GENERAL ASPECTS OF Chapter HYDROLOGY & PRECIPITATION

1

INTRODUCTION

Any form of moisture reaching the earth’s surface from the atmosphere is calledthe precipitation.

MEASUREMENT OF PRECIPITATION

All forms of precipitation are essentially measured on the basis of the verticaldepth of water that would accumulate on a level surface if the without any lossessuch as evaporation and filteration where it fell. Precipitation is usually measured inmillimeters and tenths of millimeters. If it is less than 1 mm it is recorded as trace.Rainfall with an intensity of 2.5 mm/h is called light rain, between 2.5 mm/h and 7.5mm/h it is termed as moderate rain and when it exceeds 7.5 mm/h it is termed asheavy rain. A small surface area is taken for the purpose of measurement and thevolume of precipitation water collected over that area is divided by the area to givethe depth of precipitation.

The precipitation is measured by an instrument called a raingauge. Raingauge isalso variously known as Hyetometer, ombrometer or pluviometer.

The raingauges are of two types :

1. Non-recording type or ordinary raingauges

2. Recording type or automatic raingauges.

NON-RECORDING RAINGAUGES

Standard non-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

1ChapterGENERAL ASPECTS OF

HYDROLOGY &PRECIPITATION

Syllabus : Introduction, Measurement of Precipitation , Non-recordingraingauges , Recording raingauges, Average depth of precipitation overan area, Hydrologic Cycle, Water-budget equation. Weightage – 30%

2body, a receiver with a narrowneck and handle and a splayedbase which is fixed in theground. The receiver shouldhave a narrow neck and shouldbe sufficiently protected fromradiation to minimise the loss ofwater from the receiver byevaporation. To prevent rainfrom splashing in and out, thevertical wall of the sharp edgedrim is made deep enough andthe slope of the funnel steepenough (at least 450). The rainfalling into the funnel is collectedin the receiver kept inside thebody and is measured by meansof a special measure glass(supplied along with the gauge)which is graduated in mm. The receiver has a capacity of 175mm of rain. In regions of heavy rainfall, raingauges withreceivers 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 × 60 cm × 60 cmwhich is sunk into the ground. The base of the gauge is cemented so that the rim ofthe gauge is exactly 30 cm above the ground level. The top of the gauge should beperfectly horizontal.

Symon’s Gauge isnon recordingRain Gauge.

3

IMD has changed over to the use of fiberglass reinforced polyester raingauges whichare an improved version of the Symon’s gauge, which is known as standard Rain Gauge.

GUNMETAL RIM

15

120

100

183

COLLECTOR (100 cm area)2

G.L.

BASE

BOTTLE (4 liters)

183

150

24032

0

RECEIVER

15

210

157

Indian Standard raingauge

4These gauges are available in different combinations of collector areas (100 cm2

and 200 cm2) and receiver bottle capacities (2 to 10 litres). They have the capacityto measure rainfall depths of 100 mm to 1000 mm. they conform to the Indianstandards IS : 52225-1969. The details of the collector of 100 cm2 area and thereceiver for I.S. gauge.

At the routine time of observation the funnel is removed, the receiver is taken outand the rain water collected in the receiver is carefully poured into the measure glassand read without any parallax error. When the rainfall exceeds 25 mm the measureglass will be used as many times as required. The measured rainfall in the 24 hoursending with 8.30 A.M. is recorded as the rainfall of the day on which 8.30 A.M.observation is taken. In regions may not hold the entire rainfall of the day themeasurements must be done more frequently with the last measurement being takenat 8.30 A.M. The sum of all the readings taken in the last 24 hours is recorded as therainfall of that day.

RECORDING RAINGAUGES

(i) Tipping (or tilting) buckettype

(ii) Weighing bucket type

(iii) Float type (with siphonarrangement).

1. Tipping Bucket Raingauge. Itgives intensity of rainfall. Theprinciple involved in this typeof gauge is very simple. Acontainer is divided verticallyinto two compart-ments and isbalanced in an unstableequilibrium about a horizontalaxis. In its normal position itrests against one of the twostoppers which prevent it fromtipping over completely. Therain is led from a conventional

5collecting funnel into the uppermost compartment and after a predetermined rain(usually 0.25 mm) has fallen the bucket becomes unstable in its present position andtips over to its other position of rest. The compartments of the container are so shapedthat water can now flow out of the lower one and leave it empty; meanwhile the rainfalls into the upper compartment again. The movement of buckets as it tips over can beused to operate an electric circuit and produce a record. The record thus consists ofdiscontinuous steps, the distance between each step representing the time taken forsmall amount of rain to fall.

The disadvantages of this type of gauge are as follows. If the buckets are designedto tip at a convenient frequency for a particular intensity of rainfall, they will up eithertoo soon or too late for other intensities. As a result both the intensity and amount ofrainfall recorded will be in error except during a storm which has the same intensity forwhich the buckets are designed. The record obtained from this gauge is not in aconvenient form.

The biggest advantage of the tipping bucket gauge is that it is the only recordingraingauge which can be used in remote places by installing the recorder at aconvenient and easily accessible location.

2. Weighing-Bucket Raingauge. It gives mass curve of rainfall In this type of gauge therain falling on the receiving area is collected by the funnel and is led into a storagebucket which rests on aweighing platform. Theweight of the rainfall

Received since therecording began is recordedcontinuously by transmittingthe movement of the platformthrough a system of links andlevers to a pen which makes atrace on a suitably graduatedchart secured around a drum.The drum is driven mechanicallyby a spring clock. The drummay be made to revolve once aday, once a week or once in anyother desired period.

63. Float type Raingauge. It gives mass curve of rainfall. This type of raingauge is also

known as the siphon type raingauge as it uses the siphon mechanism to empty therainwater collected in the float chamber. This is adopted by I.M.D. Rain waterentering the gauge at the top is led into the float chamber through a funnel andfilter. The purpose of the filter is to prevent dust and other particles from enteringthe float chamber which may hinder the siphon mechanism. The float chamberconsists of a float with a vertical stem protruding outside, to the top of which apen 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 connectedto the float chamber through a small opening at the bottom. This is called the siphonchamber which houses a small vertical pipe with bottom end open and the top endalmost touching the top of the chamber.

200mm

RECEIVER

FUNNEL

CLOCK DRIVEN DRUM WITH CHART

COVER

FLOAT CHAMBER

FLOAT

FILTER

SYPHONCHAMBER

Float type rainguage

Telemetering Raingauges

These raingauges are of the recording type and contain electronic units to transmitthe data on rainfall to a base station both at regular intervals and on interrogation. The

7tipping-bucket type raingauge is usually adopted for this purpose. Telemetering gaugesare of utmost use in gathering rainfall data from mountainous and generally inaccessibleplaces.

Radar Measurement of Rainfall

The meteorological radar is a powerful instrument for measuring the areal extent,location and movement of rain storms. Further, the amounts of rainfall over large areascan be determined through the radar with a good degree of accuracy.

The radar emits a regular succession of pulses of electromagnetic radiation in a

narrow beam. When raindrops intercept a radar beam, it has been shown that R 2

CZPr

where Pr = average echopower, Z= radar-echo factor, r = distance to target volume andC = a constant. Generally, the factor Z is related to the intensity of rainfall as bz alwhere a and b coefficients and I = intensity of rainfall in mm/h. The values a and b for agiven radar station have to be determined by calibration with the help of recordingraingauges. A typical for Z is 1.60200Z l .

AVERAGE DEPTH OF PRECIPITATION OVER AN AREA

The average depth of rainfall is also termed as equivalent uniform depth of rainfall.

(i) Arithmetic mean method

(ii) Thiessen Polygon method

(iii) Isohyetal method.

The first two methods are purely mechanical processes requiring no special skill orjudgment. On the other hand, the results obtained by the third method, which perhapsshould be the most accurate, depend for their accuracy upon good judgment of theperson making the computations.

1. Arithmetic Mean Method. This is the simplest of the three methods. As the nameimplies the result is obtained by dividing the sum of the rainfall amounts recordedat all the raingauge station which are located within the area under considerationby the number of stations.

n

iiP

nnnPPPP

1

21 1.............

8Where P is the average depth of rainfall and P1, P2,…Pn are the rainfalls recorded at

stations 1, 2, … etc. and n is the number of raingauge stations within the area. This methodis also known as the unweighted mean method since the same weightage is given to rainfallrecorded at all the gauges irrespective of their locations.

If the area is flat and the gauges are distributed uniformly over the area and if thevariation of individual gauge record from the mean is not too large, this method isprobably as accurate as any other method. Even in hilly terrains this method will yieldfairly satisfactory results if the orographic influences on rainfall are considered inselecting the gauge sites. This method is very rapid even by manual computations andhas got excellent adaptability to computer processing.

PROBABLE MAXIMUM PRECIPITATION (PMP)

The greatest depth of rainfall that can occur in a given duration at a given location isknown as the possible maximum, or the probable maximum precipitation, (PMP).

RAINGAUGE NETWORK

The ratio of total area of the catchment to the total number of gauges in thecatchment is defined as the raingauge density or the network density.

The adequacy of the number of gauges in the existing network may be ascertainedfollowing the procedure given by Indian Standards. The optimum numbers of raingaguesN is given by the equation

2

pC

N v

Where

p is the allowable degree of error in estimating the average depth of rainfall overthe area and Cv is the coefficient of variation of the rainfall recorded at the m existingraingauge stations given by (sx/x) × 100. If N < m, the existing network estimates theaverage depth of rainfall with an error less than the allowable value of ñ and no moregauges are required.

If N > m, the additional gauges required is given by be so located within thearea that along with the existing gauges they are evenly distributed over the entirebasin.

9HYDROLOGY

Hydrology means the science of water. It is the science that deals with theoccurrence, circulation and distribution of water of the earth and the earth’satmosphere.

Hydrologic Cycle

Water occurs on the earth in all its three states, viz. liquid, solid and gaseous, and invarious degrees of motion. Evaporation of water from water bodies such as oceansand lakes, formation and movement of clouds, rain and snowfall, streamflow andgroundwater movement are some example of the dynamic aspects of water. The variousaspects of water related to the earth can be explained in terms of a cycle known as thehydrologic cycle.

Horton’s Representation of the Hydrological Cycle

The main components of the hydrologic cycle can be broadly classified as trans-portation (flow) components and storage components as below:

10Transportation components Storage components

• Precipitation • Storage on the land surface (Depressionstorage, Ponds, Lakes, Reservoirs, etc)

• Evaporation • Soil moisture storage

• Transpiration • Groundwater storage

• Runoff

Water-Budget Equation

Catchment Area

The area of land draining into a stream or a water course at a given location is knownas the catchment area. It is also called drainage area or drainage basin. In the USA, it isknown as watershed.

The areal extent of the catchment is obtained by tracing the ridge on a topgrapic map

to delineate the catchment and measuring the area by a planimeter.

Water-Budget Equation

A catchment, in an interval of time Δt, the continuity equation for water in its variousphases is written as

Mass inflow – Mass outflow = Change in mass storage

If the density of the inflow, outflow and storage volumes are the same,

Si 0

Where i inflow volume of water into the problem area during the time period,0 Outflow volume of water from the problem area during the time period, and ΔS =

change in the storage of the water volume over and under the given area during thegiven period.

While realising that all the terms in a hydrological water budget may not be knownto the same degree of accuracy, an expression for the water budget of a catchment fora time interval Δt is written as

P – R – G – E – T = ΔS

11In this, P = precipitation, R = surface runoff, G = net groundwater flow out of thecatchment, E = evaporation, T = transpiration and ΔS = change in storage.

The storage S consists of three components as

S = Ss + Ssm + Sg

Where Ss = surface water storage,

Ssm = water in storage as soil moisture, and

Sg = water in storage as groundwater.

ΔSs = ΔSsm + ΔSg

In rainfall-runoff relationship, can be represented as

R = P – L

Where L = losses = water not available to runoff due to infiltration (causing additionto soil moisture and groundwater storage), evaporation, transpiration and surfacestorage.

12

1. A Double – Mass – Curve Analysis isuseful in

(a) Consistency Analysis

(b)Frequency Analysis

(c) Storage Computation Analysis

(d)Guessing missing data in cases ofnon – homogeneous terrain

2. Orographic rain occurs when the airis cooled sufficiently as a result of

(a) Lifting due to flow over amountain barrier

(b)Relative movement of two largeair masses

(c) V iolent upthrow of air arisingfrom localized heating

(d)Cyclonic conditions

3. An isohyet is a line joining points of

(a) Equal temperature

(b)Equal humidity

(c) Equal rainfall depth

(d)Equal evaporation

4. An isochrones is a line on the basinmap

(a) Joining raingauge stations havingequal rainfall duration

(b) Joining points having equal rainfalldepth in a given time interval

(c) Joining points having equal timeof travel of surface runoff to thecatchment outlet

(d) Joining points which are at equaldistance from the catchmentoutlet

5. A hyetograph is a graph representing

(a) Rainfall volume with time

(b)Rainfall intensity with time

(c) Rainfall volume with duration

(d)Rainfall intensity over an area

6. If ‘p’ is the precipitation, ‘a’ is the arearepresented by a rain gauge, and ‘n’is the number of rain gauges in acatchment area, then the weightedmean rainfall is

(a) (b)

(c) (d)

7. The Thiessen polygon is

(a) A polygon obtained by joiningadjoining raingauge stations

(b)A representative area used forweighing the observed stationprecipitation

Practice Problems

13(c) An are used in the construction of

depth area curves

(d)The descriptive term for shape ofa hydrograph

8. Standard rain gauge adopted in India is

(a) Natural siphon type

(b)Tipping bucket type

(c) Weighing type

(d)None of these

9. Which of the following rain gaugesgive mass curve?

(a) Natural siphon type

(b)Weighting bucket type

(c) Tipping bucket type

(d)Both (a) and (b)

10. Match List-1 with List-2 and select thecorrect answer using the codes givenbelow the lists:

List – 1 List – 2A Thiessen polygen1 Average

depth ofrainfall overan area

B Mass curve 2 Relationshipof rainfallintensity andtime

C Hyetograph 3 Relationshipof accumu-lated rainfalland time

D DAD curve 4 Relationshipof rive

runoff andtime

5 Always afailing curve

Codes:A B C D

(a) 1 5 3 2(b) 1 3 2 5

(c) 4 3 2 5

(d) 4 5 3 2


List – 1 List – 2

A Rainfall intensity 1 Isohyets

B Rainfall excess 2 Cumulativerainfall

C Rainfall averaging3 Hyetograph

D Mass curve 4 Direct runoffhydrograph

Codes:

A B C D

(a) 1 3 2 4

(b) 1 3 4 2

(c) 3 4 1 2

(d) 3 4 2 1

12. The science which deals withoccurrence, distribution andcirculation of water is called

(a) Hydrography

(b)Hydrometry

14(c) Hydrology

(d)Hydraulics.

13. IHP stands for(a) Intensive Hydraulic Press(b) International Hydrologic Publications(c) International Hydrologic Periodicals(d) International Hydrologic Programme.

14. The rainfall hyetograph is the graphdrawn between(a) cumulative rainfall and time(b) rainfall intensity and time(c) rainfall depth and area(d) rainfall intensity and cumulative rainfall.

15. The rainfall mass curve shows thevariation of(a) rainfall intensity with time(b) rainfall intensity with cumulative

rainfall(c) rainfall excess with time(d)cumulative rainfall with time.

16. Double mass curve technique is used(a) to prepare the rainfall hyetograph

from the rainfall mass curve(b) to prepare the rainfall mass curve

from the rainfall hyetograph(c) to check the consistency of record

at a suspected raingauge station(d) in developing isohyetal maps.

17. The most accurate method of finding theaverage depth of rainfall over an area is(a) Isohyetal method(b)Arithmetic mean method

(c) Thiessen polygon method(d)any of the above.

18. The Thiessen weights of 4 raingaugesA, B, C and D covering a river basinare 0.15, 0.25, 0.30 and 0.30respectively. If the average depth ofrainfall for the basin is 5 cm, and therainfalls recorded at B, C and D are 5cm, 4 cm and 5 cm respectively. Whatis the rainfall at A?(a) 5 cm (b) 6 cm(c) 7 cm (d) 8 cm.

19. For a given storm the average depthof rainfall over an area(a) decreases with increase in area(b) increases with increase in area(c) has no relation with area(d)none of the above.

20. The chart removed from a recordingtype raingauge gives(a) a rainfall hyetograph(b) an isohyetal map(c) a rainfall mass curve(d) an intensity duration curve.

21. Any cyclic trend present in the rainfalldata can be ascertained from(a) depth-area-duration curve(b) moving Average curve(c) intensity-duration curve(d)double mass curve.

22. Match List – I (Type of precipitation)with List – II (Principal causes) andselect the correct answer using thecodes given below the lists:

15List - I List – II

A. Convective 1. Temperaturedifference

B. Cyclonic 2. Mountainbarrier

C. Frontal 3. Pressuredifference

D. Orographic 4. Warm and coldair masses

Codes:A B C D

(a) 1 3 4 2(b) 4 3 1 2(c) 1 4 2 3(d) 4 3 2 1

(ESE - 2003)23. The plan area of a reservoir is 1 km2. The

water level in the reservoir is observed todecline by 20 cm in a certain period. Duringthis period the reservoir receives a surfaceinflow of 10 hectare – meters, and 20hectare – meters are abstracted from thereservoir for irrigation and power. The panevaporation and rainfall recorded duringthe same period at a nearby meteorologicalstation are 12 cm and 3 cm respectively.The calibrated pan factor 0.7. The seepageloss from the reservoir during this periodin hectare – meters is(a) 0.0(b)1.0(c) 2.4(d)4.6

(GATE – 2003 – 2 Mark)

24. Match List – I (Type of precipitation)with List – II (Principal causes) andselect the correct answer using thecodes given below the lists:

List - I List – IIA. Rainfall intensity 1. IsohyetsB. Rainfall excess 2. Cumulative

rainfallC. Rainfall 3. Hyetograph

averagingD. Mass curve 4. Direct runoff

hydrographCodes:

A B C D(a) 1 3 2 4(b) 3 4 1 2(c) 1 2 4 3(d) 3 4 2 1

(GATE – 2003 – 2 Mark)

25. A catchment is idealized as a 25 km x25 km square. It has five rain gauges,one at each corner and one at thecenter, as shown in the figure.

16During a month, the precipitation atthese gauges is measured as G1 = 300mm, G2 = 285 mm, G3 = 272 mm, G4 =290 mm and G5 = 288 mm. Theaverage precipitation (in mm, up to

the one decimal place) over thecatchment during this month byusing the Thiessen polygon methodis ______________.

GATE - I – 2017- 2 Marks)

Answer Key

1. (A) 2. (A) 3. (C) 4. (C) 5. (B)

6. (C) 7. (B) 8. (A) 9. (D) 10. (B)

11. (C) 12. (C) 13. (D) 14. (B) 15. (D)

16. (C) 17. (A) 18. (C) 19. (A) 20. (C)

21. (B) 22. (A) 23. (D) 24. (B)

25. (287.375 mm

17

Factors Affecting of Evaporation

1. Vapour Pressure

The rate of evaporation is proportional to the difference between the saturationvapour pressure at the water temperature, ew and the actual vapour pressure in theair, ea. thus,

EL = C(ew – ea)

Where

EL = rate of evaporation (mm/day)

C = a constant;

ew = saturated vapour pressure at the water surface temperature in mm of mercury,

ea = actual vapour pressure of overlying air at a specified height in mm of mercury,

Evaporation continues till ew = ea. If ew > ea condensation takes place.

2. Temperature

The same, the rate of evaporation increases with an increase in the water tem-perature. Regarding air temperature, although there is a general increase in the evapo-ration rate with increasing temperature, a high correlation between evaporation rateand air temperature does not exist. Thus, for the same mean monthly temperature, itis possible to have evaporation to different degrees in a lake in different months.

3. Wind

Wind aids in removing the evaporated water vapour from the zone of evaporationand consequently creates greater scope for evaporation. However, if the wind velocity

2ChapterEVAPORATION,

TRANSPIRATION, STREAMFLOW AND ITS EASUREMENTSyllabus : Factors Affecting of Evaporation, Types of Evapormeters, Emprical EvaporationEquations, Methods of Evaporation Estimation, Methods to Reduce Evaporation Losses,Transpiration, Evapotraspiration, Measurement of Stage, Measurement of Velocity,Area-Velocity Method, Dilution Technique of Streamflow Measurement, ElectromagneticMethod, Ultrasonic Method, MEASUREMENT OF EVAPORATION Weightage – 25%

18is large enough to remove all the evaporated water vapour, any further increase inwind velocity does not influence the evaporation. Thus, the rate of evaporation in-creases with the wind speed up to a critical speed beyond which any further increasein the wind speed has no influence on the evaporation rate. This critical wind-speedvalue is a function of the size of the water surface. For large water bodies, high-speedturbulent winds are needed to cause maximum rate of evaporation.

3. Atmospheric Pressure

The decrease in the barometric pressure, as in high altitudes, increase evapora-tion.

4. Quality of Water

When a solute is dissolved in water, the vapour pressure of the solution is less thanthat of pure water and hence causes reduction in the rate of evaporation. The percentreduction in evaporation approximately corresponds to the percentage increase in thespecific gravity. Thus, for example, under identical conditions, evaporation from seawater is about 2-3% less than that from fresh water.

5. Size of Water Body

Deep water bodies have more heat storage than shallow ones. A deep lake may storeradiation energy received in summer and release it in winter causing less evaporationin summer and more evaporation in winter compared to a shallow lake exposed to asimilar situation. However, the effect of heat storage is essentially to change the sea-sonal evaporation rates and the annual evaporation rate is seldom affected.

Types of EVAPORIMETERS

Evaporimeters are water-containing pans which are exposed to the atmosphereand the loss of water by evaporation is measured in them at regular intervals. Meteo-rological data, such as humidity, wind movement, air and water temperatures, andprecipitation are also noted along with evaporation measurement.

Many types of evaporimeters are in use and a few commonly used pans aredescribed here.

1. Class A Evaporation Pan

It is a standard pan 1210 mm diameter and 255 mm depth used by the US WeatherBureau and is known as Class A Land Pan. The depth of water is maintained between

1918 cm and 20 cm. The pan is nor-mally made sheet. Monel metalis used where corrosion is prob-lem. The pan is placed on awooden platform of 15 cmheight above the ground to allowfree circulation of air below thepan. Evaporation measurementsare made by measuring thedepth of water with a hookgauge in a stilling well.

2. ISI Standard Pan

This pan evaporimeter specified by IS: 5973–1970, also known as modified Class APan, consists of a pan 1220 mm in diameter with 255 mm of depth. The pan is made ofcopper sheet of 0.9 mm thickness, tinned inside and painted white outside. A fixedpoint gauge indicates the level of water. A calibrated cylindrical measure is issued toadd or remove water maintaining the water level in the pan to a fixed mark. The top ofthe pan is covered fully with a hexagonal wire netting of galvanized iron to protect the

US Class A Evaporation Pan

Water level in pan

GL

50

255

150

1210 Dia.Wooden

Support (SQ)

ISI Standard Evaporation Pan

1220 Thermometer clamp

ThermometerWire-mesh cover Stilling well

Fixed point gauge 102

Copper sheet thickness 0.9 10

15190

235 255 Pan10

0

200 Wooden platform75

1225 Sq

25

20water in the pan from birds. The presence of a wire mesh makes the water tempera-ture more uniform during day and night. The evaporation from this pan is found to beless by about 14% compared to that from an unscreened pan. The pan is placed over asquare wooden platform of 1225 mm width and 100 mm height to enable circulationof air underneath the pan.

3. Colorado Sunken Pan

This pan, 920 mm square and 460 mmdeep, is made up of unpaintedgalvanised iron sheet and buried intothe ground within 100 mm of the top.The chief advantage of the sunken panis that radiation and aerodynamic char-acteristics are similar to those of a lake.

Disadvantages:

(1) Difficult to detect leaks,

(2) Extra care is needed to keep the surrounding area free from tall grass, dust, etc., and

(3) Expensive to install.

4. US Geological Survey Floating Pan

With a view to simulate the characteristics of a large body of a water, this square pan(900 mm side and 450 mm depth) supported by drum floats in the middle of a raft(4.25 m × 4.87 m) is set afloat in lake. The water level in the pan is kept at the same levelas the lake leaving a rim of 75 mm. diagonal baffles provided in the pan reduce thesurging in the pan due to wave action.

Disadvantages:

1. Its high cost of installation and maintenance together with the difficulty involvedin performing measurements.

Empirical Evaporation Equations

A large number of empirical equations are available to estimate lake evaporation usingcommonly available meteorological data. Most formulae are based on the Dalton-typeequation and can be expressed in the general form

EL = Kf(u) (ew–ea)

920 Sq.

Water levelsame as GL

GL50

460

Colorado Sunken Evaporation Pan

21Where EL = lake evaporation in mm/day, ew saturated vapour pressure at the watersurface temperature in mm of mercury, ea = actual vapour pressure of overlying air ata specified height in mm of mercury, f(u) = wind-speed correction function and K = acoefficient. The term ea measured at the same height at which wind speed is mea-sured. Empirical evaporation formulas:

Meyer’s Formula (115)

161)( 9u

eeKE awML

EL = lake evaporation in mm/day,



u9 = monthly mean wind velocity in km/h at about 9 m above ground and

KM = coefficient accounting for various other factors with a value of 0.36 forlarge deep waters and 0.50 for small, shallow waters.

Rohwer’s Formula (1931)

Rohwer’s formula considers a correction for the effect of pressure in addition to thewind-speed effect and is given by

EL = 0.771(1.465–0.000732 pa)(0.44 + 0.0733 u0)(ew – ea)

EL = lake evaporation in mm/day,



pa = mean barometric breading in mm of mercury

u0 = mean wind velocity in km/h at ground level, which can be taken to bethe velocity at 0.6 m height above ground.

Methods of Evaporation Estimation

Water-Budget Method

The water-budget method is the simplest of the three analytical methods and is also theleast reliable. It involves writing the hydrological continuity equation for the lake and

22determining the evaporation from a knowledge or estimation of other variables. Thus,considering the daily average values for a lake, the continuity equation is written as

P + Vis + Vig = Vog + EL + ΔS + TL

Where p = daily precipitation

Vis = daily surface inflow into the lake

Vig = daily groundwater inflow

Vos = daily surface outflow from the lake

Vog = daily seepage outflow

EL = daily lake evaporation

ΔS = increase in lake storage in a day

TL = daily transpiration loss

All quantities are in units of volume (m3) or depth (mm) over a reference area.

EL + P + (Vis – Vos) + (Vig – Vog) – TL – ΔS

In this, the terms P, Vis, Vos and ΔS can be measured. However, it is not possible tomeasure Vig. Vog and TL and therefore these quantities can only be estimated. Transpi-ration losses can be considered to be insignificant in some reservoirs. If the unit oftime is kept large, say weeks or months, better accuracy in the estimate of EL is pos-sible. In view of the various uncertainties in the estimated values and the possibilitiesof errors in measured variables, the water-budget method cannot be expected to givevery accurate results.

Energy-Budget Method

The energy-budget method is an application of the law of conservation of energy. Theenergy available for evaporation is determined by considering the incoming energy,outgoing energy and energy stored in the water body over a known time interval.

The energy balance to the evaporating surface in a period of one day is give by

Hn = Ha + He + Hg + Hs + Hi

where Hn = net heat energy received by the water surface

= Hc(1 – r) – Hb

23

in which Hc(1 – r) = incoming solar radiation into a surface of reflection Coefficient (albedo) r

Hb = back radiation (long waver) from water body

Ha = sensible heat transfer from water surface to air

= ρ LEL, where ρ = density of water, L = latent heat of

Evaporation and EL = evaporation in mm

Hg = heat flux into the ground

Hs = heat stored in water body

H i = net heat conducted out of the system by water flow

All the energy terms are in calories per square mm per day. If the time peri-ods are short, the terms Hs can be neglected as negligibly small. All the terms ex-cept Ha can either be measured or evaluated indirectly. The sensible heat term Ha

which cannot be readily measured is estimated using Bowen’s ratio β given by theexpression

aw

awa

L

a

eeTTp

LEH

10101.6

Back radiationHb

Heat loss to airHa

Solar radiation Hc

Reflected rHc Evaporation

p L EL

(1–r) HcWater

Heat storedHsHeat flux

into the ground Hg

AdvectionHi

Energy balance in a water body

24where

pa = atmospheric pressure in mm of mercury,

ew = saturated vapour pressure in mm of mercury,

ea = actual vapour pressure of air in mm of mercury,

Tw = temperature of water surface in ºC and

Ta = temperature of air in ºC. from EL can be evauated as

)1(

LHHHH

E isgnL

Estimation of evaporation in a lake by the energy balance method has been foundto give satisfactory results, with errors of the order 5% when applied to periods lessthan a week.

Methods to Reduce Evaporation Losses

Various methods available for reduction of evaporation losses can be considered inthree categories.

1. Reduction of surface Area

Since the volume of water lost by evaporation is directly proportional to the surfacearea of the water body, the reduction of surface area wherever feasible reduces evapo-ration losses. Measures like having deep reservoirs in place of wider ones and elimina-tion of shallow areas can be considered under this category.

2. Mechanical covers

Permanent roofs over the reservoir, temporary roofs and floating roofs such as raftsand lightweight: floating particles can be adopted wherever feasible. Obviously, thesemeasures are limited to very small water bodies such as ponds, etc.

3. Chemical Films

This method consists of applying a thin chemical film on the water surface to reduceevaporation. Currently this is the only feasible method available for reduction of evapo-ration of reservoirs up to moderate size.

Certain chemicals such as cetyl alcohol (hexadecanol) and stearyl alcohol(octadecanol) form monomolecular layers on a water surface.

25Transpiration

Transpiration is the process by which water leaves the body of a living plant and reachesthe atmosphere as water vapour. The water is taken up by the plant-roof system andescapes through the leaves. The important factors affecting transpiration are: atmo-spheric vapour pressure, temperature, wind, light intensity and characteristics of theplant, such as the root and leaf systems.

Evapotraspiration

If sufficient moisture is always available to completely meet the needs of vegeta-tion fully covering the area, the resulting evapotranspiration is called potentialevapotranspiration (PET). Potential evapotranspiration no longer critically dependson the soil and plant factors but depends essentially on the climatic factors. Thereal evapotranspiration occurring in a specific situation is called actual evapotrans-piration (AET).

AET = PET mean soil moisture will be at the field. (If the water supply is less thanPET. The soil dries out and the ratio AET/PET would then be less than unity.) The de-crease of the ration AET/PET with available moisture depends upon the type of soil andrate of drying.

For clayey soils, AET/PET = 1.0 for nearly 50% drop in the available moisture.

When the soil moisture reaches the permanent wilting point, the AET reduces tozero. For a catchment in a given period of time, the hydrologic budget can be written as

P – Rs – Go – Eact = ΔS

Where

P = precipitation,

Rs = surface runoff,

Go = subsurface outflow,

Eact = actual evapotranspiration (AET) and ΔS = change in the moisture storage.This water budgeting can be used to calculate Eact by knowing or estimating other ele-ments. Generally, the sum of Rs anmd Go can be taken as the stream flow at the basinoutlet without much error.

26

Measurement of Evapotranspiration. There are mainly four methods of direct mea-surement of evapotranspiration. They are :

(i) Water-budget method,

(ii) Field experimental plots,

(iii) Soil moisture depletion studies, and

(iv) Lysimeter method.

Water-budget-method. This method, also known as inflow outflow method, is suit-able for evaluating evapotranspiration of large area such as watersheds over relativelylong period of time. The evapotranspiration can be evaluated provded all other con-stituent items of the following water balalnce equation are known

Water supply = Water disposal

Field Experimental Plots. In this method a field plot is chosen and the amounts ofwater added to the irrigation plot under observation by way of prcepitation and irriga-tion are measured along with runoff. The moisture content in various layers of the soil

0 0

50

100

20406080100

(per

cent

age)

AET

PET

FC=Field capacityPWP = Permanent wilting point

Sandy soil

Perecent available moisturePWPFC

Clayey soil

Variation of AET

27within the root zone depth are measured both at beginning and end of the crop sea-son. Then the evaportanspiration is computed as

ET = I – Q – ΔS

Where I is the total inflow in mm including precipitation and irrigation water, Q is thetotal surface runoff in mm and ΔS is the increase in soil moisture storage in mm which iscomputed from the following equation

iieibi

n

iDGMMS

1001

Where Mbi = Moisture content in per cent at the beginning of the season in ith layer

Mei = Moisture content in per cent at the end of the season in ith layer

Gi = The apparent specific gravity of the ith layer of the soil

Di = Depth of ith layer of soil, in mm, within the root zone depth

n = The number of soil layers considered in the entire root zone.

Soil moisture depletion studies. These studies involve measurement of soil moisturefrom various depths at as frequent intervals of time as possible through the growthperiod of a crop. This method gives satisfactory results for irrigated field crops grownon fairly uniform soils and where the soil moisture variations within the root zone arenot influenced by groundwater. the evapotranspiration u, for any time period betweentwo successive samplings is obtained as

iiii

n

iDGMMu

10021

1

where M1i = Soil moisture percentage at the time of first sampling in the ithe layer

M2i = Soil moisture percentage at the time of second sampling in the ithlayer

The terms Gi, Di and n have the same significance.

Lysimeter. A lysimeter (also known as evapotranspirometer) consists of a circular tankabout 60 to 90 cm in diameter and 180 cm deep. sometimes large diameters upto 3 mwith 3 in depth are also used. The lysimeter is filled with soil and individual crops ornatural vegetable, for which the evapotranspiration is required, are grown. it is buriedso that its top is in flush with the surrounding ground surface. the sides of the lysimeter

28are impervious whereas the bottom is pervious. Water passing through the soil is col-lected at the bottom and conducted through a small tube to a measuring gauge in anadjacent pit. Soil moisture measurements may be obtained from moisture sampling,weighing, or hydraulic method. In weighing type, the tank is mounted on self recordingscale. In the hydraulic method the tank is floated on water or a suitable heavy liquid andchanges in weight are recorded through pressure changes observed in a manometer.

Measurement of Stage

The stage of a river is defined as its water-surface elevation measured above a datum.This datum can be the Mean-Sea Level (MSL) or any arbitrary datum connected inde-pendently to the MSL.

Manual Gauges

1. Staff Gauge

The simplest of stage measurements are made by noting the elevation of the watersurface in contact with a fixed graduated staff. The staff is made of a durable materialwith a low coefficient of expansion with respect to both temperature and moisture. Itis fixed rigidly to a structure, such as an abutment, pier, wall, etc. The staff may bemarkings are distinctive, easy to read from a distance and are similar markings.

2. Wire Gauge

It is a gauge used to measure the water-surface elevation from above the surface suchas from a bridge or similar structure. In this, a weight is lowered by a reel to touch the

Staff Gauge

Staffgauge Abutment

Vertical staff gauge Sectional gauge

29water surface. A mechanical counter measures the rotation of the wheel which is pro-portional to the length of the wire paid out. The operating range of this kind of gauge isabout 25 m.

Automatic Stage Recorders

Automatic stage recorders overcome this basic objection of manual staff gauges andfind considerable use in stream-flow measurement practice.

Two type automatic stage recorders:

1. Float-Gauge Recorder

The float-operatedstage recorder isthe most commontype of automaticstage recorder inuse. In this, a floatoperating in a still-ing well is balancedby means of ac o u n t e r w e i g h tover the pulley of arecorder. Displace-ment of the floatdue to the rising orlowering of the wa-ter-surface eleva-tion causes an an-gular displacementof the pulley andhence of the input shaft of the recorder. Mechanical linkages covert this angular dis-placement to the linear displacement of a pen to record over a drum driven by clock-work. The pen traverse is continuous with automatic reversing when it reaches the fullwidth of the chart.

Stilling well Installation

Flush tankWalkway

Staff gaugeIntakes

Float

Counterweight

RecorderManhole

302. Bubble Gauge

In this gauge, compressed air or gas is made to bleed out at a very small rate throughan outlet placed at the bottom of the river.

Advantages :

1. There is no need for costly stilling wells

2. A large change in the stage, as much as 30 m, can be measured

3. The recorder assembly can be quite far away from the sensing point.

4. Due to constant bleeding action there is less likelihood of the getting blocked orchoked.

Instrument room

p = H

Gas circuit

Pressure point Reference level

H

1. High pressure bottle2. Gas adjustments unit3. To pressure point 4. Mercury manometer5. Recorder

1

2

3

4

5

31Measurement of Velocity

The measurement of velocity is an important aspect of many direct stream-flow mea-surement techniques. A mechanical device, called current meter, consisting essentiallyof a rotating element is probably the most commonly used instrument for accuratedetermination of the stream-velocity field. Approximate stream velocities can be de-termined by floats.

Current Meters

The most commonly used instrument in hydrometry to measure the velocity at a pointin the flow cross-section is the current meter. It consists essentially of a rotating ele-ment which rotates due to the reaction of the stream current with an angular velocityproportional to the stream velocity.

There are two main types of current meters.

1. Vertical-axis meters, and

2. Horizontal-axis meters.

1. Vertical-Axis Meters

These instruments consist of a series ofconical cups mounted around a verticalaxis. The cups rotate in a horizontalplane and a cam attached to the verticalaxial spindle records generated signalsproportional to the revolutions of thecup assembly. The Price current meterand Gurley current meter are typical in-struments under this category. The nor-mal range of velocities is from 0.15 to4.0 m/s. the accuracy of the thresholdvalue and improves to about 0.30% at speeds in excess of 1.0 m/s.

Disadvantages :- They cannot be used in situations where are appreciable vertical com-ponents of velocities. For example, the instrument shows a positive velocity when it islifted vertically in still water.

Vertical-axis Current Meter

Countingmechanism

Electricalconnection Hoist Stabilizing fin

Cup assembly 6cups on a vertical axis

Sounding weight

322. Horizontal-Axis Meters

These meters consist of a propeller mounted at the end of horizontal shaft. These comein a wide variety of size with propeller diameters in the range 6 to 12 cm, and can 4.0 m/s. Ott, Neyrtec and Watt-type meters are typical instruments under this kind. These metersare fairly rugged and are not affected by oblique flows of as much as 15º. The accuracyof the instrument is about 1% at the threshold value and is about 0.25% at a velocity of0.3 m/s and above.

Area-Velocity Method

This method of discharge measurement consists essentially of measuring the area ofcross section of the river at a selected section called the gauging site and measuringthe velocity of flow through the cross-sectional area.

The following criteria are adopted:

1. The stream should have a well-defined cross section which does not change invarious seasons.

2. It should be easily accessible all through the year.

3. The site should be in a straight, stable reach.

4. The gauging site should be free from backwater effects in the channel.

At the selected site, the section line is marked off by permanent survey markingsand the cross section determined. Towards this, the depth at various locations are

W 1 W 2 W3

Verticals

W l–1 Wl W l+1 W N–1 WN

y1

y2

y3 y4

yi–1

yi

yi+1

yN–1

N–1

A1

23

4i–1 i i+1

Boundary ofsub-section

Stream Section for Area-velocioty Method

33measured by sounding rods or sounding weights. When the stream depth is large orwhen quick and accurate depth measurements are needed, an electroacoustic instru-ment call echo-depth recorder is used.

Q = A. V

The measurement of discharge thus involves obtaining information at a numberof points over the cross-section sufficient to determine the average velocity andalso the area of flow.

The segment width should not be greater than 1/15 to 1/20 of the width of theriver.

The discharge in each segment should be less than 10% of the total discharge.

The difference of velocities in adjacent segments should not be more than 20%.

It should be noted that in natural rivers, the verticals for velocity measurementare not necessarily equally spaced. The area-velocity method as above using the cur-rent meter is often called the standard current meter method.)

Dilution Technique of Streamflow Measurement

The dilution method of flow measurement, also known as the chemical method, de-pends upon the continuity principle applied to a tracer which is allowed to mix com-pletely with the flow.

Tracers

The tracer used should have ideally the following properties:

1. It should not be absorbed by the sediment, channel boundary and vegetation. Itshould not chemically react with any of the above surfaces and also should notbe lost by evaporation.

2. It should be non-toxic.

3. It should be capable of being detected in a distinctive manner in small concen-trations.

4. It should not be very expensive.

The tracers used are of three main types:

1. Chemicals (common salt and sodium dichromate are typical)

342. Fluorescent dyes (Rhodamine-WT and Sulpho-Rhodamine B Extra are typical)

3. Radioactive materials (such as Bromine-82, Sodium-24 and Iodine-132)

Use

The dilution method has the major advantage that the discharge is estimated directlyin an absolute way. It is a particularly attractive method for small turbulent streams,such as those in mountainous areas. where suitable, it can be used as an occasionalmethod for checking the calibration, stage-discharge curves, etc. obtained by othermethods.

Electromagnetic Method

The electromagnetic method is based on the Faraday’s principle that an emf isinduced in the conductor (water in the present case) when it cuts a normal magneticfield. Large coils buried at the bottom of the channel carry a current I to produce acontrolled vertical magnetic field.

Ultrasonic Method

This is essentially an area-velocity method with the average velocity being measuredby using ultrasonic signals. The method was first reported by Swengel (1955), sincethen it has been perfected and complete systems are available commercially.

The specific advantages of the ultrasonic system of river gauging are the following:

1. It is rapid and gives high accuracy.

2. It is suitable for automatic recording of data.

3. It can handle rapid changes in the magnitude and direction of flow, as in tidalrivers.

4. The cost of installation is independent of the size of rivers.

The water which constitutes the flow in the surface stream is called streamflow. Ifthe steamflow is unaffected by the artificial diversions, storage, or other works of manin or on the stream channels, then it is called as runoff.

Evaporation is the process by which water from liquid or solid state passes intothe vapour state and is diffused into atmosphere. When water is converted from solidstate to vapour state without passing through liquid state then it is called sublimation.

35The process by which water passes from liquid to vapour state through plant metabo-lism is termed transpiration. The process by which water is evaporated from wet sur-faces and transpired by plants together is called the evapotranspiration.

MEASUREMENT OF EVAPORATION

Evaporation is usually measured either by atmometers or by the evaporationpans also known as the evaporimeters.

Atmometers. The atmometers are provided with some special surfaces which are keptwet and from these surfaces the evaporation takes place. There will be some source tocontinuously supply water and keep the evaporating surface wet. The water loss ob-served at the source is an indication of the evaporation.

Evaporation Pans. The pans are commonly use as evaporation measuring devices be-cause they are inexpensive and simple to instrument. In evaporation pans the depth ofevaporation during any time interval is measured as the drop in water surface level inthe pan in that interval standard raingauge. The observations are usually taken on dailybasis and after taking the measurement on each day the water level on daily basis andto a stipulated value by adding or removing required amount of water. Any cylindricalvessel of 1.2 to 1.8 m diameter, 0.3 m high with open top and made of galvanised ironsheet can be used as an evaporation pan. The water level within the pan is maintainedbetween 5 to 7.5 cm below the rim of the pan.

Pan Coefficient. A Criticism against the use of evaporation measurements obtainedfrom pans in the designs may be leveled on the basis that the evaporation measuredfrom a small pan will not be same as the evaporation from a large lake or reservoirsince the exposure conditions are not identical in both cases. Specifically, the heat stor-ing capacity and the heat transferred from the side and bottom of a pan are quitedifferent from those of a large lake or reservoir. Also the height of the rim above thewater surface in an evaporation pan affects the wind action over the surface. In addi-tion it creates a shadow of variable magnitude over the water surface which affectsthe radiation incident to the water surface. The ratio of reservoir evaporation to panevaporation, called the pan coefficient, is constant on annual basis and does not varyexcessively from region to region for a given type of pan.

panfromnevaporatioMeasuredreservoir from nevaporatio Actual tCoefficienpan

36There are mainly three types of pans that are in common use, namely Sunken

pan, Floating pan and Surface pan.

ISI Standard Pan. The standard pan, also known as the modified class A pan, speci-fied by IS : 5973—1970 is made of 0.9 mm thick copper sheet. It is 122 cm in diameterand 25.5 cm deep. It is tinned inside and painted white outside. It is placed on a 10 cmhigh square platform of 122.5 cm width made of wood. A fixed point gauge is kept inthe pan to indicate the water level. The top of the pan is covered fully with a hexagonalmesh of galvanised iron to protect it from birds. If is filled with water to a depth of 19cm. A calibrated cylindrical measure is used to add or remove water in order to main-tain the water level in the pan to a fixed mark indicated by the pointer gauge in thestilling well. The pan coefficient for this pan ranges from 0.65 to 1.10 and an averagevalue of 0.8 is generally recommended.

Methods of Measurement of Evaptranspiration.

(i) Water-budget method

(ii) Field experimental plots

(iii) Soil moisture depletion studies

(iv) Lysimeter method

Water-budget-method. This method, also known as inflow outflow method, issuitable for evaluating evapotranspiration of large area such as watersheds over rela-tively long period of time. The evapotranspiration can be evaluated provded all otherconstituent items of the following water Balalnce equation are known

Water supply = Water disposal

Field Experimental Plots. In this method a field plot is chosen and the amountsof water added to the irrigation plot under observation by way of prcepitation andirrigation are measured along with runoff. The moisture content in various layers ofthe soil within the root zone depth are measured both at beginning and end of the cropseason. Then the evaportanspiration is computed as

ET = I – Q – ΔS

Where I is the total inflow in mm including precipitation and irrigation water, Q isthe total surface runoff in mm and ΔS is the increase in soil moisture storage inmm which is computed from the following equation

37

iieibi

n

iDGMMS

1001

Where Mbi = Moisture content in per cent at the beginning of the season in ith layer

Mei = Moisture content in per cent at the end of the season in ith layer

Gi = The apparent specific gravity of the ith layer of the soil

Di = Depth of ith layer of soil, in mm, within the root zone depth

n = The number of soil layers considered in the entire root zone.

Soil moisture depletion studies. These studies involve measurement of soilmoisture from various depths at as frequent intervals of time as possible through thegrowth period of a crop. This method gives satisfactory results for irrigated field cropsgrown on fairly uniform soils and where the soil moisture variations within the rootzone are not influenced by groundwater. the evapotranspiration u, for any time periodbetween two successive samplings is obtained as

iiii

n

iDGMMu

10021

1

where M1i = Soil moisture percentage at the time of first sampling in the ithe layer

M2i = Soil moisture percentage at the time of second sampling in the ith layer

The terms Gi, Di and n have the same significance.

Lysimeter. A lysimeter (also known as evapotranspirometer) consists of a circu-lar tank about 60 to 90 cm in diameter and 180 cm deep. sometimes large diametersupto 3 m with 3 in depth are also used. the lysimeter is filled with soil and individualcrops or natural vegetable, for which the evapotranspiration is required, are grown. itis buried so that its top is in flush with the surrounding ground surface. The sides ofthe lysimeter are impervious whereas the bottom is pervious. Water passing throughthe soil column is collected at the bottom and conducted through a small tube to ameasuring gauge in an adjacent pit. Soil moisture measurements may be obtained frommoisture sampling, weighing, or hydraulic method. In weighing type, the tank ismounted on self recording scale. In the hydraulic method the tank is floated on wateror a suitable heavy liquid and changes in weight are recorded through pressure changesobserved in a manometer.

38

1. Wading technique is used:

(a) To determine velocity of canal seawaves during Tsunami

(b)To determine thickness of canallining in alluvial soils

(c) To measure the volume ofdredging material in harbours

(d)To determine velocity of flow in ashallow streams

2. A stilling well is required when thestage measurement is made byemploying

(a) Bubble gauge(b)Float gauge recorder(c) Vertical staff gauge(d) Inclined staff gauge

3. Penman’s equation is based on

(a) Energy budgeting only(b)Energy budgeting and water

budgeting(c) Energy budgeting and mass

transfer(d)Water budgeting and mass

transfer

4. The Penman’s evapo – transpirationequation is based on

(a) Water budget method(b)Energy balance method

(c) Mass transfer method

(d)Energy balance and mass transferapproach

5. Isopleths are lines on a map throughpoints having equal depth of

(a) Rainfall

(b) Infiltration

(c) Evapotranspiration

(d)Total runoff

6. Lysimeter and Tensiometer are usedto measure respectively, one of thefollowing groups of qunatities

(a) Capillary potential and permeability

(b)Evapotranspiration and capillarypotential

(c) Velocity in channels and vapourpressure

(d)Velocity in pipes and pressurehead

7. A lysimeter is used to measure

(a) Infiltration

(b)Evaporation

(c) Evapotranspiration(d)Radiation

8. What is the pan co – efficient for anISI pan used for measuringevaporation?

Practice Problems

39(a) 0.8

(b)0.7

(c) 0.6

(d)1

9. By using rods, the velocity obtained is

(a) Mean velocity

(b)Maximum velocity

(c) Minimum velocity

(d)Surface velocity

10. The crest gauge is used to record

(a) the lowest stage in the river

(b) the average stage in the river

(c) the peak stage in the river

(d) the stage continuously with time.

11. If N is the speed of the current meterin revolutions per second, the velocitymeasured by it is proportional to

(a) N1/2 (b) N

(c) N3/2 (d) N2

12. One cumec-day of volume is equal to

(a) 86400 m3 (b) 8.64 ha-m

(c) 24 cumec-hours (d) all the above.

13. The dilution method ofstreamgauging is best suited for

(a) small steams with fairly steadyturbulent flow

(b) meandering streams in alluvial plains

(c) streams with large depth anddischarge

(d)any type of stream.

14. When the stage in the river is 4.8 mand the water surface slope is 1 in1000 the discharge is measured to be600 m3/s. what would be thedischarge carried by the river whenthe stage is same but the watersurface slope is 1 in 1440?

(a) 300 m3/s (b) 400 m3/s(c) 500 m3/s (d) 600 m3/s

15. Which of the following is the largestunit of volume ?

(a) hectare-metre(b)km2-mm(c) million-m3

(d) cumec.

16. The runoff from a drainage basin of are4320 km2 is estimated as 10000 cumec-days. What is the depth of runoff ?

(a) 20 cm (b) 40 cm(c) 43.2 (d) 21.6 cm.

17. According to Dalton’s law,evaporation is proportional to

(a) the vapour pressure gradient(b) the difference between the

saturation vapour pressure at 1000Cand the actual vapour pressure

(c) the difference between the actualvapour pressure and thesaturation vapour pressure at 00C

(d) the difference between thesaturation vapour pressure at agiven temperature and thesaturation vapour pressure at 00C

4018. The salinity in water

(a) reduce the evaporation(b) does not affect evaporation(c) increases the evaporation(d) difficult to say.

19. The pan evaporation is denoted by Ep

and the actual evaporation by E. Then

(a) Ep > E (b) E > Ep

(c) E = Ep (d) pEE

20. The pan coefficient is defined as

(a) pE

E(b)

EE p

(c) )( EE p (d) Ep . E.

21. The chemical compound which isgenerally used to reduce theevaporation from water surface is

(a) D.D.T (b) Alum(c) Cetyl alcohol(d) Potassium dichromate.

22. Atmometer is used to measure

(a) evaporation(b) transpiration(c) evapotranspiration(d) all the above.

23. The evaporation through plants andfrom the surrounding soil together iscalled

(a) hydration(b) vapourisation(c) transpiration(d) evapotranspiration.



A Hydraulic 1 Psychrometer

conductivity

B Infiltration 2 Phytometer

capacity

C Transpiration 3 Permeameter

D Relative humidity 4 Rainfallsimulator

Codes:

A B C D

(a) 3 4 2 1

(b) 3 4 1 2

(c) 4 3 2 1

(d) 3 2 4 1

41

Answer Key

1. (D) 2. (B) 3. (C) 4. (D) 5. (C)

6. (B) 7. (C) 8. (A) 9. (A) 10. (C)

11. (B) 12. (D) 13. (A) 14. (C) 15. (C)

16. (A) 17. (A) 18. (A) 19. (A) 20. (A)

21. (C) 22. (A) 23. (D) 24. (A)

Explanations

Sol. 14. (C)

Q S

2 2

1 1

1Q S 10001440

1Q S 14401000

21000Q 6001440

3500m / sec

42

MEASUREMENT OF INFILTRATION

INFILTROMETERS

Types of Infiltration

(1) Flooding type infiltometers

(2) Rainfall simulators.

In the flooding type of infiltrometers water is applied in the form of a sheet usuallywith a constant depth of flooding. It may use asingle ring or two rings to delineate the samplearea. In the former case it is known as a simpleinfiltrometer or a tube infiltrometer. In the lattercase it is called a double ring infilrometeri. In therainfall simulators water is applied by sprinklingat a uniform rate that is in excess of infiltrationcapacity.

Tube Infiltrometer. This consists of ametal cylinder of diameter 25 to 30 cm andlength 50 to 60 cm, with both ends open. It isdriven into a level ground such that about 10cm of the cylinder is above the ground.

A major drawback of the tubeinfiltrometer is that the infiltrated waterpercolated laterally at the bottom of the ring.

30cm

10cm

50cm

G.L.

Tube infiltrometer

3Chapter

HYDROGRAPHS,RUNOFF AND INFILTRATIONSyllabus : Measurement Of Infiltration, Infiltration Indices, Infiltration,Infiltration Capacity, Measurement Of Infiltration. Weightage – 30%

43thus the tube is not truly representing the area through which the infiltration is takingplace. This drawback is rectified to a large extent in the double ring infiltrometer.

Double Ring Infiltrometer. This is the most commonly used flooding typeinfiltrometer. It consists of two concentric rings driven into soil uniformly without tilland disturbing the soil to the least to a depth of about 15 cm. The diameters of the rings

may vary between 25 to 60 cm.

INFILTRATION INDICES

The infiltration capacity curves which are developed either from infiltrometertests or the hydrograph analyses methods can be used to estimate the runoff from agiven storm. The infiltration rate curve appropriate to the soil, vegetation, andantecedent moisture conditions existing at the time of occurrence of storm issuperimposed on the rainfall hyetograph with base lines coincident. The area of therainfall hyetograph above the infiltration curve would then represent the runoff volumewhose time distribution may be obtained through the application of unit hydrograph

Double ring infiltrometers

30cm

10cm

15cm

G.L.

60cm

44principle. The rainfall volume below the infiltration curve represents the total depth ofinfiltration during the storm. The problem, however, lies in selecting the appropriateinfiltration curve representative of the condition existing at the start of the storm.Attempts have been made to correlate the parameters of the infiltration rate curvewith soil types, land use patterns and the soil moisture, soil moisture is often quantifiedthrough the antecedent precipitation index.

-index. The index is an average rainfall intensity above which the rainfallvolume equals the runoff volume. The rainfall hyetograph is plotted on a timebase and a horizontal line is drawn such that the shaded area above the line exactlyequals the measured runoff. Since the unshaded area below the line is alsomeasured rainfall but did not appear as runoff it represents all the losses includingdepression storage, evaporation, interception as well as infiltration. However,infiltration is the largest loss compared to the other losses. The -index canavailable. Then probably a relationship between the size of the flood, antecedentsoil moisture and the -index could be developed. Since the data very large, the-index will be very useful in predicting the infiltration from to define the patternof rainfall excess.

W-index. The W-index is a refined version of -index. It excludes the depressionstorage and interception from the total losses. It is the average infiltration rate during

the time rainfall intensity exceeds the capacity rate. That is

tSQP

tFW )(

Where

F = the total infiltration,

t = time during which rainfall intensity exceeds infiltration,

P = total precipitation corresponding to t,

Q = the total storm runoff and S is the volume of depression storage andinterception.

Thus W-index is essentially equal to -index minus the depression and interceptionstorage. While the segregation of infiltration and retention seems to be a refinement,the task of estimating retention is so difficult that combining it with infiltration is probablyequally satisfactory which explains the popularity of -index.

45Wmin-index. This is the lowest value of W-index which is observed under very

wet initial conditions. Under these conditions since the retention rate is very low W-index and -index tend to be equal. This index is principally used in studies of maximumflood potential.

COMPONENTS OF RUNOFF

According to the source from which the flow is derived, the total runoff isvisualised to consist of surface runoff, subsurface runoff, and groundwater runoff.

Schematic representation of runoff process

TOTAL PRECIPITATIONON THE BASIN

CHANNELPRECIPITATION

OVERLANDFLOW

INFILTRATION ABSTRACTIONS LIKE INTERCEPTION AND

EVAPORATION

CHANNELFLOW

CHANNEL FLOW

INTERFLOW OR SUB-SURFACE RUNOFF

DEEP PERCOLATION

RAPIDINTERFLOW

DELAYEDINTERFLOW

GROUND WATERRUNOFF

SURFACERUNOFF

CHANNELFLOW

BASE FLOW

DIRECT RUNOFF(QUICK FLOW)

TOTAL RUNOFF A THE BASIN OUTLET

46InfiltrationInfiltration is the flow of water into the ground through the soil surface. The distribu-tion of soil moisture within the soil profile during the infiltration process. When wateris applied at the surface of a soil, four moisture zones in the soil.

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

Zone 2 Beneath zone 1, there is a transition zone.

Zone 3 The next lower zone is the transmis-sion zone where the downward mo-tion of the moisture takes place, themoisture content in this zone isabove field capacity but below satu-ration. Further, it is characterized byunsaturated flow and fairly uniform

moisture con-tent.

Zone 4T h elast zone isthe wettingzone. The soilmoisture isthis zone willbe at or nearfield capacityand the mois-ture contentdecreases with the depth. The boundary of the wetting

zone is the wetting front where a sharp discontinuity exists between the newlywet soil and original moisture content of the soil. Depending upon the amountof infiltration and physical properties of the soil, the wetting front can extendfrom a few centimeters to metres.

Infiltration Capacity

The maximum rate at which a given soil at a given time can absorb water is defined asthe infiltration capacity. It is designated as fp and is expressed in units of cm/h.

1 Saturation Zone2 Transition Zone

3 Transmission Zone

4 Wetting Zone

Wetting Front

Dep

th

Moisture content0

Distribution of Soil Moisturein the Infiltration Process

Input

Spill

Wiregauze

to storage

An Analogy forInfiltration

47The actual rate infiltration f can be expressed as

f = fp when i > fP

and f = i when i < fp

where i = intensity of rainfall. The infiltration capacity of a soil is high at the beginningof a storm and has an exponential decay as the time elapses.

MEASUREMENT OF INFILTRATION

INFILTROMETERS

(1) flooding type infiltometers

It is known as a simple infiltrometer or a tube infiltrometer.

(2) rainfall simulators.

It is called a double ring infilrometeri. In the rainfall simulators water is applied bysprinkling at a uniform rate that is in excess of infiltration capacity.

In the flooding type of infiltrometers water is applied in the form of a sheet usuallywith a constant depth of flooding. they may use a single ring or two rings to delineatethe sample area.

48

1. When the outflow from a storagereservoir is uncontrolled as in a freelyoperating spillway, the peak ofoutflow hydrograph occurs at

(a) The point of intersection of theinflow and outflow hydrographs

(b)A point, after the intersection ofthe inflow and outflowhydrographs

(c) The tail of inflow hydrograph(d)A point, before the intersection of

the inflow and outflowhydrographs

2. A watershed got transformed fromrural to urban over a period of time.The effect of urbanization on stormrunoff hydrograph from thewatershed is to

(a) Decrease the volume of runoff(b) Increase the time to peak

discharge(c) Decrease the time base(d)Decrease the peak discharge

3. The ratio of actual evapo – transpirationto potential is in the range

(a) 0.0 to 0.4(b)0.6 to 0.9(c) 0.0 to 1.0(d)100 to 2.0

4. S – Curve Hydrograph is thehydrograph

(a) Producing 1 cm of runoff over the basin(b)Of flow from a 1 cm intensity rain

of infinite duration(c) Having a volume of 1 cm3

(d)Of the total storm duration in anysingle storm rainfall

5. Steep rise in the flow – mass curveduring a certain period indicates:

(a) Very high evaporation lossesduring that period

(b)Flash floods during that period(c) Sudden spurt in irrigation demand

during that period(d)Sudden rise in demand for water

to meet hydropower generation

6. The shape of the recession limb ofhydrograph depends on

(a) Basin as well as stormcharacteristics

(b)Storm characteristics only(c) Basin characteristics only(d)Base flow only

7. V iewing watershed as a system,which one of the followingassumptions is made in the UnitHydrograph theory?

Practice Problems

49(a) Non – linearity(b)Both linearity and time variance(c) Both time invariance and non –

linearity(d)Both linearity and time invariance

8. Which one of the followingcharacteristics described awatershed system in system’sparlance?

(a) Linear(b)Non – linear(c) Linear and time – invariant(d)Non – linear and time – variant

9. Total area of DRH is equal to

(a) Total rainfall(b)Total transpiration(c) Total evapotranspiration(d)Run – off volume

10. Match the following:-

P. Rainfall 1. Isohyetointensity

Q. Rainfall excess 2. Cumulativerainfall

R. Rainfall 3. Hyetographaveraging

S. Mass curve 4. Direct run –offhydrograph

Codes:

P Q R S(a) 1 3 2 4(b) 3 4 1 2

(c) 1 2 4 3(d) 3 4 2 1

11. What is unity in a unit run – offhydrograph?

(a) Duration of storm(b)Area of basin(c) Depth of run – off(d)Base period of hydrograph

12. The ratio of total channel length to thedischarge area is called…

(a) Drauage volume(b)Runoff volume(c) Drainage density

(d)Mass curve density

13. Infiltration capacity of the soil isdefined as

(a) the depth of water absorbed bythe soil during the storm

(b) the intensity of rainfall abovewhich the rainfall volume equalsthe observed runoff volume

(c) the maximum rate at which the soilabsorbs water

(d) the permeability of the soil invertical direction.

14. A 6 h storm with hourly intensities of7, 18, 25, 12, 10 and 3 mm/hproduced a runoff of 33 mm. then the-index is

(a) 7 mm/h (b) 3 mm/h

(c) 10 mm/h (d) 8 mm/h.

5015. -index is defined as

(a) the difference between maximumand minimum infiltration capacities

(b) the difference between the totalrainfall and the total runoff dividedby the duration of the storm

(c) the rainfall intensity above whichthe rainfall volume equals theobserved runoff volume

(d) the minimum infiltration rateduring the storm.

16. W-index will be always

(a) equal to -index

(b)more than -index

(c) less than -index

(d)a constant fraction of -index.

17. A 6 h storm with a uniform of 1.5 cm/h produced a runoff depth of 72 mm.the average infiltration rate duringthis storm is

(a) 3 mm/h (b) 6 mm/h(c) 9 mm/h (d) 12 mm/h.

18. A 6 h storm of uniform intensity anda total of 8 cm produced a runoff of 5cm. what is the runoff produced by a12 h storm of uniform intensity and

a total rainfall of 11 cm assuming -index to be same during both thestorm ?

(a) 5 cm (b) 8 cm(c) 11 cm (d) 9 cm.

19. Under identical conditions if theinfiltration capacity measured bydouble ring infiltrometer is fd and thatmeasured by a rainfall is fs, thenwhich of the following is true ?

(a) fs < fd

(b) fs > f4

(c) fs = fd

(d) difficult to tell.

20. Interception loss when expressed asper cent of storm rainfall

(a) increases with increase in stormrainfall

(b)decreases with increase in stormrainfall

(c) decreases with decrease in stormrainfall

(d) is dependent of storm rainfall.

21. Direct runoff is the sum of

(a) the surface runoff and the base flow

(b) the base flow and thegroundwater runoff

(c) the delayed subsurface runoff andthe deep percolation

(d) the surface runoff and the promptsubsurface runoff.

22. The base flow is the differencebetween

(a) the total runoff and the directrunoff

(b) the total runoff and the delayedsubsurface runoff

51(c) the prompt subsurface runoff and

the delayed subsurface runoff

(d) the total runoff and the promptsubsurface runoff.

23. The base flow of a stream represents

(a) the groundwater runoff and theprompt subsurface runoff

(b) the groundwater runoff and thesubsurface runoff

(c) the groundwater runoff and thedelayed subsurface runoff

(d) the runoff due to snow melt.

24. Virgin flow means

(a) the flow in the streamcorresponding to no infiltrationand evaporation losses

(b) the flow in the stream which is notaffected by the works of man

(c) the flow in the stream which doesnot contain the flow from thewatershed leakage of theneighbouring basin

(d)none of the above.

25. The water year in India may be taken as

(a) from June to May(b) from January to December(c) from April to March(d) from October to September.

26. The subsurface runoff is also knownas

(a) Interflow(b) Storm seepage

(c) Secondary base flow(d) All the above.

27. Flow mass curve is the graph drawnbetween

(a) the flow rate and time(b) cumulative volume of flow and time(c) cumulative volume of flow and

cumulative time(d)cumulative discharge and time.

28. Flow mass curve is used

(a) to determine the storage capacityof the reservoir to meet a givenuniform demand

(b) to check the consistency of theflow record at a given site

(c) to develop the unit hydrograph(d) to develop the synthetic unit

hydrograph.

29. The flow mass curve is an integralcurve of

(a) rainfall hyetograph(b) infiltration curve(c) discharge hydrograph(d) rating curve of the gauging site.

30. Flow duration curve is the graphdrawn between

(a) the discharge in the stream and time(b) the accumulated discharge and time(c) the discharge and the per cent of

time such discharge is equaled orexceeded

(d)cumulative volume of flow andtime.

5231. If Q0 and Qt are the discharges at

times t0 and t on the recession limbof the hydrograph and if a is acoefficient with positive value, then

(a))(

0

0ttat eQQ (b)

)(

0

0ttat eQQ

(c)2)(

0

0ttat eQQ (d)

20 )(

0

ttat eQQ

32. The inflection point on the recession sideof the hydrograph indicated the end of

(a) the baseflow(b) the direct runoff(c) the overland flow(d) rainfall.

33. The concept of unit hydrograph wasfirst introduced by

(a) Dalton (b) Sherman(c) Horton (d) Thiessen.

34. The unit hydrograph is the graphicalrelation between the timedistributions of

(a) total rainfall and total runoff(b) total rainfall and direct runoff(c) effective rainfall and total runoff(d) effective rainfall and direct runoff.

35. The word unit in the unit hydrographrefers to the

(a) unit depth of runoff(b)unit duration of the storm(c) unit base period of the

hydrograph(d)unit area of the basin.

36. If Krs, Kri adn Krb denote the recessionconstants of surface runoff, interflowand baseflow respectively, which ofthe following ti true

(a) Krs > Kri > Krb

(b) Krs < Kri < Krb

(c) Krs > Kri < Krb

(d) Krs < Kri > Krb.

37. The peak ordinate of a 4 h unithydrograph of a basin is 270 m3/s.then, the peak ordinate of 8 h unithydrograph of the same basin will be

(a) 270 m3/s(b) less than 270 m3/s(c) more than 270 m3/s(d) (270 × e-4/8) m3/s.

38. The peak discharges in 4 and 8 hoursunit hydrographs of a basin occur att1 and t2. Then

(a) t1 = t2

(b) t1 > t2

(c) t1 < t2

(d) difficult to guess.

39. The base period of a 6 h unithydrograph of a basin is 84 h. then a12h hydrograph derived from this 6h unit hydrograph will have a baseperiod of

(a) 90 h (b) 78 h

(c) 42 h (d) 168 h

40. The basic principles of unithydrograph theory are

53(a) linearity and time invariance(b) nonlinearity and time invariance

(c) nonlinear time variance andlinearity

(d) nonlinear time variance andnonlinearity.

41. A storm occurring over a basin for aperiod of 6 h with uniform intensityproduced an effective rainfall of 15cm and the peak flow of 930 m3/s.over of 7.5 cm occurs, what is thecorresponding peak flow ?

(a) 930 m3/s (b)1860 m3/s(c) 2790 m3/s (d) 465 m3/s

42. The 4 h unit hydrograph of a basin canbe approximated as a triangle with abase period of 48 h and a peakordinate of 200 m3/s. then the areaof the basin is

(a) 1728 Km2 (b) 3456 Km2

(c) 864 Km2 (d) 5184 Km2

43. The upper limit on the area of thebasin for the applicability of unithydrograph is generally taken to be

(a) 100 Km2 (b) 2500 Km2

(c) 5000 Km2 (d) 10000 Km2

44. The direct runoff hydrograph of abasin can be approximated as atriangle with a base period of 80 h anda peak flow of 200 m3/s occurring at16th hour. If the area of the basin is1440 km2, what is the depth of runoffindicated by the hydrograph ?

(a) 1 cm (b) 10 cm(b) 2 cm (d) 20 cm

45. The S-curve hydrograph is thesummation of the

(a) unit hydrograph(b) total runoff hydrograph

(c) effective rainfall hyetograph

(d) baseflow recession curve.

46. The S-curve hydrograph is used

(a) to estimate the peak flood flow of abasin resulting from a given storm

(b) to develop synthetic unithydrograph

(c) to convert the unit hydrograph ofany given duration into a unithydrograph of any other desiredduration

(d) to derive the unit hydrographfrom complex storms.

47. The lag time of the basin is the timeinterval between

(a) the centroid of the rainfalldiagram and the peak of thehydrograph

(b) the beginning and end of directrunoff

(c) the beginning and end of effectiverainfall

(d) the inflection points on the rising andrecession limbs of the hydrograph.

5448. The following four hydrological

features have to be estimated ortaken as inputs before one cancompute the flood hydrograph at anycatchment outlet:1. Unit hydrograph2. Rainfall hydrograph3. Infiltration Index4. Base flowThe correct order in which they haveto be employed in the computation is(a) 1, 2, 3, 4 (b)2, 1, 4, 3(c) 2, 3, 1, 4 (d)4, 1, 3, 2


List – 1 List – 2(Name of Scientist) (Contribution to

field of Hydr-ology)

A Dalton 1 Unithydrograph

B Snyder 2 EvaporationC Balney-Criddle 3 Empirical

floodformula

D Sherman 4 Syntheticunithydrograph

5 Consumptiveuse equation

Codes:A B C D

(a) 2 4 5 1

(b) 1 4 3 2(c) 2 3 5 1(d) 1 3 4 5

50. Match List-1 with List-2 and selectthe correct answer using the codesgiven below the lists:

List – 1 List – 2 (Equation) (Applicability/

Principle ofEquation)

A Anemometer 1 HumidityB Rain Simulator 2 Evapotrans-

pirationC Lysimeter 3 InfiltrationD Hygrometer 4 Wind speed

Code:A B C D

(a) 4 3 1 2(b) 4 3 2 1(c) 3 4 1 2(d) 3 4 2 1


List – 1 List – 2A index 1 Used measure-

ment ofevapotranspira-tion for givenvegetation

B Lysimeter 2 Used for flowmeasurement ofa steam

55C Dilution 3 Average rainfall

technique above which therainfall is equal torunoff volume

D USLE 4 Replates to soilloss estimation from

a catchment

Codes:

A B C D

(a) 4 2 1 4

(b) 3 1 2 4

(c) 3 2 1 4

(d) 4 1 2 3

52. The rainfall during three successive 2hour periods are 0.5, 2.8 and 1.6 cm.The surface runoff resulting from thisstorm is 3.2 cm. The – index valueof this storm is

(a) 0.20 cm/hr

(b)0.28cm/hr

(c) 0.30cm/hr

(d)0.80cm/hr

(GATE – 2004 – 2 Marks)

53. Match List – I (Contains parameters)with List – II (methods/instruments)and select the correct answer usingthe code given below the lists:

List - I List – II

A. Streamflow 1. Anemome-velocity nter

B. Evapo– transp- 2. Penman’s

iration rate method

C. Infiltration rate 3. Horton’smethod

D. Wind velocity 4. Currentmeter

Codes:

A B C D(a) 1 2 3 4(b) 4 3 2 1(c) 4 2 3 1(d) 1 3 2 4

(GATE – 2012 – 2 Marks)

The drainage area of a watershed is 50km2. The – index is .5 cm/hour and thebase flow at the outlet is 10 m3/s. Onehour unit hydrograph (unit depth = 1 cm)of the watershed is triangular in shapewith a time base of 15 hours. The peakordinate occurs at 5 hours.

54. The peak ordinate (in m3/s/cm) of theunit hydrograph is

(a) 10.00(b)18.52(c) 37.03(d)185.20

(GATE – 2012 – 2 Marks)

Storm – I of duration 5 hrs. gives adirect run – off of 4 cm and has anaverage intensity of 2 cm/hr.

Storm – II of 8 hr. duration gives a run– off of 8.4 cm. (Assume – index issame for both the storms).

5655. The value of – index is

(a) 1.2

(b)1.6

(c) 1

(d)1.4

(GATE – 2013 – 2 Marks)

56. The direct runoff hydrograph inresponse to 5 cm rainfall excess in acatchment in shown in the figure. Thearea of the catchment (expressed inhectares) is ___________

57. Total area of DRH is equal to

(a) Total rainfall(b)Total transpiration(c) Total evapotranspiration(d)Run – off volume

Statement for linked question 58 and 59:-

A 4 hour UH of a catchment istriangular in shape with base of 80 hours.The area of catchment is 720 km2. The baseflow and index are 30 m3/s and 1 mm/h, respectively. A storm of 4 cm occursuniformly in 4 hours over the catchment.

59. The peak discharge of 4 hour UH is

(a) 40 m3/s

(b)50 m3/s

(c) 60 m3/s

(d)70 m3/s

59. The peak flood discharge due tostorm is

(a) 210 m3/s

(b)230 m3/s

(c) 260 m3/s

(d)720 m3/s

60. Peak of a flood hydrograph due to asix – hour storm is 470 m3/sec. Theaverage depth of rainfall is 8.0 cms.Assume an infiltration loss of 0.25cm/hour and a constant base flow of15 m3/sec. The peak discharge of a 6hour unit hydrograph for thiscatchment, will be

(a) 50 m3/sec(b)60 m3/sec(c) 70 m3/sec

(d)90 m3/sec

5761. The average rainfall for a 3 hour

duration storm is 2.7 cm and the lossrate is 0.3 cm/hour. The floodhydrograph has base flow of 20 m3/sand produces a peak flow of 210 m3/s. The peak of a unit hydrograph is

62. During a storm event in a certainperiod, the rainfall intensity is 3.5 cm/hour and the – index is 1.5 cm/hour.The intensity of effective rainfall (incm/hour, up to one decimal place) forthis period is _____________.

(GATE - II – 2017 - 1 Mark)63. Match List-1 with List-2 and select the

correct answer using the codes givenbelow the lists:

List – 1 List – 2A Unit hydrograph 1 Design floodB Synthetic unit 2 Permeability

hydrographC Darcy law 3 Ungauged

basinD Rational method 4 1 cm of

runoffCodes:

A B C D

(a) 2 3 4 1

(b) 4 3 2 1

(c) 2 1 4 3

(d) 4 1 2 3



A Index 1 Dependableflow

B Stope area 2 Reservoirmethod regulation

C Flow-duration 3 Steadycurve stream

dischargeD Flow- duration 4 Runoff

curve volume5 Unsteady

streamdischarge

Codes:A B C D

(a) 3 5 1 4(b) 3 1 2 4(c) 4 1 2 3(d) 4 5 1 3

65. Match the two listList – 1 List – 2

A Hydrograph 1 DRHdue to conti-nuous effective

B Hydrograph due 2 S-curve to an ERH

C Unit hydrograph 3 IHUwith ordinatesexpressed aspercent of totaldirect runoff

58

Answer Key

1. (A) 2. (C) 3. (C) 4. (B) 5. (B)

6. (C) 7. (D) 8. (C) 9. (D) 10. (B)

11. (C) 12. (C) 13. (C) 14. (D) 15. (C)

16. (C) 17. (A) 18. (A) 19. (A) 20. (B)

21. (D) 22. (A) 23. (C) 24. (B) 25. (A)

26. (D) 27. (B) 28. (A) 29. (C) 30. (C)

31. (B) 32. (C) 33. (B) 34. (D) 35. (B)

36. (A) 37. (B) 38. (C) 39. (A) 40. (A)

41. (D) 42. (A) 43. (C) 44. (C) 45. (A)

46. (C) 47. (A) 48. (C) 49. (D) 50. (B)

51. (B) 52.(C) 53.(C) 54.(B) 55.(A)

56.(21.6) 57.(D) 58.(B) 59.(A) 60.(C)

61. (2.0) 62. (0.9904) 63. (B) 64. (D) 65. (A)

D Unit hydrograph 4 Distribution

whose duration graph

of effective

rainfall is infini-

tesimally small

Codes:

A B C D

(a) 2 1 4 3

(b) 1 2 4 3

(c) 2 3 4 1

(d) 1 4 3 2

59

Explanations

Sol. 14. (D)

Runoff = 33 mm

Duration of Strom = 6 hour

Total Precipitation = (7 + 18 + 25 + 12 + 10 + 3) × 1 = 75mm

index e

P Rt

75mm 33mm 7mm/h6

But intensity of rainfall are given

7 mm/hr 18 mm/hr

25 mm/hr 12 mm/hr

10 mm/hr 3 mm/hr

that means, intensity of rainfall 7 mm/h and 3 mm/hr causes infiltration i.e. notcausing any runoff contribution.

So, these intensities of rainfall will not consider, in computing index .

index

18 25 12 10 334

65 33 324 4

8mm / hr

Sol. 17. (A)

Total Precipitation

60= 1.5 × 6 cm = 90 mm

Runoff = 72 mm

Avg. infiltration rate

index90 72 18 3

6 6

3mm / hr

Sol. 18. (A)

During 6 hour storm.

Total Precipitation = 8 cm

Total runoff = 5 cm

index8 5 3 0.5

6 6

cm/hr

During 12 h storm

Total precipitation = 11 cm

Runoff = ?

index = 0.5 cm/h

( index is same for both storm)

indexe

P R 11 R 0.5cm / ht 12

11 12 0.5 R

5cm R

Sol. 39. (A)

A & B are 6 h unit hydrograph with base period each 84 hours.

61

R = A+B

A B

O 6 12 90 96

84 hour90 hour

The curve R is resultant curve of A & B of 12 hour hydrograph, with base period 90hour.

Sol. 42. (A)

200 m /sec3

1 cm

4 h

48

48 ho

t

Area of unit hydrograph

1 Bast height2

31 48 200m / Sec2

317280000m

Area of unit hydrograph

= 1 cm × Area of catchment

6217280000 m2 = 1 cm × Area

317280000m Area1cm

2Area 1728000000m

2Area 1728km

Sol. 43. (C)

Upper limit on the area of the basin for the applicability of unit hydrograph isgenerally taken as 500 km2 and lower limit is 2 km2.

Sol. 44. (C)

200 m /sec3

pnet

80

80 ho

t

16

Area of basin = 1440 km2

pnet = ?

Area of direct runoff hydrograph

(D.R.H)

31 80 60 60sec 200m /sec2

3=28800000m

Area of basin × pnet = 28800000 m3

633

6 2

28800000mpnet1440 10 m

= 0.02 m

= 0.02 × 100 2cm

Sol. 52. (C)

Rainfall intensity

0.5 2.8 1.6, , cm/hr2 2 2

Runoff = 3.2 cm

Total precipitation

= 0.5 + 2.8 + 1.6 = 4.9

index4.9 3.2 0.28cm/hr

6

Rainfall intensity of 0.52

is less than 0.28,

So, index will be

index2.8 1.6 3.2

4

4.0 3.2 0.3cm/hr4

Sol. 54. (B)

Area of catchment = 50 km2

index = .5 cm/hour

Base flow = 10m3/sec

D = 1 hour

Base time = 15 hour

64

peak1 15 60 60 Q2

= Area of catchment × 1 cm

peakQ 18.52

Sol. 55. (A)

index5 2 4

5

10 4 1.2 cm/hr5

Sol. 56.

Area pf catchment × 5 cm

1 6 60 60 12

210800mArea 216000m5cm

Area 21.6 hr.

Sol. 58. (B)

D = 4 h

Area of catchment = 720 km2

Base flow = 30m3/sec

index = 1 mm/hr

P = 4 cm

Area of catchment × 1 cm

1 80 60 60 Q2

656

3720 10 2Q m / sec100 80 60 60

3Q 50m / sec

Sol. 59. (A)

pnet 14cm 4 3.6cm

10

Peak ordinate of DRH

= 50 × 3.6 = 180 m3/sec

Peak ordinate of flood hydrograph

= 180 + 30 3210 m /sec

Sol. 60.(C)

Peak of flood hydrograph = 470m3/sec

Rainfall depth = 8 cm

index = 0.25 cm/hour

Base flow = 15 m3/sec


= 470 – 15 = 455 m3/sec

pnet = 8 – 0.25 × 6

pnet = 6.5 cm


3455 70 m /sec6.5

Sol. 61.

Peak flor of DRH

66= 210 – 20 = 190 m3/sec

pnet = 2.7 – 0.3 × 3 = 1.8

Peak flow of U.H

3190 105.5 m /sec1.8

Sol. 62.

index = 1.5 cm/hour

Intensity of rainfall = 3.5 cm/hour

indexP R

t

P R1.5t t

R1.5 3.5t

R 3.5 1.5 2 cm/ht

67

FLOOD ROUTING

Flood routing is the technique of determining the flood hydrograph at a section of ariver by utilizing the date of flood flow at one or more upstream sections.

Flood routing may be divided into two basic types namely

(1) the reservoir routing and

(2) the channel routing or the streamflow routing.

The reservoir routing analyses the effect of reservoir storage on the floodhydrograph, while the channel routing analyses the effect of storage of a specifiedchannel reach on the flood hydrograph.

RESERVOIR ROUTING

Letting I and Q to denote the inflow into and outflow from a reservoir, and S the storage inthe reservoir, the continuity equation in the differential form for the reservoir is given by

dtdSQI –

Alternatively, the same can be written as

StQtI –

Where

I = is the average inflow rate in a small time interval t,

Q = the average outflow rate in the same time interval

4ChapterFLOODS ROUTING

ANDFLOOD CONTROL

Syllabus : Flood Routing, Reservoir Routing, Channel Routing, UnitHydrograph Application. Weightage – 15%

68S = the corresponding change in the storage of the reservoir during the same

time interval. if the inflow and outflow have straight line variation within the time interval

122121 –

2–

2SStQQtII

Proper units must be chosen for storage to maintain compatibility. For example, if I andQ are expressed in m3/s and Ät in days, then storage must be expressed in cumec-days.

CHANNEL ROUTING

The channel routing, or stream channel routing as it is called often. Usesmathematical relations to calculate outflow from a stream channel once inflow, lateralcontributions and channel characteristics are known.

When there is no lateral inflow into the channel reach, the unsteady flow in the channelis described by the following two equations which are known as Saint-Venant equations.

0tA

xQ

0

fo SSxyg

xVV

tV

Where

x = the distance measured along the flow direction from some reference pointto the section under consideration

Q = the discharge at the section

A = area of the flow at the section

t = the time variable

V = the average velocity of flow for the section

y = the depth of flow at the section

g = acceleration due to gravity

So = channel bed slope, and

Sf = slope of the energy line.

69The estimation of the design flood can be grouped as under.

(i) Increasing the observed maximum flood by a certain percentage

(ii) Envelope curves

(iii) Empirical flood formulae

(iv) Rational Method

(v) Unit hydrograph application

(vi) Frequency analysis (or Statistical methods).

UNIT HYDROGRAPH APPLICATION

It can be applied to the design storm to yield the design flood hydrograph. Thestorm with a specified return period which is adopted for the estimation of design floodmay be called a design storm. The design storm may be determined after acomprehensive study of major storms recorded in the region.

One of the important assumptions of unit hydrograph theory is the principle oflinearity. This assumption is not strictly valid. If the unit hydrograph used in the designflood estimation is based on average storms only, the peak discharge in the design floodhydrograph is increased by certain percentage before it is actually implemented in thedesign to account for the effects of non-linearity in the basin. The percentage increasedepends on the size of the storm used for derivation of unit hydrograph and the size ofthe design adopted for the design.

70

1. The standard project flood is

(a) Same as the probable maximumflood

(b)Same as the design flood

(c) Smaller than the probablemaximum flood

(d)Larger than the probablemaximum flood by a factorimplying safety factor

2. Dickens formula predicts maximumflood discharge, Q, in terms of thearea, A, and the coefficient, c, as Q =cAn. The value of n is

(a) 0.25

(b)0.50

(c) 0.67

(d)0.75

3. Muskingham method for routing offlood

(a) Is used for routing floods throughreservoirs

(b) Is a method of routing that usescontinuity and momentumequations

(c) Is a hydrologic method of routingfloods through streams

(d) Is one in which only energyequation is used

4. The return period for the annualmaximum flood of a given magnitudeis 8 years. The probability that thisflood magnitude will be exceededonce during the next 5 years is

(a) 0.625(b)0.3664(c) 0.487(d)0.529

5. A linear reservoir is one in which

(a) Storage varies linearly with time(b)Storage varies linearly with

outflow rate(c) Storage varies linearly with inflow

rate(d)Storage varies linearly with

elevation

6. A flood wave with a known inflowhydrograph is routed through a largereservoir. The outflow hydrographwill have

(a) Attenuated peak with reducedtime – base

(b)Attenuated peak with increasedtime – base

(c) Increased peak with increasedtime – base

(d) Increase peak with reduced time– base

Practice Problems

717. Probability of a 10 – year flood to occur

at least once in the next 5 years is

(a) 35% (b)40%(c) 50% (d)65%

8. Return Period Refers to

(a) The probability of Exceedance ofan event

(b)The Probability of Non –Exceedance of an event

(c) The Inverse of the Probability ofExceedance of an event

(d)The Inverse of the Probability ofNon – Exceedance of an event

9. The Muskingham method of floodrouting is a

(a) Form of hydraulic routing of aflood

(b)Form of reservoir routing(c) Complete numerical solution of St.

Venant equations(d)Hydrological channel routing

method

10. The design flood commonly adopted inIndia for barrages and minor dams is

(a) Probable maximum flood(b)A flood of 50 – 100 years return

period(c) Peak flood(d)Standard project flood or a 100 –

year flood, whichever is higher

11. In a linear reservoir, the

(a) Volume varies linearly withelevation

(b)Outflow rate varies linearly withstorage

(c) Storage varies linearly with time(d)Storage varies linearly with inflow

rate

12. Probability of a 10 years flood to occurat least once in the next 4 years is

(a) 25% (b)35%(c) 50% (d)65%

13. When the outflow from a storagereservoir is uncontrolled in a freelyoperating spillway, the peak ofoutflow hydrograph occurs at

(a) Point of intersection of the inflowand outflow hydrographs

(b)A point, after the intersection ofinflow and outflow hydrographs

(c) The tail of inflow hydrographs(d)A point before the intersection of

inflow and outflow hydrograph

14. A linear reservoir is one in which

(a) The volume varies linearly withelevation

(b) The storage varies linearly with time(c) The storage varies linearly with

the outflow rate(d)The storage varies linearly with

the inflow rate

15. In reservoirs with an uncontrolledspillway, the peak of the plottedoutflow hydrograph

(a) Lies outside the plotted inflowhydrograph

72(b)Lies on the recession limb of the

plotted inflow hydrograph(c) Lies on the peak of the inflow

hydrograph(d) Is higher than the peak of the

plotted inflow hydrograph.

16. The hydrologic routing methods arebased on

(a) continuity equation only(b)discontinu-e equation and

momentum equationonly(c) continuity equation and energy

equation(d) momentum equation only.

17. The peak of the outflow hydrographcoincides with the intersection ofinflow and outflow hydrographs inthe case of

(a) reservoir routing with controlledoutflow

(b) reservoir routing withuncontrolled outflow

(c) channel routing(d) any flood routing.

18. Modified Puls method of reservoirrouting is also known as

(a) Inflow storage discharge method(b) Muskingum method(c) Storage indication method(d) none of the above.

19. Hydraulic routing methods make use of

(a) energy equation only(b) continuity equation only

(c) momentum equation only(d) both continuity and momentum

equations.

20. In the channel routing by theMuskingum method, the value of therouting coefficients C0 and C1 areestimated as – 0.2 and 0.5respectively. The value of the thirdcoefficient C2 would be

(a) 0.2 (b) – 0.5(c) 0.3 (d) 0.7.

21. The condition satisfied by the threerouting coefficients of theMuskingum method is

(a) C0 + C0 + C2 = 0(b) C0 + C1 + C2 = 1(c) C0C1C2 = 1

(d) 2

1

1

0

CC

CC

22. For natural stream channels the valueof the Muskingum parameter x willgenerally be

(a) between 0 and 0.3(b) between 0.3 and 0.6(c) more than 0.5(d) more than 1.0.

23. The most commonly used probabilitydistribution to fit the flood data is

(a) Normal distribution(b)Gambel’s extreme value

distribution(c) Log-Normal distribution(d)Gamma distribution.

7324. A flood with a return period of 100

years is the flood which occurs

(a) every 100th year

(b) the maximum observed flood inthe past 100 years

(c) once in every 100 years on theaverage

(d) only after 100 years in theimmediate future.

25. The probability that a T, year floodoccurs in any year is

(a) rT

1(b)

21

rT

(c) log

rT1

(d) rTe .

26. A structure is designed for Tr yearflood,. It has an estimated useful lifeperiod on N years. Then theprobability that it will not fail duringits life period is given by

(a)

rT11 (b)

N

rT

11

(c) N

rT

111 (d) rT

N

11

27. A structure with a useful life period ofN years is designed for a Tr year flood.Then the risk in the design is given by

(a) rT

1(b)

N

rT

1

(c) N

rT

111 (d) N

rT

11

28. The Dicken’s formula for flood peakis given by

(a) Q = C A1/3 (b) Q = C A2/3

(c) Q = C A1/4 (d) Q = C A3/4

29. Consider the following statements:1. A 100-year flood discharge is greater

than a 50-year flood discharge2. 90% dependable flow is greater

than a 50% dependable flow3. Evaporation from salt-water

surface is less than that fromfreshwater surface.

Which of these statements are true?(a) 1 and 2(b)2 and 3(c) 1 and 3(d)1, 2 and 3

74

Answer Key

1. (C) 2. (D) 3. (C) 4. (C) 5. (B)

6. (B) 7. (B) 8. (C) 9. (D) 10. (D)

11. (B) 12. (B) 13. (A) 14. (C) 15. (B)

16. (A) 17. (B) 18. (C) 19. (D) 20. (D)

21. (B) 22. (A) 23. (B) 24. (C) 25. (A)

26. (B) 27. (C) 28. (D) 29. (C)

75

Explanations

Sol. 4. (B)

Given return period (T) = 8 years.

1 1PT 8

1 7q 18 8

1

1 4c5,1P 5 f q

1

1 4

c1 758 8

0.3664 1cn n

Sol. 7. (B)

T = 10 year

1P10

9q10

at least once in 10 year i.e. Risk.

Risk = 1 – qn

76591 100

10

= 40%

Sol. 12. (B)

T = 10 years

Risk 491 100

10

= 34.39 35%

GENERAL ASPECTS OF Chapter HYDROLOGY & PRECIPITATION

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