitable location in the downstream, a pickpick-up weir is constructed. This weir diverts the water
from the river to the canal.

Choice between the systems

Direct irrigation scheme is adopted in the circumstances where the river is perennial and has a
normal flow throughout the irrigation season, never less at any time than the requirements of the
fields. On the contrary, storage irirrigation system is adopted when the river flow is either not
perennial, or where flow is insufficient during certain parts of the crop season for irrigation

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requirements. In a multistage river valley development a combined storage-cum--diversion

scheme is more useful.

Lecture 37
Lift Irrigation

Lift irrigation is practised when the water –supply

supply is at too low a level to run by gravitation on to
the land. In such a circumstances water is lifted up by mechanical means. Irrigation from wells is
an example of lift irrigation, in which sub-soil
sub soil water is lifted up to the surface and is then
conveyed to the agricultural fields.

In lift irrigation water is lifted from a river or a canal to the bank to irrigate the lands which are
not commanded by gravity flow. Lift irrigation is being increasingly practiced in India. Lift
irrigation also includes tube well irrigation but the latter is not feasible in areas where scarcity of
water exists, climate is dry and groundwater is low, i.e. groundwater is in insufficient quantity
and of unsuitable quality. Lift canal then constitutes the only means of extension of irrigation to
such perched lands. A lift canal can cater for much larger areas than a tube well and is suitable
where supplies either from a river or a canal are available for lifting to higher elevation.

It is also known as pumped irrigation and is further classified as (a) lift irrigation from surface
source and (b) lift irrigation from ground source.

In case of lift irrigation from surface source, the water is lifted with the help of pumps and
discharged into lift canal. The source
source of water in this case may be river or parent gravity flow
canal. The water in the lift canal flows under gravity to distributaries and minors, from where it
flows into fields through water courses. In case after some travel the canal again comes in deep
cutting, the lifting of water may be further required. In this type of irrigation, the water may be
lifted at one point or at different stages, depending upon the topography of the area. This point
should be noted that after lift, the water flows under gravity
gra in lift canals.

In the lift irrigation from ground sources, the water is lifted by means of deep tubetube-wells, which
are installed in the fields itself. The tube-well
tube well water is used in irrigating the fields around it. The
water flows from tube-wellwell into masonry channels or other water courses. This method is very
common and economical for irrigation purpose. Farmers may have their own tube tube-wells or take
water from govt. owned tube-wells.
wells.

Lecture 38

Bandhara Irrigation

Bandhara Irrigation: a special irrigation

irrigation scheme. Adopted across small perennial rivers. This
system lies somewhere between inundation type and perennial type of irrigation. A Bandhara is
a low masonry weir (obstruction) of height 1.2m to 4.5m constructed across the stream to divert

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water into a small canal. The length of the main canal is usually restricted to about 8km.
Economical and can irrigate a small area up to 400 ha.  Trapezoidal section – u/s is vertical d/s
slope is 1:2 to 1:5 Crest width = (H)1/2, minimum of 1.2m Discharge Q = 1.7 L (h)3/2

Bhandara Irrigation

This method of irrigation is followed in Central Maharashtra and is commonly known there as
the `Phad‘

1. Bandhara is a special type of irrigation scheme, between inundation type and permanent
type irrigation.
2. It is essentially a minor irrigation scheme wherein small streams which otherwise allow
their flows to be wasted are dammed at the places by bandhara and canals are taken off
from them for irrigation of small areas.
3. A series of such dams or bandharas are constructed
constru and water available in the monsoon is
thus made use.

Advantages of Bandhara irrigation

1. The system of irrigation has low initial cost.

2. Small quantities of water which would have otherwise gone waste is utilised to a
maximum in this system.
3. Length of canal and distribution system being small, seepage and evaporation losses are
very less.
4. The area to be irrigated being close to the source, it yields a high duty and intensive
irrigation.

Disadvantages of Bandhara irrigation

1. The irrigation area for one Bandhara is more or less fixed and, hence even if greater
quantity of water is available for irrigation it goes waste.
2. If the river is of non-perennial
perennial type, the supply of water becomes seasonal and unreliable
in summer.

FUNCTIONS OF IRRIGATION SOILS

The functions of irrigation soils for crop growth are:

(i) Adequate moisture holding capacity to meet water requirements of crops,

(ii) Provide proper circulation of air to a suitable depth for the development of root
system of plants,
(iii) Free from harmful concentration of soluble salts and parasites,
(iv) Promote growth of bacteria which make the soil richer in organic matter for crop
growth,
(v) Supply nutrients for crop yield,
(vi) Resistant to soil erosion or soil depletion under the cropping system,

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(vii) Suitable for agricultural implements, and

(viii) Suitable for providing anchor for plant roots.

MAINTAINING SOIL FERTILITY

Soil is a store of nutrients for crop growth. It supplies 13 out of 16 elements needed continuously
in balanced proportions for the nutrition of higher plants, in addition to mechanical support,
sunshine, adequate oxygen around plant roots, optimum amount of soil water and non non-existence
of toxic substances to plants such as soluble aluminum, excess of soluble salts etc. the five
factors of soil formation i.e. parent material, climate, topography, bio
bio-sphere
sphere and time determine
time and place. All plants depend on food produced by them
the kind of soil existing at any one time
through photosynthesis, as such balanced nutrition through fertilizer addition to plants is
essential. The nutrients in the soil get consumed if the same type of crop is grown repeatedly.
Soil fertility
ity can be maintained by the methods as follow:

1. Rest to the land. Sufficient rest to the soil is necessary for recuperation after harvesting
the crop and before the next crop is grown.
2. Additions of manures. Manures and fertilizers supply deficient plant nu nutrient in the soil.
3. Crop rotation. Crop pattern or crop rotation is so selected that optimum yields are
obtained per unit of water applied. Crop rotation implies changing the crops to be grown
in the land every year for the three reasons,
(a) Since different crops
rops require different nutrient and in different proportions, by
changing the crop every year the same type of nutrient is not used up from the soil
every year.
(b) Crop rotation keeps the soil free from certain soil diseases. If the same crop is grown
every year, the crop may impart some disease associated with it to the soil. Thus crop
rotation controls insects pests, weed growth and plant diseases, and
(c) Different crops have different depths of root zones. By growing different crops, some
of which may have shashallow
llow roots while the other deep roots, optimum utilization of
the available moisture of the soil is made. Proper crop rotation helps maintain soil
structure, improve tilth, increase moisture holding capacity of the soil, maintain
adequate supply of organic matter and nitrogen, and improve quality of crop products.

SOIL WATER

Soil water is defined as suspended water in the uppermost belt of soil of zone of aeration lying
near enough to surface to be discharged into atmosphere by transpiration of plants or bby
evaporation from the soil. It includes gravitational water, capillary water and hygroscopic water.

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1. Gravitational water. It is the water in the unsaturated zone in excess of hygroscopic and
capillary water which moves out of the soil under favourable drainage conditions.
2. Capillary water. It is the water held by surface tension in the capillary spaces and as a
continuous film around the particles, free to move under thehe influence of capillary forces
and available to plants.
3. Hygroscopic water. It is the water held in static state with the atmospheric water vapour.
Hygroscopic coefficient is the percentage of moisture the dry soil will take up from the
saturated atmosphere in order to be in equilibrium with atmospheric saturated water
vapour at that particular temperature. Hygroscopic water is not capable of any movement
by gravity or capillary forces.
water on the basis of
Water is also classified as unavailable, available and superfluous water
availability of soil water to crops.

4. Unavailable water. It refers to hygroscopic water which is not available to crops due to
its inability to move by gravity or capillary forces. It is the soil moisture held so firmly by
molecular forces that it cannot ordinarily be absorbed by plant roots with sufficient
rapidly to produce growth.
5. Available water. It refers to the capillary water which readily contributes to the plant
roots. Plants wilt if the capillary water is used up. The available water in the capillary

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water zone is limited up to a permanent wilting point and accordingly it forms the line of
demarcation between the capillary water and hygroscopic water.
6. Superfluous water. It refers to the gravitational water which drains down so deeper that
plant roots cannot draw it.
Lecture 39
Water requirement of crop

Water is essentially a basic input influencing assured crop production. Water dissolves mineral
nutrients which move in the plant along with its stem. At the end of the life cycle of a plant,
water is also a constituent of the economic product, which may be seed, stem, leaf, flower or
fruit.

The functions of irrigation water in crop production are: (i) Triggers activity in a seed, setting a
chain of biochemical reactions, (ii) dissolves mineral nutrients for their rise from the soil to the
plant, (iii) promotes chemical action within the plant for its growth, (iv) promotes and supports
life of bacteria beneficial to plant growth, (v) helps temperature control of the soil as also
minimizes the effect of frost, and (vi) at the end of life cycle of the plant, water is still a
constituent of the product.

Water requirement of a crop is defined as the quantity of water required by a crop in a given
period of time for normal growth under field conditions. It includes evaporation and other
economically unavoidable waste. Usually water requirement of a crop is expressed in water
depth per unit area.

The total water requirement may be defined as the quantity of water needed for potential
production per unit of land for sustained production and is sum of three entities, viz.
consumptive use (A), application and conveyance losses (B), and other special needs (C).

Water requirement, WR = A + B + C.

It is also equal to water requirement – effective rainfall + contribution

ion of groundwater.

Unit water requirement is defined as weight of water actually used by plants in producing unit
weight of dry matter.

Principal crops and Crops Seasons

Crops can be classified in the following ways:

1. Agricultural classification
2. Classification
cation based on crop seasons
3. Classification based on irrigation requirement

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Agricultural Classification

This consists of the following types of crops.

(i) Field crops: Such as wheat, rice, maize, barley, oats, great millet, spiked millet,
gram, pulses etc.
(ii) Commercial crops: Such as sugar cane, cotton, tobacco, hemp, sugar beat etc.
(iii) Oil seed crops: Such as mustard, ground nut, sesame, linseed, caster etc.
(iv) Horticulture crops: Consisting of various fruit crops, various vegetable crops and
flower crops.
(v) Plantation crops: Such as tea, coffee, cocao, coconut, rubber etc.
(vi) Forage crops: Such as fodder, grass etc.
(vii) Miscellaneous crops: Such as medicinal crops, aromatic crops, sericulture crops,
condiments and spices.

Classification based on crop seasons

Based on crop season, crops are classified as follows.

(i) Rabi crops or winter crops: These crops are sown in autumn (or October) and are
harvested in spring (or March). Various crops that fall under this category are: gram,
wheat, barely, peas, mustard, tobacco, linseed, potato etc.
(ii) Kharif crops or monsoon crops: These crops are sown by the beginning of the
southwest monsoon and are harvested in autumn. These consist of rice, maize, spiked
millet, great millet, pulses, ground net etc.
(iii) Perennial crops: These are the crops that require water for irrigation throughout the
year. Examples of perennial crops are: sugar cane, fruits, vegetables etc.
(iv) Eight months crops: These crops such as cotton require quire irrigation water for 8
months.

Classification based on irrigation requirements

Based on irrigation requirements crops can be classified as (i) dry crops, (ii) wet crops and (iii)
garden crops.

Dry crops are the one which do not require water for irrigation;
irrigation; only rain water is sufficient for
their growth. Wet crops are those which cannot grow without irrigation. Garden crops require
irrigation throughout the year.

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Lecture 40 & 44, 45

Definitions

Gross commanded area (GCA) It is defined as the gross irrigable area which can be
commanded by irrigation. The boundary of GCA is usually defined by the drainage on either side
across which irrigation cannot be extended economically.

Gross irrigation area (GIA) It is defined as the gross commanded area less such areas within
irrigation limits as may be excluded, for any reason, for irrigation by the canal, project or
scheme.

Culturable command area it is that portion of the culturable irrigable area which is commanded
by irrigation, i.e. gross commanded area less the area of unculturable land included in the gross
area such as areas under small drainage, ponds, abadies, reserve forest, alkali soils, roads,
railway lines and watercourses,, etc. In the formulation of projects and schemes, CCA is roughly
taken as 90 to 80 per cent of GCA depending on the configuration of land.

Intensity of irrigation It is defined as the percentage of CCA proposed to be irrigated annually

ensive irrigation system the intensity may be 100 percent or even more
or seasonly. In intensive
depending on multiple cropping once or twice under different crops in the same area. In
protective irrigation scheme, the intensity may be low to extend the benefits to wider area.
fall and its distribution has a marked relation to intensity of irrigation. In high rainfall areas
Rainfall
intensity is high because water needs are less and in semi-arid arid areas intensity is low due to
frequent irrigation needs. With the adoption of water saving methods
thods such as sprinkler irrigation
and drip irrigation, more area can be brought under irrigation with the available water and hence
higher intensity of irrigation. High intensity has to be accompanied by efficient drainage
ater logging.
otherwise fraught with risk of water

Area to be irrigated It is the area to which irrigation is be done, i.e. the product of CCA and
intensity of irrigation. It is normally worked out separately for each crop season.

Crop ratio crop ratio or Kharif-Rabi

Rabi ratio is defined as the ratio between the areas anticipated to
be irrigated in these two crops. Crop ratio adopted in Punjab and Haryana is 1:1.5, Uttar Pradesh
1:1.2, Maharastra, Madhya Pradesh, Orissa, West Bengal, Andhra Pradesh 1:1.

Overlap allowance crop of one season may extend into the other season. In such a period of
overlapping, both the crops require irrigation simultaneously. Thus there is extra demand during
this period. To cater for this, usually 5 percent of canal discharge is provided as overlap
allowance which implies that the canal discharge is increased by 5 percent for this period of
overlap.

Time factor it is expressed as the number of days the canal has actually run to the number of
days the canal was designed to run for a particular
particular period of watering. Alternatively, it is defined

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as the number of days the canal is in flow to the base days. For example, time factor of a canal
run for 12 days, out of 20 days it was supposed to run is 20/20.

Capacity factor it is defined as the rate of mean supply to authorized full supply or capacity of
the canal at a certain point. Thus if it were possible to run a canal system at full supply discharge
or closed, then the capacity factor and the time factor would be the same. Owing to rotational
working, branches and distributaries do not run for the same number of days as either the main
canal or each other. The volume of discharge of a canal is given in cumec days by the sum of
daily discharges for the period in question. It is equal to (i) average discharge multiplied by the
number of days the canal is in flow, and (ii) mean discharge multiplied by the number of days in
the crop.

Crop period it is the time that a crop takes from the instant of its sowing to that of its harvesting.

Base period it is the time in days between first watering of a crop at the time of sowing to its last
watering before harvesting, i.e. the number of the days in a crop or more precisely the number of
days over which duty
uty is measured.

Base period of a crop is thus slightly less than its crop period, but for practical purposes it is
generally taken equal to the crop period. It number 183 days for Kharif and 182 days for Rabi.

Delta It is an expression used in irrigation practice to mean the depth of water that would result
over a given area from a given discharge for a certain length of time. Alternatively, it may be
defined as the total volume of water delivered divided by the area over which it has been spread.

Delta is the total depth of water in cm required by a crop to come to maturity. It depends on the
amount of each watering and the interval between successive watering during the base period.
Delta is stated with reference to the place at which it is measured, that is, delta at farm, delta at
outlet, delta at distributary head, and delta at the head of main canal.

Duty duty or duty of water is the relation between the area irrigated, or to be irrigated, and the
quantity of waterr used, or required to irrigate it for the purpose of maturing its crop. When
applied to a channel it is the area irrigated during a base period divided by the mean supply
utilized in cumecs. It may be defined as the area irrigated by unit discharge which means the
number of hectares under a particular crop brought to maturity by a constant supply of 1 cubic
metre of water per second flowing continuously for the base period.

Duty of the channel is usually calculated on the head discharge. Duty based on disc
discharge passed
through the outlet and thus excluding all losses in the canal system is called the outlet discharge
factor.

Duty is expressed in (i) water depth units, (ii) depth-area units per unit area, (iii) area per unit
rate of flow or per unit volume of water, and (iv) volume of water or rate of flow per unit area.

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In practice duty is expressed in hectares per cubic metre per second and is represented by D.

If 1 cubic metre/sec water flowing continuously

continuously for a base period B, matures 175 hectares, then
the duty of water for that crop is defined as 175 hectares per cumec to the base of B days. It is a
measure of degree of efficient utilization of irrigation water from a canal.

Cumec day A unit of volume used in irrigation practice and means the volume of water resulting
from a discharge of 1 cumec for one day. It amounts to 8.64 hectare metres.

Relation between duty and delta

Let D = duty of water (hectares/cumec), B = base period (days),

(days and ∆ = delta of water (m)

Volume of one cubic metre flowing for one day = 1 x 24 x 60 x 60 = 86, 400 m3 = 8.64 ha m.

Volume of one cubic metre flowing for B days = 8.64 B (ha m)

By the definition of duty (D), one cubic metre = 104 D m2 of area supplied for B days matures D
hectares of land.

Total depth of water applied on the land = volume / area = 86400 B/104 D = 8.64 B/D m

Now, delta is total depth of water, ∆ = 8.64 B/D m

Or D = 8.64 B/∆ m

∆ = 864 B/D cm

Where, ∆ = delta (cm), B = base period (days), and D = duty (hectares/cumec)

Gross duty it is the duty of water measured at the source of diversion or irrigation supplies.

Nominal duty it is the duty sanctioned as per schedule of an irrigation department

Economic water duty it is the duty of water which results in the maximum yield. (i) per unit
area when land is the limiting factor, and (ii) per unit of irrigation water when water is the
limiting factor.

Designed duty it is the duty of water assumed in an irrigation project for d

designing
esigning capacities of
the channels.

Lecture 41
FACTORS AFFECTING DUTY

1. Methods and system of irrigation ;

2. Mode of applying water to the crops ;
3. Methods of cultivation ;

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4. Time and frequency of tilling ;

5. Types of the crop ;
6. Base period of the crop ;
7. Climatic conditions of the area;
8. Quality of water ;
9. Method of assessment ;
10. Canal conditions;
11. Character of soil and sub-soil
soil of the canal ;
12. Character of soil and sub-soil
soil of the irrigation fields.

Crop Seasons of India

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METHODS OF IMPROVING DUTY

1. Suitable method of applying water to the crops should be used.

2.The land should be properly ploughed and levelled before sowing the crop. It should be given
good tilth.
3.The land should be cultivated frequently, since frequent cultivation reduces loss of moisture
specially when the ground water is within capillary reach of ground surface.
4.The canals should be lined. This reduces seepage and percolation losses. Also, water can be
conveyed quickly, thus reducing evaporation losses.
5.Parallel canals should be constructed. If there are two canals running side by side, the F.S.L.
will be lowered and the losses will thus be reduced.
6. The idle length of the canal should be reduced.
7.The alignment of the canal either in sandy soil or in fissured rock should be avoided.
8.The canal should be so aligned that the areas to be cultivated are concentrated along it.
9.The source of supply
ly should be such that it gives good quality of water.
10. The rotation of crops must be practiced.
11. Volumetric method of assessment should be used.

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12. The farmers must be trained in the proper use of water, so that they apply correct quantity of
water at correct timing
13. The land should be redistributed to the farmers so that they get only as much land as they are
capable of managing it.
14.Research stations should be established in various localities to study the soil, the seed and
conservation of moisture. The problems concerning the economical use of water should be
studied at research stations.
15. The canal administrative staff should be efficient, responsible and honest. The operation of
canal system should be such
uch that the farmers both at the head of the canal as well as at the tail
end get water as and when they need it.
Paleo it is the first watering before the crop is sown in order to add sufficient water to
unsaturated zone of the soil for initial growth of the crop.

Kor watering it is the first watering after the crop has grown to a few centimeters or the second
watering from the beginning, the first ( before crop is grown) being paleo. The depth of water
applied is the maximum and is designated as kor depth.. All subsequent watering are of lesser
depth, the least being at the time when the crop is mature. The portion of the base period in
which kor watering is needed is termed as kor period. While designing capacity of an irrigation
ken into account in view of maximum water requirement during this
canal, kor watering is taken
period.

Demand it is defined as the amount of water needed for irrigation based on elements of time and
quantity. It is related to a particular point along the irrigation system such as demand at
distributary head, demand at head of canal.

Consumptive use of water

Evapo-transpiration
transpiration or consumptive use of water by a crop is the depth of water consumed by
evaporation and transpiration during crop growth, including water consumed by accompanying
weed growth. Water deposited by dew or rainfall, and subsequently evaporating without entering
the plant system is part of consumptive use. When the consumptive use of the crop is known, the
water use of large units can be calculated.

Evaporation

Evaporation is the transfer of water from the liquid to the vapour state. The rate of evaporation
from water surface is proportional to the difference between the vapour pressure at the surface
and the vapor pressure in the overlaying air (Dalton’s law). When irrigation water is applied by
flooding methods, large amounts of water are lost by
by direct evaporation from soil surface without
having passed through the roots stems and leaves of the plants.

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Transpiration and Transpiration Ratio

Transpiration is the process by which plants dissipate

dissipate water from the surface of their leaves,
stalks and trunks in the process of growth. As much as 99% of total water received by a plant
through its roots is lost to the atmosphere by this process. The transpiration ratio is the ratio of
the weight of water transpired by the plant during its growth to the weight of dry matter
produced by the plant exclusive of roots.

Factors affecting consumptive use of water

1. Evaporation, which depends upon humidity

2. Mean monthly temperature
3. Growing season of crop and cropping pattern
4. Monthly precipitation in the are
5. Irrigation depth or the depth of water applied for irrigation
6. Wind velocity in the locality
7. Soil and topography
8. Irrigation practices and methods of irrigation

Potential Evapo-transpiration
transpiration (PET) and Actual Evapo-transpiration
Evapo transpiration (AET)

Evapo-transpiration is the total loss of water from farm land as evaporation and from plants
grown on it as transpiration. If sufficient moisture is always available to completely meet the
needs of the plants, the resulting evapo-transpiration
evapo is called potential evapo
evapo-transpiration
(PET).

The real evapotranspiration occurring in a specific situation is called actual evapo

evapo-transpiration

Lecture 42
Irrigation Efficiencies

Efficient use of irrigation water is an obligation of each user as well as the planners. Even under
the best method of irrigation, not all the water applied during irrigation is stored in the root zone.
In general, efficiency is the ratio of water output to the water input and is expressed as
percentage. The objective of efficiency concepts is to show when improvements can be made
which will result in more efficient irrigation. The following are the various types of irrigation
efficiencies:

(i) Water conveyance efficiency (ηc)

This takes into account the conveyance or transit losses and is determined from the following
expression:

ηc = ∗ 100

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where ηc = water conveyance efficiency

= water delivery to the farm or irrigation plot

= water supplied or diverted from the river or reservoir

(ii) Water application efficiency (ηa)

Water application efficiency is the ratio of the quantity of water stored into the root zone of the
crops to the quantity of water delivered to the field. This focuses the attention of the suitability of
the method of application of water to the crops. It is determined from the following expression:

ηa = ∗ 100

where ηa = water application efficiency

= water stored in the root zone during the irrigation

= water delivered to the farm.

The common sources of loss of irrigation water during water application are (i) surface run off Rf
from the farm and (ii) deep percolation Df below the farm root-zone soil. Hence

Wf = Ws + Rf + Df
−(Rf + Df)
ηa = ∗ 100

and in a well designed surface irrigation system, the water application efficiency should be
atleast 60%; in sprinkler irrigation system this efficiency is about 75%.

The common factors which are responsible for low water application efficiency are: (i) irregular
land surfaces, (ii) shallow soils underlain by gravels of light permeability, (iii) either very small
or excessively large irrigation streams, (iv) non-attendance
nce of water during irrigation, (v) long
irrigation runs, (vi) wrong irrigation methods, (vii) improper preparation of land, (viii) compact
impervious soil, (ix) steep slopes of land surfaces and (x) excessive single application.

(iii) Water use efficiency (ηu)

It is the ratio of water beneficially used, including leaching water, to the quantity of water
delivered, and is determined from the following expression:
ηu = ∗ 100

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where ηu = water use efficiency

= water used beneficially or consumptively

= water delivered

(iv) Water stored efficiency (ηs )

The concept of water stored efficiency gives an insight to how completely the required water has
been stored in the root zone during irrigation. It is determined from the following expression:

ηs = ∗ 100

where ηs = water storage efficiency

= water stored in the root zone during irrigation

= water needed in the root zone prior to irrigation = (Field capacity – Available moisture)

(v) Water distribution efficiency (ηd)

Water distribution efficiency evaluates the degree to which water is uniformly distributed
throughout the root zone. Uneven distribution has many undesirable results. The more uniformly
the water is distributed;
stributed; the better will be the crop response. It is determined from the following
expression.

ηd = [1 − ] ∗ 100

where ηd = water distribution efficiency

y = average numerical deviation in depth of water stored from average depth stored during
irrigation

d = average depth of water stored during irrigation

the efficiency provides a measure for comparing

comparing various systems or methods of water
application, i.e. sprinkler compared to surface, one sprinkler system compared to the other
system or one surface method compared to other surface method.

(vi) Consumption use efficiency (ηcu)

It is given by

ηcu = ∗ 100

where or Cu = normal consumption use of water

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= net amount of water depleted from root zone of soil.

The efficiency, therefore, evaluates the loss of water by deep percolation and by excessive
surface evaporation following irrigation.

Determination of irrigation requirements of crops

In order to determine the irrigation requirements of certain crop, during its base period, the
following terms are required:

(i) Effective rainfall (Re)

Effective rainfall is that part of the precipitation falling during the growing period of a crop that
is available to meet the evapo-transpiration
transpiration needs of the crop.

(ii) Consumptive irrigation requirement (CIR)

Consumptive irrigation requirement is defined as the amount of irrigation water that is required
to meet the evapo-transpiration
transpiration needs of the crop during its full growth. Therefore,

CIR = Cu – Re

Where Cu is the consumptive use of water

(iii) Net irrigation requirement (NIR)

Net irrigation requirement is defined as the amount of irrigation water required at the plot to
transpiration needs of water as well as other needs such as leaching etc. thus
meet the evapo-transpiration

NIR = Cu – Re + water lost in deep percolation for the purpose of leaching etc.

(iv) Field irrigation requirement (FIR)

Field irrigation requirement is the amount of water required to meet ‘net irrigation requirements’
plus the water lost in percolation in the field water courses, field channels and in field
applications of water. If ηa is the water application efficiency, we have

FIR =
ηa

(v) Gross irrigation requirement (GIR)

Gross irrigation requirement is the sum of water required to satisfy the field irrigation
requirement and the water lost as conveyance losses in distributaries upto the field. If ηc is the
water conveyance efficiency, we have

GIR =
ηc

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IRRIGATION RELATIONSHIP

Saturation capacity. The soil whose all soil pores are filled with water and there is no air left in
it is termed as saturated soil. Most crops except rice cannot withstand saturated soil condition for
more than 2 – 5 days. Saturation capacity, termed as saturated point or maximum moisture
holding capacity, is the amount of water required to fill all the pore spaces between the soil
particles by replacing the entire air held in pore spaces. It is the upper limit of possible moisture
content. It is expressed as equivalent cm of water per metre of soil depth.

Field capacity.. Is defined as the amount of water held in the soil after excess gravitational water
has been drained, i.e. the moisture percentage of the soil expressed on dry weight basis in the
field 2 or 3 days after the soil profile is thoroughly wetted by rain or irrigation water, provided
there is no watertable within capillary reach of the root zone:
ℎ
Field capacity = * 100
ℎ

Soil moisture content. The amount of water present in a soil is termed as soil moisture content,
expressed as mm of water depth present in 1 m depth of soil. It is not constant but varies with
time.

Permanent wilting point. When the water is rendered insufficient to meet the requirement of
the plant, its green leaves turn yellow and the plant wilts and finally dies out. The soil water
ent wilting point. In any given soil, all
content at this stage, when the plant dries is called permanent
forms of vegetation wilt when the moisture content is reduced to a certain percentage known as
wilting coefficient percentage or wilting coefficient. It is defined as the moisture content of soil
expressed as a percentage
centage of the dry weight, at the time when the leaves of a plant growing in the
soil first undergo a permanent reduction in their moisture content as a result of the deficiency I
the soil moisture supply. Alternatively it is defined as the percentage moisture in a soil at which
the plants wilt and fail to recover the turgidity when placed in an atmosphere saturated with
vapour.

The permanent wilting percentage of soils varies greatly for different soil. It may vary from 3 to
23 percent for a coarse sandy soil
oil to fine clay respectively, corresponding to field capacity range
of 2 to 40 percent.

Available moisture. It the amount of water in the soil at any time in excess of the wilting
coefficient, expressed as percentage by weight of dry soil or as equivalent of water per unit depth
of soil. Alternatively it is the difference in soil moisture form field capacity to wilting point.

Available moisture usually considered is the amount retained between field capacity or the
moisture content 2 or 3 days after a soil has been wetted to a depth of 1 m, and the permanent
wilting percentage or that moisture content at which plants cease to grow.

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Readily available moisture. It is the portion of available moisture that is most easily extracted
by plants. It varies from 75 % to 80 % of the available moisture.

Soil moisture deficiency. Also called field moisture deficiency. It is the amount of water that
ring moisture content of a soil to its field capacity.
must be applied to a soil to bring

Moisture equivalent. It is the ratio of water which a soil after saturation will retain against a
centrifugal force of 1000 times the force of gravity to weight of the soil when dry. Moisture
equivalentt is slightly less or roughly equal to field capacity for medium textured soil.

Moisture equivalent field capacity = 1.8 to 2 permanent wilting point.

Depth of water stored in root zone.

zone. The depth of eater stored in root zone of a soil containing
water up to field capacity is estimated as under.

Assume d = depth of root zone (m), F = field capacity (expressed as ratio), γ = density of soil,
and w = unit weight of water.

Considering unit area of 1 m2 of soil area,

ℎ
F=
ℎ

ℎ
=
∗1∗

Therefore, Weight of water retained in unit area = F γ d.

Depth of water stored (which is available for evapo-transpiration)

evapo transpiration) = F γ d / w (m)

Available moisture depth = γ d / w (field capacity – wilting point moisture content)

Optimum moisture content. For satisfactory growth, plants require readily available watewater in
the soil, i.e. water which can be easily extracted by them. Excessive flooding and deficient soil
moisture are not conductive to plant growth. Thus, optimum moisture content in the soil is

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required for optimum plant growth. The optimum moisture percentage is the moisture content in
the soil corresponding to which optimum growth of plants takes place.

Root zone depth. It is the depth below the ground surface in which the crops develop roots
system to derive water for growth. The plants may have (i) tap roots that penetrate deeply into
the soil under favourable conditions, (ii) shallow primary roots, and (iii) lateral roots.

In clay soils, roots cannot penetrate to considerable depth. A high watertable limits the root
growth due to lack of aeration. Roots of perennial field crops and fruit trees extend to a
considerable depth under favourable conditions.

Crops in general develop most of their root system in the upper portion of the root zone. The
depth to which irrigation water percolates limits the depth of root zone. The usual depth of root
zone is 1.25 m. The first quarter of this depth contributes 40 percent of the total water
requirement, the next 30 per cent and the next 30 percent and the further next 20 percent and the
last quarter only 10 percent. The size of the wet bulb formation more or less tallies with this
moisture absorption pattern.

FREQUENCY OF IRRIGATION

Soil moisture in the root zone varies between field capacity (upper limit) and wi wilting point
(lower limit). The soil moisture is not allowed to deplete upto wilting point. For satisfactory
growth the moisture content in the soil should be between field capacity and optimum moisture
content, which is the readily available moisture. Thus irrigation is aimed at supplying the amout
of water such that water content is equal to field capacity.

After irrigation, moisture content starts falling as water is consumed by plants through their roots
and when the soiloil moisture in the soil reaches optimum value, next irrigation is required to raise
moisture content again to field capacity of the soil. The moisture will again start falling
necessitating fresh irrigation application when the soil moisture content reach reaches optimum
moisture content level. Thus water contained in the root and the consumptive use of water by
plants dictates the frequency of irrigation.

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Irrigation may be started when soil moisture is about

about halfway between the field capacity and
wilting point. Adequate amount of water is applied to bring soil moisture to field capacity. The
depth of water to be stored in root zone during each irrigation is given by the relation:

D = γ d / w (F – Mo)

Where, D = depth of water (m), γ = unit weight of soil (g), d = depth of root zone (m), w = unit
weight of water (g), F = field capacity (expressed as ratio), and Mo = optimum moisture content
(expressed as ratio)

Frequency of irrigation, f = D/Cu

Where f = frequency of irrigation, D = depth of water (cm), and Cu = rate of daily consumptive
use (cm)

Lecture 43
Module 5
Canals: Types of canals, Alignment
lignment of canals, definition of gross command area, cultural
command area, intensity of irrigation, time factor, crop factor. Unlined and lined canals.
methods problems
Standard sections. Design of canals by Kenedy’s and Lacey’s methods-problems

Reservoirs: Definition, investigation

nvestigation for reservoir site, storage zones determination of
storage capacity using mass curves, economical height of dam.

CANALS

A canal is an artificial channel, generally trapezoidal in shape constructed on the ground to carry
water to the fields either from the river or from a tank or reservoir.

TYPES OF CANALS

Canal can be classified in the following ways:

(a) Classified based on the nature of source of supply

(1) Permanent canal.

A canal is said to be permanent when it is fed by a permanent source of supply. The canal is a
well made up regular graded channel. It has also permanent masonry works for regulation and
distribution of supplies. A permanent canal is also sometimes known as perennial canal when the
sources from which canal takes is an ice fed Perennial River.

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(2) Inundation canal

Inundation canals usually draw their supplies from rivers whenever there is a high stage in the
river. They are not provided with any headworks for diversion of river water to the canal. They
are, however, provided with a canal head regulator. The head of the canal has to be changed
sometimes to suit the changing pattern of river course.

(b) Classification based on financial output

(1) Productive canal

Productive canals are those which yield net revenue to the nation after full development of
irrigation in the area.

(2) Protective canal

Protective canal is a sort of relief work constructed with the idea of protecting a particular area
from famine.

(c) Classification
cation based on the function of the canal
(1) Irrigation canal
(2) Carrier canal
(3) Feeder canal
(4) Navigation canal
(5) Power canal

An irrigation canal carries water to the agricultural fields. A carrier canal, besides doing
irrigation, carries water for another canal. Upper Chenab canal in West Punjab is the example of
one such canal. A feeder canal is constructed with the idea of feeding two or more canals.
Examples of such canals are: Rajasthan feeder canal and Sirhind feeder.

(d) Classification based on boundary surface of the canal

Based on the type of boundary surface, canals may be of the following types:

(1) Alluvial canals

An alluvial canal is the one which is excavated in alluvial soils, such as silt, and which carry a lot
of silt along with water. The boundary or theth perimeter of such a channel is therefore made of
silt, commonly known as ‘alluvium’. The silt content of water flowing in such a channel may
vary along its length. The silt content may increase if the velocity of flow is such that it scours
the bed and sides
ides of the canal. Similarly the silt content of water may decrease if the velocities
are such that ‘silting’ takes place. The aim of the designer would be to design the channel in such
a way that a non-silting non-scouring
scouring velocity is obtained for the given design discharge.

(2) Non-Alluvial canals

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A non-alluvial
alluvial canal is the one which is excavated in non-alluvial
non alluvial soils, such as loam, clay, hard
soil (murram), rock etc. Such channel generally do not have silting problems as water can flow
with non-silting
silting velocities without fear of scouring of sides and bed on the canal.

(3) Rigid boundary canals

Rigid boundary canals are those which have rigid sides and rigid base, such as lined canals.

(e) Classification
ation based on the discharge and its relative importance in a given network
of canals
(1) Main canal

Main canal generally carries water directly from the river or reservoir. Such a canal carries heavy
supplies and is not used for direct irrigation except in eexceptional
xceptional circumstances. Main canals act
as water carriers to feed supplies to branch canals and major distributaries.

(2) Branch canal

Branch canal are the branches of the main canal in either direction taking off at regular intervals.
In general, branch canals
als also do not carry out any direct irrigation, but at times direct outlets
may be provided. Branch canals are usually feeder channels for major and minor distributaries.
They usually carry a discharge of over 5 cumecs.

(3) Major distributaries

Major distributaries usually called Rajbha, take off from a branch canal. They may also
sometimes take off from the main canal, but their discharge is generally lesser than branch
canals. They are real irrigation channels in the sense that they supply waterr for irrigation to the
field through outlets provided along them. Their discharge varies from ¼ to 5 cumecs.

(4) Minor distributaries or minors

Minor distributaries or minors take off from branch canals or from distributaries. Their discharge
is usually less than ¼ cumecs. They supply water to the water courses through outlets provided
along them.

(5) Water course or field channel

A water course or field channel is a small channel which ultimately feeds the water to irrigation
fields. Depending upon the size and extent of the irrigation scheme, a field channel may take off
from a distributary or minor. Sometimes, it may even take off from the branch canal for the field
situated very near to the branch canal.

(f) Classification based on canal alignment

According to thee alignment, a canal may be classified as

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(1) Ridge canal

A ridge canal or a watershed canal is aligned along a watershed and runs for most of its length on
a watershed. When a channel is on the watershed, it can command areas on both banks and hence
a large area can be brought under cultivation. Also no drainage can intersect a watershed and,
hence the necessity of constructing cross drainage works is obviated.

When the watershed takes a very sharp loop, the

the canal should be aligned straight to save
considerable idle length.

The canal has also to leave watershed to by-pass

by pass towns and villages situated at the watershed.

(2) Contour canal

contour canal. When the

A channel aligned nearly parallel to the contours of the area is called a contour
canal takes off from a river in a hilly area, it is not possible to align the canal on the watershed as
the watershed on the top of the hill may be very high and the areas which need irrigation are
canal is then aligned roughly parallel to the contours of the area.
concentrated in the valley. The canal
The contour chosen for the alignment should be so placed as to include all culturable area of the
valley on the side of the canal.

ground level on the other side is quite

The contour canal can irrigate only on one side. As the ground
high, there is no necessity of a bank on this side. Hence, a contour canal is sometimes
constructed with one bank only, and is known as a single bank canal. However, when both the
banks are provided, it is known as a double
dou bank canal.

(3) Side slope canal

It is a channel aligned roughly at right angles to the contours of the country and is neither on the
watershed nor in the valley. Such a channel would be roughly parallel to the natural drainage of
the country and hence, itt does not intercept any cross drainage. However, it has very steep bed
cross-drainage.
slope, since the direction of the steepest slope of the ground is at right angles to the contours of
the country.

CANAL ALIGNMENT

A canal has to be aligned in such a way that it covers the entire area proposed to be irrigated,
with shortest possible length and at the same time its cost including the cost of cross drainage
works is minimum. A shorter length of canal ensures less loss of head due to friction and smaller
loss of discharge
harge due to seepage and evaporation, so that additional areas can be brought under
cultivation.

General Consideration for Alignment

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1. The alignment of the canal should be such as to ensure (i) the most economical way of
distributing the water to the land, (ii) As high a command as possible, and (iii) minimum
number of cross drainage works.
2. The alignment of a canal on a watershed, being the most economical, is preferred. As a
general rule, the entire wate
watershed lying in a command should be occupied by
distributaries.
3. The length of the main canal from the point where it takes off from a river to a point
where it mounts on a watershed should be minimum.
4. The contour alignment should be changed this way or that way in order to reduce the
number of cross-drainage
drainage works to a minimum.
5. The alignment should avoid villages, roads, cart tracks, cremation places, places of
worship and other valuable properties.
6. The alignment should pass through the balanced depth of cutting. If not, it should involve
minimum depth of cutting or minimum height of filling.
7. The number of kinds and acute should be minimum.
8. Idle length of canal should be minimum and branches etc. should be economically
planned.
9. The alignment should not be made in a rocky, brakish or cracked strata.

Alignment of a Field Channel or Water Course

Though the maintenance of a field channel is the responsibility of the farmers, its alignment
should have the following features:

1. They should be laid along the field boundaries.

2. They should be capable of supplying sufficient water to the tail end
3. Separate field channels should be provided for high and low lands.
4. The field channels should not pass through rocky, brakish or cracked strata.

Lecture 46
DESIGN OF ALLUVIAL CHANNELS

The channel or canal which takes off from a river has to draw a fair share of silt flowing in the
river. This silt is carried either in suspension or along the bed of the canal. The silt load carried
by the channel imposes a difficult problem in a channel design in alluvial soils. The velocity to
be allowed in a channel design should be such that the silt flowing in the channel is not dropped
on the bed. In case of channel silts up, its capacity reduces and so it will irrigate less area. Also
the velocity should not be large enough to erode away the bed and sides of the channel. If the
sides and bed of a channel are eroded away, the cross cross-section
ection increases and besides other
damages because of scour, its full supply depth decreases; it can, therefore, command much less

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area. A velocity which will just keep the silt in suspension, without scouring the channel is
known as non-silting and non-scouring
scouring velocity.

For the design of an irrigation channel, the design discharge Q, and the surface and soil
properties such as the rugosity coefficient N and silt factor f are known. The problem consist
consists in
determination of the four unknowns:

(i) Area for cross-section

section (A),
(ii) Hydraulic mean depth (R),
(iii) Velocity of flow (V),
(iv) Bed slope (S).

To start with, following two equations are available:

Q = A * V …….(continuity equation)

V = f (N,R,S) …….(flow equation)

The flow equation may be Manning’s equation or Kutter’s equation or any other similar flow
equation. However, since there are four unknowns, two more equations must be available for the
complete and unique solution. The additional two equations may be obtained
obtained from the following
criteria:

(i) Providing channel of best discharging section thus getting another equation between
A and R
(ii) Limiting equation of velocity from considerations of scouring and silting
(iii) Governing the slope by the available ground slope
(iv) Fixing
g a suitable B/D ratio on experience

Criterion (i) is not good for alluvial soil where a non-silting

non non-scouring
scouring velocity is a must.
Criteria (ii) and (iii) are utilized by Kennedy’s method of design of channels. Wood’s table gives
a table of suitable B/D D ratio for various discharges. However, Lacey’s theory furnishes four
equations for the complete determination of the four unknowns, without depending upon the
earlier flow equation by Manning’s or Kutter.

Lecture 47
KENNEDY’S THEORY

Kennedy selected a number of sites on Upper Bari Doab Canal System, one of the oldest in
Punjab for carrying out investigations about velocity and depth of the channel. The sites selected
by him did not require any silt clearance for more than thirty years and were thus supposed to be
flowing with non-silting
silting and non-scouring
non scouring velocity. Kennedy’s study revealed the following:

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1. The flowing water has to counteract some amount of friction against the bed of the canal.
This gives rise to vertical eddies rising up gently to the surface. These eddies are
responsible for keeping most of the silt in suspension. Some eddies may start from sides
but these are for most of its part horizontal and so do not have any silt
ilt supporting power.
The silt supporting power is, therefore, proportional to the bed width of the stream and
not to its wetted perimeter.
2. He also defined critical velocity as non-silting
non non-scouring
scouring velocity and gave a relation
between critical velocity to the depth of flowing water. The relation is
V0 = 0.55 D0.64
In general, V0 = CDn
Where V0 = Critical Velocity,
D = Depth of water over bed portion of a channel,
n = any index number.
Since the equation has been derived on the basis of observations on one canal system
only, it is applicable to only those channels which are flowing in sandy silt of the same
quality or grade as that of Upper Bari Doab Canal System.
Kennedy later realized the importance of silt grade on critical velocity and introduced a
factor m known as critical velocity ratio (C.V.R.) in his equation. The equation is then
written as
V0 = 0.55 m D0.64
ℎ
Where m = C.V.R. =
Sand coarser than the standard, m varies from 1.1 to 1.2
Finer than the standard, m varies from 0.9 to 0.8
Generally, in a system of canal, higher C.V.R. is assumed in head reaches and lower
value of C.V.R. is assumed towards
towar its tail end.
Kennedy made use of Kutter’s equation for finding the mean velocity of flow in the
channel:

1 0.00155
23+ + =C√
V= 0.00155
√
1+ (23+ )
√

Design procedure
Case 1: Given Q, N, m and S (from L-section)
L
1. Assume a trail value of D in metres
0.64
2. Calculate the velocity V0 from the equation, V0 = 0.55 m D
3. Get area of section A from the continuity equation: A =
4. Knowing D and A, calculate the bed width B from geometry of canal section. The
side slope of the canal in alluvial soil is assumed to be ½:1 when the canal runs for
some time.
2
A = BD + , from which B can be calculated
2

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5. Calculate the perimeter and the hydraulic mean depth from the following relations:
+ 2
P = B + D√5 and R = = 2
B + D√5
6. Calculate the actual mean velocity of flow (V) from Kutter’s equation. If the value of
velocity (V) is the same as V0 found in step 2, the assumed depth is correct. If not,
repeat the calculations with a changed value of D till the two velocities are the same.

Case 2: Given Q,N, m and B/D ratio from Wood’s table

1. Calculate A in terms of D.
Let = x or B = D x
2 2
A = BD + =x 2+ = 2(x + 0.5)
2 2
2. The value of velocity V0 is known in terms of D by Kennedy’s equation
V0 = 0.55 m D0.64
Substitute the values of V0 and A in the continuity equation and solve for D.
Thus Q = A * V0 = 2(x + 0.5) * 0.55 m D0.64
Q = 0.55 m (x + 0.5) 2.64
1
Hence D = [ /(0.55 ( + 0.5))]2.64
In the above relation Q, m and x are known. Hence D is determined.
3. Knowing D, calculate B and R from the following relations:
2
+
B = xD and R = 2
B + D√
√5
4. Calculate the velocity V0 from Kennedy’s equation
V0 = 0.55 m D0.64
5. Knowing V0 and R, determine the slope S from Kutter’s flow equation. The equation
can be solved by trial and error.

Drawbacks in Kennedy’s theory

1. Kennedy did not notice the importance of B/D ratio

2. He aimed to find out only the average regime conditions for the design of a channel.
3. No account was taken of silt concentration and bed load, the complex silt carrying
phenomenon was incorporated in a single factor m.
4. Silt grade and silt charge were not defined
5. Kennedy did not give any slope equation
6. Kennedy used Kutter’s equation for the determination of the mean velocity and,
therefore, the limitations of Kutter’s equation got incorporated in Kennedy’s theory of
channel design.

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Lecture 48
LACEY’S REGIME THEORY

“ Dimensions, width, depth and slope of a regime channel to carry a given discharge loaded with
a given silt charge are all fixed by nature.” This idea was first put forward by Lacey. Lacey
succeeded in evolving more generally applicable equations based on his own experiments and
the experiments of the past investigators.

Regime channel: Lacey defined regime channel as a stable channel transporting a regime silt
charge. A channel will be in regime if it flows in unlimited incoherent alluvium of the same
character as that transported and the silt grade and silt charge are all constant.

Incoherent alluvium. It is a soil composed of loose granular graded material which can be
scoured with the same ease with which it is deposited.

Regime silt charge. It is the minimum transported load consistent with fully active bed.

Regime silt grade. This indicates the gradation between the small and the big particles. It should
not be taken to mean the average mean diameter of a particle.

Regime conditions. A channel is said to be in regime when the following conditions are
satisfied.

1. The channel is flowing in unlimited incoherent alluvium of the same character as that
transported.
2. Silt grade and silt charge are constant.
3. Discharge is constant

If the above three conditions are met with fully, then the channel is said to be in true regime.

Initial regime. One of the conditions of attaining regime of a channel is that ththere should be
freedom for the channel to form its own section. Initial regime is the state of channel that has
formed its section only and yet not secured the longitudinal slope.

Final regime. When a channel is constructed with defective slope, it tries to throw off the
incoherent silt on the bed to increase their slopes. To attain the final regime, the channel forms
its section first before the final slope. The channel after attaining its section and longitudinal
slope will be said to be in final regime.

Permanent regime. When a channel is protected on the bed and side with some kind of
protecting material, the channel section cannot be scoured up and so there is no possibility of
change of section or longitudinal slope; the channel will then be said
said to be in permanent regime.
Regime theory is not applicable to such channels.

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There is only one section and only one longitudinal slope at which the channel will carry a
particular discharge with
ith a particular silt grade. Natural silt transporting channels have a
tendency to assume a semi-elliptical
elliptical section. The coarser the silt, greater is the waterway of such
a channel and narrower the depth. The finer the silt, greater is the depth and the ch channel closely
approximates a semi-circle.

cross-section
If a channel is constructed with too small a cross section for a particular discharge and the slope
steeper than required, scour will occur till final regime is attained. Similarly silting will occur in
a channel till final regime is attained in a channel with a wider cross-section
section and flatter slope
than required.

Lacey also state that the silt is kept in suspension due to the force of vertical eddies. According
to him, the eddies are generated from bed and sides
sides,, both normal to surface of generation. Hence,
vertical component of eddies generated from sides will also support the silt. Lacey, therefore,
assumed hydraulic mean depth (R) as variable, unlike Kennedy who assumed depth D as
variable. Since Lacey assumed a semi-ellipse as the cross-section section of a regime channel,
assumption R as a variable seem to more logical.

Lacey’s regime equations

Summary of Lacey’s formulae

Serial no Designation of formula Expression in M.K.S. units

1 V-f-R 2
V=√
5
2 A-f-V A 2 = 140.0 5
2 1
3 V-R-S V = 10.8 3 3

4 P-Q P = 4.75 √
5 V-Q-f 1
2 6
V=( )
140
6 S-f-R 23
S=
4980 2
7 S-f-q 5
3
S = 0.000172
3

8 S-f-Q 5
2
S=
3340 6
9 Regime scour depth relation 1
R = 0.47 ( )3
1
10 Regime scour depth relation 2 3
R = 1.35 ( )

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Lacey’s theory applied to channel design

The channel section can be designed on Lacey’s theory by using various equations developed
above. The design procedure involves no trial and error steps. For the channel design, the
discharge Q and mean diameter of silt particles md or silt factor f should be known.

Channel design procedure

1. Calculate the silt factor, f = 1.76 √

1
2 6
2. Compute velocity, V = ( )
140
3. Determine area, A = Q/V
4. Compute perimeter, P = 4.75 √
5. Find out bed width B and depth D of the channel section since A and P are known. The
side slope of an irrigation channel is usually ½:1. Hence
2
Area, A = BD + and perimeter, P = B + D√5 D
2
−√ 2−6.944
Hence D = ; B = P – 2.236 D
3.472
2 + 2
6. Calculate R = 5 also calculate R = 2

2 B + D 2.236
Both the values of R should be the same; this will provide a numerical check from steps 1
to 5.
5
2
7. Find the slope S = 1
3340 6

Comparison of Kennedy’s and Lacey’s theories

1. Kennedy introduced the term C.V.R. (m) in his equation to make it applicable for
channels of different grades of silt, but he did not give any idea to measure the value of
m. Lacey introduced the concept of silt factor f in his equations and suggested a method
of determining the value of f by relating it with particle size.
2. Kennedy assumed that silt is kept in suspension because of eddies generated from the bed
only, and so he proposed a relation between V and D. Lacey assumed that silt is kept in
suspension because of the normal components of eddies generated from the entire
perimeter and so he proposed a relation between V and R.
3. Kennedy assumed Kutter’s formula for finding the value of mean velocity where in the
value of N is to be assumed arbitrarily.
arbitrarily. Lacey gave his own formula for the velocity and
thus a designer has not to choose anything arbitrarily.
4. Kennedy gave no formula for determination of longitudinal slope of the canal. The slope
to be given to the canal is based on experience or on Wood’s d’s table. Lacey gave a formula
for the longitudinal slope of a regime channel.

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5. Lacey proposed that the shape of a regime channel should be a semi ellipse. Since the
channel section is trapezoidal in shape, it can never attain final regime. Kennedy simply
gave the idea that a non-silting
silting and non
non-scouring channel will be a regime channel.
6. Lacey’s theory as applied to channel design does not involve any trial and error
procedure whereas kennedy’s theory in involves
volves a trial and error procedure for design of
channel.
7. Lacey made a distinction between two types of resistance in alluvial channels, one
determined by grain size and the other due to irregularities of the channel. Kennedy did
not make any such distinction.
8. Basic concept of the theories is the same that the silt remains in suspension due to the
force of vertical eddies.

Defects in Lacey’s theory

1. The theory does not give a clear description of physical aspects of the problem.
2. It does not define what actually governs the characteristics of an alluvial channel.
3. The derivation of various formulae depends upon a single factor f and dependence on
single factor f is not adequate. There are different phases of flow on bed and sides and
hence different values of silt factor for bed and side should have been used.
4. Lacey’s equations do not include a concentration of silt as variable
5. Lacey did not take into account the silt left in channel by water that is lost in absorption
which is as much as 12 to 15 % of the total discharge of channel.
6. The effect of silt attrition was also ignored. The silt size does actually go on decreasing
by the process of attrition among the rolling silt particles dragged along the bed.
7. Lacey did not properly define the silt grade and silt charge.
8. Lacey, however considers that a regime channel is inherently free from external restraint
and shock and has, therefore, a constant Na for a given size of material. In so far as
regime channel is a sediment transporting channel and will normally have a changing
pattern of bed ripple formation, this statement is unlikely to be correct.
9. Lacey introduced semi-ellipse
ellipse as ideal shape of a regime channel which is not correct.
10. Strictly speaking an artific
artificial channel is not a regime channel, and regime theory is not
applicable to it.

Lecture 49

Reservoirs

Introduction

A reservoir is created with the impounding of runoff from the catchment upstream by the
construction of a dam across a river or stream. Storage is done during the period when the flow is
in excess of the demand for release during the lean supply period so as to maintain continuo
continuous
Hydel power generation, besides meeting requirements of irrigation, water supply etc.

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The reservoir thus provides

(i) Protection from flood as the flood water escaping downstream is moderated so as to
be compatible with the flood carrying capacity of the river channel downstream,
(ii) Water stored is utilized for irrigation, power generation, water supply, navigation,
fishing, recreation, etc.,
(iii) Improvement in climate,
(iv) Lessening river pollution,
(v) Check on spread of diseases due to improved water supply and sanitation
(vi) Reduction in river section downstream of the dam thereby making available
considerable land from river bed for cultivation

The reservoir on the other hand have certain disadvantages, viz.,

(i) Submergence ce of fertile valley lands,

(ii) Displacement of large population from reservoir area and their resettlement cost at
new areas,
(iii) Reduced capacity to flush pollution and salt from estuary,
(iv) Possible adverse effects on the ecology of the project area,
(v) Entrapping of fertile sand which has manorial value for cultivation in command area.
Lack of silt adversely effects fishing, and
(vi) Flooding of forest and displacement of wild life.

Classification of reservoirs

1. Classification on the basis of the purpose served

Storage reservoir

Also termed as conservation or impounding reservoir. Storage reservoir stores surplus water
during the period of excess flow for a considerable period of time so as to maintain continuous
supply for irrigation, Hydel power generation, municipal water water supply, industrial purposes,
navigation etc. during the period of lean supply in the river but when demand is keen.

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Flood control reservoir

Flood control reservoir holds a part volume ( in excess of the safe carrying capacity of the river
channel downstream) temporarily during floods and releases latter, when flood recedes, as
rapidly as the channel capacity permits. Flood control reservoir thus reduces flood stage and
mage downstream. Construction of reservoir solely for flood control
consequently avoids damage
measures is not advisable except in very special cases due to the high cost of construction. Flood
control reservoirs may be either retarding reservoirs or detention reservoirs.

(i) Retarding g reservoirs: Retarding reservoirs are those at which no gates are provided
in the outlets to regulate the release but the discharging capacity of the outlets and
spillway is so fixed that it is not in excess of the flood carrying capacity of the river
nnel downstream. With the rise in the reservoir level, the amount of water released
channel
is such as would not create flooding in the areas downstream. They are preferred on
small rivers. The suitable location of a retarding reservoir is immediately upstream on
the city to be protected from floods or above the confluence of two or more streams.

(ii) Detention reservoirs: Detention reservoirs have gated outlets and store water for a
relatively brief period of time so as to provide greater flexibility in the operation of
reservoirs. They are specially suitable when the area under control increases in size
and protected area is wide spread.

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Advantages: (i) profitable venture and prime necessity, (ii) maximum benefits from the
stored water by way of irrigation, power, flood control, navigation, public water supply, etc.,
and (iii) lot of indirect benefits such as development of fisheries and tourism.

Limitations: (i) High initial cost, (ii) cost be comes disproportionate to the results aimed at in
becomes
case the capacity required for flood control cannot be utilized completely or partly, (iiii)
usually involve submergence and consequent uprooting of population and consequent their
rehabilitation and resettlement
lement problems.

2. Classification on the basis of regulation

Distribution reservoir

It is a reservoir connected with water supply project of a city or a town. It is usually of limited
storage capacity, used primarily to cater for fluctuations in demand which may occur over short
periods of several hours to several days and also a local storage to take care of emergency in the
event break in main supply line or failure of the pumping plant etc.

Single purpose reservoir

It is the reservoir to serve only one purpose

purpose which may be flood control or irrigation or power
generation, municipal water supply, recreation etc.

Multipurpose reservoir

Also termed as multiuse reservoir. In this reservoir, the storage and release ca cater for a
combination of two or more purposes such as irrigation, Hydel power generation, flood control,
public water supply, navigation, recreation, fisheries etc. A reservoir which is designed for one
of the purposes put provides other incidental benefits
benefits because every reservoir serves or should
serve more than one purpose on account of indirect benefits that may result from its operation, is
also referred to a multipurpose reservoir. Keeping with the diverse purposes served by a
multipurpose reservoir, it is usually of large capacity so as to (i) reserve a certain minimum
storage capacity at all times for flood control, (ii) provide steady supply for kharif and rabi crops,

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(iii) generate maximum firm power. Usually, these enormous benefits accruing from a
multipurpose project repay the cost of building the dam within a couple of years.

Balancing reservoir

It is a reservoir, usually of limited capacity, located downstream of a main reser

reservoir to (i) store
the water let down from the reservoir in excess of that required for irrigation or additional power
generation, (ii) provide flexibility of operation to the distribution system, and (iii) at certain
locations primarily for permitting regu lated supply to the power penstocks with a view to cater
regulated
for the fluctuations in requirement of water supply to the turbines depending on power load.

Auxiliary reservoir

Also termed as compensatory reservoir. This is a reservoir which supplements and absorbs the
spill of a main reservoir.

System of reservoir

These consist of a group of single or multipurpose reservoirs which may be operated in an

integrated manner for optimum utilization of water resources of the river system.

3. Classification on the basis of water quality

Fresh water reservoir: It is the reservoir the water of which is replenished at least once,
preferable twice in a year by the continuous cycle of inflow and outflow due to low capacity-
inflow ratio. It provides quality water nearest to the fresh water.

Stored water reservoir: It is the reservoir which stores almost entire inflow, the bulk of which
is received during monsoon. Evidently the capacity
capacity-inflow ratio is high. The quality of water
may not be comparable to the fresh water reservoir also possibly due to bad odour and taste
imparted by contamination.

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Lecture 50

Storage zones of a reservoir

Max. Reservoir level

Dam
Spill storage Normal Reservoir level

Valley storage Max. Head for

Live storage Sluice way
power generation
Natural river surface Min. Reservoir
before dam level

Maximum flow water surface Min. head

Dead storage
Dead storage in river without dam

Dead storage in a reservoir, about 10% of the gross storage capacity, is provided to cater for
sediment deposition by the impounded sediment-laden
sediment laden waters as very small portion of the
ment passes down through dam outlets and for the minimum drawdown in the case of power
sediment
projects. It is equivalent to the volume of sediment expected to be deposited in the reservoir
during the designed life of the reservoir, usually taken as 100 years. The level attained by the
reservoir corresponding to the dead storage is termed as dead storage level. It is the level below
which a reservoir is not susceptible to release water by the in in-built
built outlet means. In power
projects, releases are allowed up to minimum
minimum drawdown level only instead of dead storage level
so as to maintain the minimum head required for power generation.

Live storage

It is the storage capacity of the reservoir above dead storage level which constitutes useable
portion of the total storage. It is thus the difference of gross storage capacity and the sum of dead
storage capacity and inactive capacity for hydro power generation. Inactive capacity is the
storage capacity exclusive of dead storage below which evacuation is not contemplated beca because
of minimum irrigation and power load requirement. However, in practice dead storage capacity
and inactive capacity are considered the same. Live storage has to be sufficient so that the project
is successful for (i) 75% of its life period in an irrigation project, (ii) 90% of its life period in an
Hydel power project and (iii) 100% of its life period in water supply project.

Flood storage

Flood storage is the storage contained between the maximum reservoir level and the full
reservoir level. It is the storage space provided in a reservoir for storing flood water temporarily
to moderate the releases downstream. Flood storage varies with the spillway capacity for a given

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design flood. When the spillway capacity is equal to the inflow design flood, there is no flood
storage and the full reservoir level and maximum reservoir level are the same. Effective flood
storage is the difference between the amount of water stored in the reservoir
reservoir during the period of
flood peak reduction and the amount of natural valley storage during the same period.

Induced surcharge storage

The storage between FRL and MWL of a reservoir which may be induced by regulating the
outlet gates after the reservoir is filled up to full reservoir level.

Bank storage

It is the storage absorbed and stored in the bed and a bank of a stream, Lake, in the voids in the
soil cover in a reservoir and becomes available in whole or in part as seepage water when the
water level drops down in the reservoir. Bank storage increases the reservoir capacity over and
above that given by the elevation storage curve.

Valley storage

Rivers in dry season flowing within its banks have small water flow compared to that when in
floods. Valley storage is the storage in the river or stream in floods after it has overflowed its
banks. It is an important element in the design of large size flood control reservoirs where the
valley storage at the time of flood is of significant proportions in regard to the reservoir storage
volume.

Conservation storage

Water impounded in a reservoir for conventional uses such as irrigation, power generation,
industrial uses, municipal water supply, etc. is termed as conservation storage.

Carry over storage

Also
so called ‘over year storage’. The storage left over in a reservoir at the end of the depletion
period of a year which is available for use in later years. Alternatively, it is the storage collected
during surplus for making up deficiencies in dry or lean year.

Cyclic storage Water stored in a reservoir during periods of more than average supply and the
over year storage
releasing of it for use during periods of insufficient supply; also called over-year

Effective power storage Storage between minimum and maximum reservoir level for power
generation

Effective flood control storage Storage which represents the difference between the amount of
water stored in the reservoir during the period of flood peak reduction and the amount of natura
natural
valley storage during the same period.

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Coefficient of storage For surface waters, a coefficient to express the relation of storage capacity
in a reservoir, to the mean annual inflow in the reservoir; also called ‘reservoir factor’

Maximum water level

Spill zone
Full reservoir level

Flood control zone

Conservation zone
Ma
Maximum drawdown level
Buffer zone
Dead storage level
Dead
storage zone

In multipurpose reservoirs, five basic storage zones of reservoir space for use in operating the
reservoir for various functions are indicated in the figure.

1. Spill zone. It is the storage space above flood control zone between FRL and MWL as is
occupied mostly during high floods.
2. Flood control zone. It is thee storage space earmarked as temporary storage for absorbing
high flows for alleviating downstream flood damages.
3. Conservation zone. It is the storage zone between FRL and dead storage level used for
conservation of water to meet various future demands.
4. Buffer zone. It is the storage space above dead storage level which is used to satisfy only
very essential water needs in case of extreme situation
5. Dead storage zone. It is the lowest zone meant to absorb sediment inflow. It is not
susceptible to release by inin-built outlet means.

Active capacity:: It is the storage available for project purposes; usually between the lowest
allowable level of release (min. drawdown level) and the highest controlled water surface (static
full pool level) also called effective storage, useful storage, usable storage, effective capacity or
useful capacity.

Area capacity curves: The graph of area of water spread and the storage volume of reservoir as
function of elevation.

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Lecture 51

Design capacity of reservoirs

The design capacity of storage reservoirs is to ensure that the water will be replenished often
enough; at least once preferable twice, in a year to classify it as fresh water reservoirs and
accordingly the drainage area must be large enough. The design capacity of a reservoir is fixed in
long-term
consideration of the various factors such as (i) long term precipitation records for the catchment,
(ii) long term runoff data at or near the site, (iii) sediment yield into the reservoir from
catchment, (iv) area and capacity curves, (v) trap efficiency, (vi) losses in the reservoir, (vii)
maximum requirement of water for different uses to be met from the reservoir and direct fflows of
the river irrespective of the availability of assured supplies, (viii) consideration of dependability,
(xi) upstream use, and (x) density current aspects and location of outlets. Usually the storage
capacity is determined by mass curve method.

Mass curve
Demand curve
Uniform demand
Accumulated discharge Mm3

Inflow curve

Storage

month

It is a curve of the accumulated total flow against time. The characteristics of the curve are that
(i) It rises continuously as it shows accumulated inflow; (ii) the rise is steep if the rate of inflow
is high. Steepness of the curve indicates the rate of inflow at that time interval, (iii) it is
horizontal if there is no inflow during a certain period. The relatively dry periods ar
are indicated as
hollows on the curve, (iv) any point on it represents the total flow, in million cubic metres, from
beginning of the period upto the given time, and (v) slope of a tangent to it at this point indicates
the discharge in cumecs.

Demand curve

It is a curve of the accumulated demand with time. In other words it is a plot of accumulated
outflow against time. The demand curve is a straight line, rising from the origin, if there is a

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constant rate
te of demand. In case there is a variable rate of demand, the demand curve is curved
one.

Storage capacity determination

The storage capacity of a reservoir to meet the demand of continuous supply is determined with
the help of the observed discharge data of a stream or a river on which the dam is to be built. The
flow values for the driest years in as long a period as is available, say 25 to 30 years; minimum
10 years is considered. The storage capac
capacity is determined by the following methods:

1. Analytical method: The inflow and demand values in each month are determined. The
demand includes prior rights, if any, evaporation losses, rainfall etc. The deficit and
surplus of water which is the departure of inflow volume from the demand volume is
determined. The maximum value of cumulative excess of demand over the inflow
represents the minimum storage necessary to meet the demand. The cumulative excess
inflow volume starting from each demand withdrawal from storage is also determined
which indicates the filling up on the reservoir and the volumes in excess of storage to be
spilled over as also whether the reservoir fills up after a demand and when.
2. Mass curve method: In this method the storage capacity is determined
termined from mass curve
of inflow and demand. As examination of the demand curve and inflow curve drawn to
the same scale would indicate when storage is required. When inflow rate is low, i.e. the
inflow curve is flat but at that time if the slope of demand curve is more, it indicates that
storage is required to meet the demand.
3. Bar graph method. In this method, the average inflows of the driest year are plotted as
ordinates against time as abscissa to get a stepped graph known as bar grap
graph. The area in
the diagram represents the total volume of water which enters the reservoir in that year.
Likewise, the average demand is then plotted on the graph, straight line if demand is
uniform and stepped in the case of non-uniform demand. The area off maximum deficit
i.e., between the demand and inflow represents to scale the minimum storage capacity of
the reservoir. The storage required to meet the maximum uniform demand that can be
met with without loss of water over spillway is given by the area of surplus over the
demand.

Safe yield

Reservoir yield is the amount of water that can be released from a reservoir in a certain interval
of time. The time interval is a month or a year for large conservation reservoirs. Reservoir yield,
depending upon inflow, vary from time to time. Firm or safe yield is the maximum amount of
supply or yield guaranteed during a critical dry period. Firm power generation is determined
corresponding to firm yield. Secondary yield is the quantity of water available in excess of safe
yield during the periods of high flows. The safe yield from a reservoir of a given storage capacity
is determined by mass curve method by (i) drawing inflow mass curve, (ii) drawing mass

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demand curve. On it draw straight lines from a common origin to represent the demands at
various rates, (iii) from high points on mass inflow curve, draw tangents such that their
maximum departure from the mass curve does not exceed the specified reservoir capa capacity, (iv)
the slopes of each of these tangents are measured, (v) the slope indicates the yield as can be
attained in each year from the reservoir of a given capacity, and (vi) the slope of the most flat
demand line is the safe yield.

References:

T1. K. Subramanya, “Engineering Hydrology”, Tata McGraw Hill Publishers, New Delhi.

T2. Jayarami Reddy, “A Text Book of Hydrology”, Lakshmi Publications, New Delhi.

T3. Punmia and LalPandey, “Irrigation and Water Power Engineering” Lakshmi Publications,
New Delhi.

R1. H.M. Raghunath, “Hydrology”, Wiley Eastern Publication, New Delhi.

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