Chapter 1: HYDROLOGICAL CYCLE AND PRECIPITATION 7 · chapter 1: hydrological cycle and precipitation The word hydrology means science of water which deals with the spatial and temporal

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2 Career Avenues GATE Coaching by IITians

Chapter 1: HYDROLOGICAL CYCLE AND PRECIPITATION 7

HYDROLOGIC CYCLE ......................................................................................................................................... 7

PRECIPITATION ................................................................................................................................................ 8

MEASUREMENT OF PRECIPITATION ................................................................................................................. 9

ESTIMATION OF MISSING DATA ..................................................................................................................... 15

ABSTRACTIONS FROM PRECIPITATION .......................................................................................................... 16

EVAPORATION ............................................................................................................................................... 17

SUMMARY ..................................................................................................................................................... 17

FORMULA ...................................................................................................................................................... 17

TIPS................................................................................................................................................................ 18

PREVIOUS YEAR GATE QUESTIONS................................................................................................................. 19

Chapter 2: INFILTRATION AND RUNOFF .................... 21

INFILTRATION ................................................................................................................................................ 21

RUNOFF ......................................................................................................................................................... 21

RUNOFF—RAINFALL RELATIONS .................................................................................................................... 22

STAGE DISCHARGE RELATIONSHIP ................................................................................................................. 22

SUMMARY ..................................................................................................................................................... 23

TIPS................................................................................................................................................................ 23

PREVIOUS GATE QUESTIONS .......................................................................................................................... 24

Chapter 3: HYDROGRAPHS ....................................... 29

Hydrographs .................................................................................................................................................. 29

Hydrograph Analysis ...................................................................................................................................... 30

Unit Hydrograph ............................................................................................................................................ 31


Synthetic Unit Hydrograph ............................................................................................................................ 35

Use and Limitations of Unit Hydrograph ........................................................................................................ 35

PROBLEM ....................................................................................................................................................... 36

SUMMARY ..................................................................................................................................................... 43

TIPS................................................................................................................................................................ 44


Chapter 4: Floods and Flood Estimation ................... 54

Floods ............................................................................................................................................................ 54

Planning of Reservoirs ................................................................................................................................... 58

Flood Routing ................................................................................................................................................ 59

PROBLEM ....................................................................................................................................................... 60

SUMMARY ..................................................................................................................................................... 61

FORMULA ...................................................................................................................................................... 62

TIPS................................................................................................................................................................ 62


Chapter 5: Well Hydraulics ....................................... 65

Ground Water Resources ............................................................................................................................... 65

Well Irrigation ................................................................................................................................................ 66

Occurrence of Ground Water ......................................................................................................................... 67

Well Hydraulics .............................................................................................................................................. 70

Equilibrium Equations .................................................................................................................................... 71

Well Interference ........................................................................................................................................... 74

Pumping Tests (Or Aquifier Tests) .................................................................................................................. 75

SUMMARY ..................................................................................................................................................... 77


FORMULA ...................................................................................................................................................... 78

TIPS................................................................................................................................................................ 78


Chapter 6: Introduction to Irrigation ........................ 80

INTRODUCTION ............................................................................................................................................. 80

STORAGE OF SOME MAJOR DAMS (2) ............................................................................................................ 80

IMPACT OF IRRIGATION ON HUMAN ENVIRONMENT .................................................................................... 80

WATER RESOURCES OF INDIA ........................................................................................................................ 81

NEED OF IRRIGATION IN INDIA ...................................................................................................................... 83

IDEAL WEATHER FOR KHARIF AND RABI SEASONS ......................................................................................... 84

CROPS OF KHARIF SEASON ............................................................................................................................ 84

CROPS OF RABI SEASON................................................................................................................................. 86

OTHER MAJOR CROPS .................................................................................................................................... 88

MULTIPLE CROPPING ..................................................................................................................................... 89

SUMMARY ..................................................................................................................................................... 90

TIPS................................................................................................................................................................ 90

Chapter 7: Water Requirements of Crops ................. 91

SOIL–WATER RELATIONSHIPS ........................................................................................................................ 91

ROOT-ZONE SOIL WATER ............................................................................................................................... 93

INFILTRATION ................................................................................................................................................ 96

CONSUMPTION USE (OR EVAPOTRANSPIRATION) ......................................................................................... 98

IRRIGATION REQUIREMENT ........................................................................................................................... 99

FREQUENCY OF IRRIGATION ........................................................................................................................ 101

SUMMARY ................................................................................................................................................... 116


FORMULA .................................................................................................................................................... 117

TIPS.............................................................................................................................................................. 117

PREVIOUS YEAR GATE QUESTIONS............................................................................................................... 118

Chapter 8: Canal Irrigation...................................... 127

ESTIMATION OF DESIGN DISCHARGE OF A CANAL ....................................................................................... 128

SUMMARY ................................................................................................................................................... 136

FORMULA .................................................................................................................................................... 136

TIPS.............................................................................................................................................................. 136


Chapter 9: Canal Head Works ................................. 139

LOCATION OF HEADWORKS ON RIVERS ....................................................................................................... 139

DIFFERENT UNITS OF HEADWORKS .............................................................................................................. 140

FISH LADDER ................................................................................................................................................ 141

CANAL HEAD REGULATOR ............................................................................................................................ 142

DESIGN OF WEIR .......................................................................................................................................... 144

WEIR CREST, GLACIS, AND IMPERVIOUS FLOOR ........................................................................................... 145

UPSTREAM AND DOWNSTREAM LOOSE PROTECTION ................................................................................. 146

SUMMARY ................................................................................................................................................... 152

FORMULA .................................................................................................................................................... 152

TIPS.............................................................................................................................................................. 153

PREVIOUS YEAR GATE QUESTIONS: ............................................................................................................. 153

Chapter 10: Gravity Dams ....................................... 159

FORCES ON A GRAVITY DAM ....................................................................................................................... 159


STRESS ANALYSIS OF GRAVITY DAMS .......................................................................................................... 165

ELEMENTARY PROFILE OF A GRAVITY DAM ................................................................................................. 170

SUMMARY ................................................................................................................................................... 180

FORMULA .................................................................................................................................................... 180

TIPS.............................................................................................................................................................. 180

PREVIOUS YEAR GATE QUESTIONS: ............................................................................................................. 181

Chapter 11: Water Logging ..................................... 182

WATERLOGGING .......................................................................................................................................... 182

ECONOMICS OF CANAL LINING .................................................................................................................... 185

DRAINAGE OF IRRIGATED LANDS ................................................................................................................. 186

SUMMARY ................................................................................................................................................... 194

FORMULA .................................................................................................................................................... 194

TIPS.............................................................................................................................................................. 195

Chapter 12: Methods of Irrigation .......................... 196

METHODS OF IRRIGATION ........................................................................................................................... 196

WATER USE SUBSYSTEMS ............................................................................................................................ 196

SURFACE IRRIGATION .................................................................................................................................. 197

SUMMARY ................................................................................................................................................... 203

TIPS.............................................................................................................................................................. 203



Chapter 1: HYDROLOGICAL CYCLE AND PRECIPITATION

The word hydrology means science of water which deals with the spatial and temporal

characteristics of the earth’s water in all its aspects such as occurrence, circulation, distribution,

physical and chemical properties, and impact on environment and living things.

HYDROLOGIC CYCLE

The total water of earth, excluding deep ground water, is in constant circulation from the earth

(including oceans) to atmosphere and back to the earth and oceans. This cycle of water amongst

earth, oceans, and atmospheric systems is known as hydrologic cycle. Figure shown below is an

enormously simplified sketch of the hydrologic cycle for which sun is the source of energy. The

hydrologic cycle can be visualized to begin with the evaporation(due to solar heat) of water from

the oceans, streams and lakes of the earth into the earth’s atmosphere. The water vapor, under

suitable conditions, get condensed to form clouds moving with wind all over the earth’s surface

and which, in turn, may result in precipitation (in the form of rain water, snow, hail, sleet etc.)

over the oceans as well as the land surface of the earth. Part of the precipitation, even while

falling, may evaporate back into the atmosphere.

Another part of the precipitation may be intercepted by vegetation on the ground or other

surfaces. The intercepted precipitation may either evaporate into the atmosphere or fall back on

the earth’s surface. The greater part of the precipitation falling on the earth’s surface is retained

in the upper soil from where it may return to the atmosphere through evaporation and

transpiration by plants and/or find its way, over and through the soil surface as runoff, to stream

(or river) channels and the runoff thus becoming stream flow. Yet another part of the

precipitation may penetrate into the ground to become part of the ground water. The water of

stream channels, under the influence of gravity, moves towards lower levels to ultimately meet

the oceans. Water from ocean may also find its way into the adjoining aquifers. Part of the

stream water also gets evaporated back into the atmosphere from the surface of the stream. The

ground water too moves towards the lower levels to ultimately reach the oceans. The ground

water, at times, is a source of stream flow.

Hydrologic system is defined as a structure or volume in space surrounded by a boundary that

receives water and other inputs, operates on them internally, and produces them as outputs.

The global hydrologic cycle can be termed a hydrologic system containing three sub-systems:

the atmospheric water system, the surface water system, and the subsurface water system.

Another example of the hydrologic system is storm-rainfall-runoff process on a watershed.

Watershed (or drainage basin or catchment) is a topographic area that drains rain water falling

on it into a surface stream and discharges surface stream flow through one particular location

identified as watershed outlet or watershed mouth. The term ‘watershed’ used for the catchment

area should be distinguished from the watershed used in the context of canal alignment


PRECIPITATION

The atmospheric air always contains moisture. Evaporation from the oceans is the major source

(about 90%) of the atmospheric moisture for precipitation. Continental evaporation contributes

only about 10% of the atmospheric moisture for precipitation.

The common forms of precipitation are drizzle or mist (water droplets of diameters less than 0.5

mm), rain (water drops of size between 0.5 mm and 6.0 mm), snow (ice crystals combining to

form flakes with average specific gravity of about 0.1), sleet (rain water drops, falling through air

at or below freezing temperatures, turned to frozen rain drops), and hail (precipitation in the

form of ice balls of diameter more than about 8 mm).

Most of the precipitation, generally, is in the form of rains. Therefore, the terms precipitation and

rain fall are considered synonymous. Rainfall, i.e., liquid precipitation, is considered light when

the rate of rainfall is upto 2.5 mm/hr, moderate when the rate of rainfall is between 2.5 mm/hr

and about 7.5 mm/hr, and heavy when the rate of rainfall is higher than about 7.5 mm/hr.

The temporal variation of annual rainfall at a given place is expressed in terms of the coefficient

of variation, Cv defined as


The coefficient of variation of the annual rainfall for different places may vary between15 (for

regions of high rainfall) and 70 (for regions of scanty rainfall) with an average value of about 30.

MEASUREMENT OF PRECIPITATION

PRECIPITATION GAUGES

Precipitation (of all kinds) is measured in terms of depth of water (in millimeters) that would

accumulate on a level surface if the precipitation remained where it fell. A variety of instruments

have been developed for measuring precipitation (or precipitation rate) and are known as

precipitation gauges or, simply, rain gauges which are classified as either recording or non-

recording rain gauges.

Non-recording rain gauges only collect rain water which, when measured suitably, gives the total

amount of rainfall at the rain gauge station during the measuring interval. The Indian

Meteorological Department has adopted Symon’s rain gauge. A glass bottle and funnel with

brass rim are put in a metallic cylinder such that the top of the cylinder is 305mm above the

ground level. Rain water falls into the glass bottle through the funnel. The water collected in the

bottle is measured with the help of a standard measuring glass jar which is supplied with the rain

gauge. The jar measures rainfall in millimeters. At each station, rainfall observations are taken

twice daily at 8.30 a.m. and 5.30 p.m.

Recording rain gauges automatically record the intensity of rainfall and the time of its occurrence

in the form of a trace (or graph) marked on a graph paper wrapped round a revolving drum.

Following three types are the most widely used recording rain gauges:

Tipping bucket rain gauge,

Weighing bucket rain gauge, and

Siphon rain gauge.


Tipping bucket rain gauge: A 300 mm diameter funnel collects rain water and conducts it to one

of the two small buckets which are so designed that when 0.25 mm of rainfall is collected in a

bucket, it tilts and empties its water into a bigger storage tank and, simultaneously, moves the

other bucket below the funnel. When any of the two buckets tilts, it actuates an electric circuit

causing a pen to make a mark on a revolving drum. The recording equipment can be remotely

located in a building away from the rain gauge. At a scheduled time, the rain water collected in

the storage tank can be measured to yield total rainfall in the measuring duration. The rainfall

intensity (and also the total rainfall) can be estimated by studying the record sheet on which each

mark indicates 0.25 mm of rain in the duration elapsed between the two adjacent marks.

Weighing bucket rain gauge: This gauge has a system by which the rain that falls into a bucket

set on a platform is weighed by a weighing device suitably attached to the platform. The

increasing weight of rain water in the bucket moves the platform. This movement is suitably

transmitted to a pen which makes a trace of accumulated amount of rainfall on a suitably

graduated chart wrapped round a clock driven revolving drum. The rainfall record of this gauge

is in the form of a mass curve of rainfall. The slope of this curve at any given time gives the

intensity of rainfall at that time.

Siphon rain gauge: This gauge is also called float type rain gauge as this gauge has a chamber

which contains a light and hollow float. The vertical movement of float on account of rise in the

water level in the chamber (due to rain water falling in it) is transmitted by a suitable mechanism

to move a pen on a clock-driven revolving chart. The record of rainfall is in the form of a mass

curve of rainfall and, hence, the slope of the curve gives the intensity of rainfall.

Bureau of Indian Standards has laid down the following guidelines for selecting the site for rain

gauges (IS : 4897-1968):

The rain gauge shall be placed on a level ground, not upon a slope or a terrace and never

upon a wall or roof.

On no account the rain gauge shall be placed on a slope such that the ground falls away

steeply in the direction of the prevailing wind.

The distance of the rain gauge from any object shall not be less than twice the height of the

object above the rim of the gauge.

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.

In hills where it is difficult to find a level space, the site for the rain gauge shall be chosen

where it is best shielded from high winds and where the wind does not cause eddies.

The location of the gauge should not be changed without taking suitable precautions.

Description of the site and surroundings should be made a matter of record.


AVERAGE DEPTH OF PRECIPITATION OVER AN AREA

The information on the average depth of precipitation (or rainfall) over a specified area on either

the storm basis or seasonal basis or annual basis is often required in several types of hydrologic

problems. The depth of rainfall measured by a rain gauge is valid for that rain gauge station and

in its immediate vicinity. Over a large area like watershed (or catchment) of a stream, there will

be several such stations and the average depth of rainfall over the entire area can be estimated by

one of the following methods:

Arithmetic Mean Method

This is the simplest method in which average depth of rainfall is obtained by obtaining the sum

of the depths of rainfall (say P1, P2, P3, P4 .... Pn) measured at stations 1, 2, 3, .....n and dividing

the sum by the total number of stations i.e. n. Thus,

This method is suitable if the rain gauge stations are uniformly distributed over the entire area

and the rainfall variation in the area is not large.

Theissen Polygon Method

The Theissen polygon method takes into account the non-uniform distribution of the gauges by

assigning a weightage factor for each rain gauge. In this method, the entire area is divided into

number of triangular areas by joining adjacent rain gauge stations with straight lines, as shown in

Fig. 2.7 (a and b). If a bisector is drawn on each of the lines joining adjacent rain gauge stations,

there will be number of polygons and each polygon, within itself, will have only one rain gauge

station. Assuming that rainfall Pi recorded at any station i is representative rainfall of the area Ai

of the polygon i within which rain gauge station is located, the weighted average depth of rainfall

P for the given area is given as


Isohyetal Method

An isohyet is a contour of equal rainfall. Knowing the depths of rainfall at each rain gauge

station of an area and assuming linear variation of rainfall between any two adjacent stations, one

can draw a smooth curve passing through all points indicating the same value of rainfall, Fig. 2.7

(c). The area between two adjacent isohyets is measured with the help of a planimeter. The

average depth of rainfall P for the entire area A is given as

Since this method considers actual spatial variation of rainfall, it is considered as the best method

for computing average depth of rainfall.

Example 2.1

The average depth of annual precipitation as obtained at the rain gauge stations for a specified

area is as shown in Fig. 2.7 (a). The values are in cms. Determine the average depth of annual

precipitation using (i) the arithmetic mean method, (ii) Theissen polygon method, and (iii)

isohyetal method.

Solution

(i) Arithmetic mean method:

Using Eq. (2.2), the average depth of annual precipitation,

(ii) Theissen polygons for the given problem have been shown in Fig. 2.7 (b). The

computations for the average depth of annual precipitation are shown in the following Table:


PRECIPITATION GAUGE NETWORK

The spatial variability of the precipitation, nature of the terrain and the intended uses of the

precipitation data govern the density (i.e., the catchment area per rain gauge) of the precipitation

gauge (or rain gauge) network. Obviously, the density should be as large as possible depending

upon the economic and other considerations such as topography, accessibility etc. The World

Meteorological Organization (WMO) recommends the following ideal densities (acceptable

values given in brackets) of the precipitation gauge network (3):

For flat regions of temperate, Mediterranean, and tropical zones, 600 to 900 sq. km (900–

3000 sq. km) per station.


For mountainous regions of temperate, Mediterranean, and tropical zones, 100 to 250 sq. km

(250 to 1000 sq. km) per station.

For small mountainous islands with irregular precipitation, 25 sq. km per station.

For arid and polar zones, 1500 to 10,000 sq. km per station.

At least ten per cent of rain gauge stations should be equipped with self-recording gauges to

know the intensities of rainfall. The Bureau of Indian Standards (4) recommends the following

densities for the precipitation gauge network:

In plains: 520 sq. km per station;

In regions of average elevation of 1000 m: 260 to 390 sq. km per station; and

In predominantly hilly areas with heavy rainfall: 130 sq. km per station.

For an existing network of raingauge stations, one may need to know the adequacy of the

raingauge stations and, therefore, the optimal number of raingauge stations N required for a

desired accuracy (or maximum error in per cent, ) in the estimation of the mean rainfall. The

optimal number of raingauge stations N is given as

Example 2.2

A catchment has eight rain gauge stations. The annual rainfall recorded by these gauges in a

given year is as listed in column 2 of the following Table.


ESTIMATION OF MISSING DATA

The continuity of a record of precipitation data may have been broken with missing data due to

several reasons such as damage (or fault) in a rain gauge during a certain period. The missing

data is estimated using the rainfall data of the neighboring rain gauge stations. The missing

annual precipitation Px at a station x is related to the annual precipitation values, P1, P2, P3 ......Pm

and normal annual precipitation, N1, N2, N3 ......Nm at the neighboring stations 1, 2, 3, ......M

respectively. The normal precipitation (for a particular duration) is the mean value of rainfall on

a particular day or in a month or year over a specified 30-year period. The 30-year normals are

computed every decade. The term normal annual precipitation at any station is, therefore, the

mean of annual precipitations at that station based on 30-year record. The missing annual

precipitation Pxis simply given as

If the normal annual precipitations at various stations are within about 10% of the normal annual

precipitation at station x, i.e., Nx. Otherwise, one uses the normal ratio method which gives


DEPTH-AREA-DURATION (DAD) ANALYSIS

Depth-area-duration (DAD) curves, Fig. 2.9, are plots of accumulated average precipitation

versus area for different durations of a storm period. Depth-area-duration analysis of a storm is

performed to estimate the maximum amounts of precipitation for different durations and over

different areas. A storm of certain duration over a specified basin area seldom results in uniform

rainfall depth over the entire specified area. The difference between the maximum rainfall depth

over an area P0 and its average rainfall depth P for a given storm, i.e., P0 – P increases with

increase in the basin area and decreases with increase in the storm duration. The depth-area-

duration curve is obtained as explained in the following example:

ABSTRACTIONS FROM PRECIPITATION

Prior to rain water reaching the watershed outlet as surface runoff or stream flow, it has to satisfy

certain demands of the watershed such as interception, depression storage, evaporation and

evapo-transpiration, and infiltration. A part of precipitation may be caught by vegetation on the

ground and subsequently get evaporated. This part of precipitation is termed intercepted

precipitation or interception loss (which, incidentally, is the gain for the atmospheric water)

which does not include through-fall (the intercepted water that drips off the plant leaves to join

the surface runoff) and stem flow(the intercepted water that runs along the leaves, branches and

stem of the plants to reach the ground surface.


EVAPORATION

Evaporation is the physical phenomenon by which a liquid is transformed to a gas. The rate of

evaporation of precipitation depends on (i) the vapor pressure of water, (ii) prevailing

temperature, (iii) wind speed, and (iv) atmospheric pressure. Transpiration is a phenomenon due

to which water received by the plant through its root system leaves the plant and reaches the

atmosphere in the form of water vapor. Evaporation and transpiration are usually considered

together as evapo-transpiration (or consumptive use).

SUMMARY

• The common forms of precipitation are drizzle or mist, rain, snow, sleet and hail.

• Three types are the most widely used recording rain gauges: (i) Tipping bucket rain gauge,

(ii) Weighing bucket rain gauge and (iii) Siphon rain gauge.

• As per the World Meteorological Organization (WMO) the ideal densities (acceptable values

given in brackets) of the precipitation gauge network :

• For flat regions of temperate, Mediterranean, and tropical zones, 600 to 900 sq. km

(900–3000 sq. km) per station.

• For mountainous regions of temperate, Mediterranean, and tropical zones, 100 to 250

sq. km (250 to 1000 sq. km) per station.

• For small mountainous islands with irregular precipitation, 25 sq. km per station.

• For arid and polar zones, 1500 to 10,000 sq. km per station.

As per the Bureau of Indian Standards the densities for the precipitation gauge network:

• In plains: 520 sq. km per station;

• In regions of average elevation of 1000 m: 260 to 390 sq. km per station; and

• In predominantly hilly areas with heavy rainfall: 130 sq. km per station.

The rate of evaporation of precipitation depends on (i) the vapor pressure of water, (ii)

prevailing temperature, (iii) wind speed, and (iv) atmospheric pressure.

Evaporation and transpiration are usually considered together as evapo-transpiration (or

consumptive use).


FORMULA

Coefficient of variation, Cv defined as

Arithmetic Mean Method

Theissen Polygon Method

Isohyetal Method

The normal annual precipitation considering the mean of annual precipitations (< 10%

variation) :

The normal annual precipitation considering the normal ratio method (> 10% variation) :


TIPS

This is comparatively an easy chapter to prepare for the exam and a one mark question

can be expected once in two years from this chapter.

An important question would be finding the missing data from the chapter.

PREVIOUS YEAR GATE QUESTIONS

1. While applying the rational formula for computing the design discharge, the rainfall

duration is stipulated as the time of concentration because

(A) this leads to the largest possible rainfall intensity

(B) this leads to the smallest possible rainfall intensity

(C) the time of concentration is the smallest rainfall duration for which the rational

formula is applicable

(D) the time of concentration is the largest rainfall duration for which the rational

formula is applicable.

2. The intensity of rainfall and time interval of typical storm are

Time interval Intensity of rainfall

(Min) (mm/min)

0-10 0.7

10-20 1.1

20-30 2.2

30-40 1.5

40-50 1.2

50-60 1.3

60-70 0.9

70-80 0.4

The maximum intensity of rainfall for 20 min duration of the storm is

(A) 1.5 mm/min (B) 1.85 mm/min

(C) 2.2 mm/min (D) 3.7 mm/min


Answers:

1. (c) Time of concentration (tc) is the time when entire catchment starts contributing to

surface run-off at channel and this is the minimum time required. So, smallest rainfall

duration for which the rational formula is applicable.

2. (b)

Maximum Intensity is for time interval 20-40 minutes where it is

= (2.2*10 + 1.5*10) / (10 + 10)

= 1.85 mm/min

Documents

Chapter 1: HYDROLOGICAL CYCLE AND PRECIPITATION 7 · chapter 1: hydrological cycle and precipitation The word hydrology means science of water which deals with the spatial and temporal