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HydrologyBy Sandeep Jyani Sir

25-06-2019

REPRESENTATION OF RAINFALL DATA

1. MASS CURVE➢ It is a plot of cumulative rainfall against

time.

➢ Mass curves of rainfall are very useful in extracting the information on the duration and magnitude of a storm.

➢ Also, intensities at various time intervals in a storm can be obtained by the slope of the curve.

➢ For non-recording rain gauges, mass curves are prepared from knowledge of the approximate beginning and end of a storm and by using the mass curves of adjacent recording gauge stations as a guide.

2

Time(days)Acc

um

ula

ted

pre

cip

itat

ion

Civil Engineering by Sandeep Jyani

REPRESENTATION OF RAINFALL DATA

2. HYETOGRAPH➢ It is the plot of average

intensity of rainfall against time.

➢ Area of hyetograph gives total depth of rainfall.

3

Time(hours)

Inte

nsi

ty (

cm/h

r) i3

i2

i1

t3t2t1


∅ − 𝑰𝒏𝒅𝒆𝒙

It is an approximate infiltration rate (infiltration capacity) so defined that the area of hyetograph above ∅ index gives value of direct run off/ surface runoff

Hence the area of hyetograph below the ∅ index gives depth of infiltration.

4

Time(hours)

Inte

nsi

ty (

cm/h

r) i3

i2

i1

t3t2t1

∅ − 𝑰𝒏𝒅𝒆𝒙Cm/hr


REPRESENTATION OF RAINFALL DATA2. HYETOGRAPH

➢ Plot the hyetograph for the following

5

Cumulative rainfall

0 6 9 15

Time (hr) 0 3 6 9


AVERAGE PRECIPITATION

1. The following methods are used to measure the average precipitation over an area:

2. Arithmetic Mean Method

3. Thiessen polygon method

4. Isohyetal method

6Civil Engineering by Sandeep Jyani

AVERAGE PRECIPITATION1. ARITHMETIC MEAN METHOD

• Simplest method for determining areal average.• This method does not take into account rain

gauges falling outside the catchment.• Least accurate method.• Yields fairly accurate results for plain areas with

uniform rainfall distribution.• If there are small variations in rainfall values

measured by stations, then this method can be used.

• Let P1, P2…………..Pn are the rainfall values obtained from stations within a catchment, then average precipitation as per ARITHMETIC MEAN METHOD is given by

P’ = 𝑷𝟏+𝑷𝟐+𝑷𝟑………………..𝑷𝒏

𝒏


Que. 1 Using Arithmetic Average Method, find average rainfall over a catchment.

The rain gauge data is: 12.6, 18.8, 14.8, 10.4 and 16.2 mm.

Sol. P’ = 𝑃1+𝑃2+𝑃3………………..𝑃𝑛

𝑛

P’ = 12.6+18.8+14.8+10.4+16.2

5

P’ = 14.56mm

8


2. THIESSEN POLYGON METHOD➢ This method assumes that any point in

the watershed receives the same amount of rainfall as that measured at the nearest rain gauge station.

➢ Here, rainfall recorded at a gauge can be applied to any point at a distance halfway to the next station in any direction.



2. THIESSEN POLYGON METHOD• STEPS TO BE FOLLOWED

i. Draw lines joining adjacent gauges.

ii. Draw perpendicular bisectors to the lines created in step i)

iii. Extend the lines created in step ii) in both directions to form representative areas for gauges.

iv. Compute representative area for each gauge


Draw Theissen polygon for the following rain gauge stations

12


2. THIESSEN POLYGON METHOD

If there are ‘n’ number of rain gauge stations in and around the catchment area, and if A1, A2…….An be the respective areas of Thiessen Polygon then, average precipitation is given by –

P’ = 𝑃1𝐴1+𝑃2𝐴2+𝑃3𝐴3……………..𝑃𝑛𝐴𝑛

𝐴1+𝐴2+𝐴3…….𝐴𝑛


Que. 2 Using Thiessen Polygon Method, find average rainfall over a catchment.

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RAIN GAUGE STATION

A B C D E

POLYGON AREA (km2)

40 45 38 30 43

PRECIPITATION(mm)

30.8 33.4 34.6 32.6 24.6


Solution –

P’ = 6085.6

193= 31.53mm

15

RAIN GAUGE STATION

POLYGON AREA(km2)

PRECIPITATION(mm)

AREA*PRECIPITATION(km2.mm)

A 40 30.8 1232

B 45 33.4 1503

C 38 34.6 1314.8

D 30 32.6 978

E 43 24.6 1057.8

SUMMATION 193 6085.6


AVERAGE PRECIPITATION3. ISOHYTEL METHOD

➢ An isohyet is a line joining points of equal rainfall magnitude.

➢ The Isohyetal method is used to estimate the mean precipitation across an area by drawing lines of equal precipitation.

➢ This method uses topographic and other data to yield reliable estimates

16

CATCHMENTISOHYETES

ISOHYETAL areaSTATION


AVERAGE PRECIPITATION3. ISOHYTEL METHOD

If P1, P2, P3……….Pn are the isohytesand if A1, A2, A3….An-1 are inter isohyet area respectively, then the average precipitation is given by –

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P’ = 𝑨𝟏

𝑷𝟏+𝑷

𝟐

𝟐+𝑨𝟐

𝑷𝟐+𝑷

𝟑

𝟐…………………𝑨𝒏−𝟏

𝑷𝒏−𝟏+𝑷𝒏𝟐

𝑨


18

AREA b/w ISOHYETES

(km2)

22 80 110 89 70


(cm)

12.5 13.5 14.5 15.5 16.5

PRODUCT

A×(𝑷𝟏+𝑷𝟐)

𝟐

275 1080 1595 1379.5 1155

ISOHYETES 12 13 14 15 16 17

Solution : Pav = ∑𝐴 (𝑃1+𝑃2)/2

∑𝐴

= 275+1080+1595+1379.5+1155

22+80+110+89+70

= 5484.5371

= 14.78cmCivil Engineering by Sandeep Jyani

HYDROGRAPH

• It is a plot of discharge against time.

• Components of Hydrograph➢ Hydrograph generally contains the following three parts.

1. Rising Limb2. Peak (or Crest) Segment3. Falling (or Recession) Limb

19D

isch

arge

m3 /

sec

Time(days)Start of rainfall

A B

Start of direct runoff

HYDROGRAPH

1. A – Start of rainfall

2. ABEF – Base Flow

3. B – Start of Direct Runoff

4. BC – Rising limb

5. CPD – Crest Segment

6. P – Peak of hydrograph

7. C and D – Inflection points

8. DE – Recession Limb

9. E – Point where overland flow seizes

10. D – Point which marks end of direct runoff

11. BCPDE – Direct Runoff Hydrograph

20

Time(days)

Recession limb

Rising limb

Peak segment

A

B

C

P

D

E

F

Dis

char

ge m

3 /se

c


HYDROGRAPH

• Components of Hydrograph1. Rising Limb

➢ It is the ascending curved portion of the hydrograph.

➢ The rising limb rises slowly in the early stage of the flood but more rapidly toward the end portion.

➢ The shape of rising limb depends on duration and intensity distribution of rainfall.

➢ This is because in early stages the losses is more and water reaches to the stream faster.

21

Dis

char

ge m

3 /se

c

Time(days)

Rising limb

HYDROGRAPH

• Components of Hydrograph2. Peak Segment

➢ Peak segment is shown by inverted U in the hydrograph.

➢ Peak of hydrograph occurs when all parts of basins contribute at the outlet simultaneously at the maximum rate.

➢ Depending upon the rainfall-basin characteristics, the peak may be sharp, flat or may have several well defined peaks.

22

Time(days)

Dis

char

ge m

3 /se

c


HYDROGRAPH

• Components of Hydrograph3. Recession Limb

➢ Recession Limb represents the withdrawal of water from the storage built up during the early phase of hydrograph.

➢ It extends from the point of inflection at the end of the crest to the beginning of the natural groundwater flow.

➢ The shape of recession limb depends upon basin characteristics only and independent of the storm.

23

Time(days)

Recession limb

Dis

char

ge m

3 /se

c

HYDROGRAPH

BASE PERIOD:• Time between start of direct run off to end of direct run off

• The time interval from the rainfall excess to the peak is essentially dependent on the basin and storm characteristics

DIRECT RUN OFF:• It is the part of precipitation which reaches the stream

immediately after the rainfall.

BASE FLOW:• It is the part of rainfall which reaches the stream long time after

the rainfall.


FACTORS AFFECTING HYDROGRAPH1. Catchment parameter

a) Shape of catchment• Hydrograph from a catchment with broader end near the

outlet will have a higher and early peak, as compared to a catchment with narrow end towards the outlet.

2. Form factor

• 𝑭𝒐𝒓𝒎 𝒇𝒂𝒄𝒕𝒐𝒓 =𝑨𝒗𝒆𝒓𝒂𝒈𝒆 𝒘𝒊𝒅𝒕𝒉

𝑨𝒗𝒆𝒓𝒂𝒈𝒆 𝒍𝒆𝒏𝒈𝒕𝒉

• It is the ratio of average width of the catchment and the average length of the catchment.

• Higher is the form factor, lower is the base period, lower is the infiltration and higher is runoff discharge.

25

FACTORS AFFECTING HYDROGRAPH

3. Compaction factor

• 𝑪𝒐𝒎𝒑𝒂𝒄𝒕𝒊𝒐𝒏 𝒇𝒂𝒄𝒕𝒐𝒓 =𝒑𝒆𝒓𝒊𝒎𝒆𝒕𝒆𝒓 𝒐𝒇 𝒄𝒂𝒕𝒄𝒉𝒎𝒆𝒏𝒕

𝒑𝒆𝒓𝒊𝒎𝒆𝒕𝒆𝒓 𝒐𝒇 𝒇𝒐𝒓𝒎 𝒄𝒊𝒓𝒄𝒍𝒆

• It is the ratio of perimeter of the catchment to the perimeter of form circle.➢Form circle is the circle with same area as that of the

catchment.

• As compaction factor decreases, base period decreases, infiltration decreases and runoff discharge increases.


FACTORS AFFECTING HYDROGRAPH

4. Catchment slope• As slope increases flow velocity increases, base period

decreases, infiltration decreases and peak of hydrograph increases.

5. Land use• Urbanization reduces infiltration and increases flow

velocity thereby increasing the base period and increasing the peak of hydrograph.

6. Rainfall properties• As intensity of rainfall increases, peak of hydrograph

increases.• When storm movement is downstream, peak of

hydrograph is higher as compared to when storm movement is upstream.

27

UNIT HYDROGRAPH• It is defined as a direct runoff hydrograph,

resulting due to 1cm of effective rain applied uniformly at a uniform intensity.

• It is expressed as D hr UH

• Effective intensity in D hr UH is 𝟏

𝑫𝒄𝒎/𝒉𝒐𝒖𝒓

• Area if Unit hydrograph= 𝟏 𝒄𝒎 ×𝒂𝒓𝒆𝒂 𝒐𝒇 𝒄𝒂𝒕𝒄𝒉𝒎𝒆𝒏𝒕

• 4 hour UH will have a higher peak because the area of both the UH are same but base period of 4 hour is less

• Unit hydrograph is applicable for the area greater than 2 km2 and less than 5000km2

because rainfall is not uniformly distributed on areas greater than 5000km2

28

Time(days)

1/D

cm

/hr


Important Points

1. Isovels: Velocity

2. Isochrones: travel time from a common point

3. Isochrome: equal to time of travel

4. Isohytes: rainfall

5. Isoniff: snowfall amount

6. Isohels: sunshine

7. Isohaline: salinity

8. Isobath: Depth in sea

9. Isotherm: Temperature

10. Isopluvial map: lines of equal depth of rainfall of given duration and frequency


Que 1. Unit Hydrograph theory was enunciated by

a) Merril Bernard

b) W.W. Horner

c) Le-Roy K. Shermen

d) Robert E. Horten.


Que 1. Unit Hydrograph theory was enunciated by

a) Merril Bernard

b) W.W. Horner




Que 2. The theory of infiltration capacity was given by

a) Merril Bernard

b) W.W. Horner




Que 3. Hydrology helps in

a) predicting maximum flows

b) deciding the minimum reservoir capacity

c) forecasting the availability of quantity of water at reservoir site

d) all the above


Que 4. Pick up the correct statement from the following :

a) Rain which is intercepted by buildings, vegetations and other objects, is generally known as rainfall interception

b) The difference between the total rainfall and intercepted rainfall, is generally called ground rainfall

c) When rainfall exceeds the interception rainfall, water reaches the ground and infiltration starts

d) The maximum rate of absorbing water by the soil in any given condition, is known as infiltration capacity

e) All the above



a) Rain which is intercepted by buildings, vegetations and other objects, is generally known as rainfall interception

b) The difference between the total rainfall and intercepted rainfall, is generally called ground rainfall

c) When rainfall exceeds the interception rainfall, water reaches the ground and infiltration starts

d) The maximum rate of absorbing water by the soil in any given condition, is known as infiltration capacity

e) All the above


Que 5. The surface Run-off is the quantity of water

a) absorbed by soil

b) intercepted by buildings and vegetative cover

c) required to fill surface depressions

d) that reaches the stream channels


Que 6. Pick up the correct equation from the following :

a) Run off = Surface run off + Ground water flow

b) Run off = Surface run off - Ground water flow

c) Run off = Surface run off / Ground water flow

d) Run off = Surface run off x Ground water flow


Que 7. Infiltration capacity of soil depends upon

a) number of voids present in the soil, arrangement of soil particles

b) shape and size of soil particles

c) compaction of the soil particles

d) all the above.



a) Absolute humidity at a given temperature is equal to weight of moisture present in a unit volume

b) Relative humidity is the ratio of actual vapourpressure and saturation vapour pressure at the same temperature

c) Relative humidity is the ratio of the weight of the vapours present per unit volume to the weight of vapours which could be contained at the same temperature when fully saturated

d) All the above.


Que 9. In India, rain fall is generally recorded at

a) 8 AM

b) 12 Noon

c) 4 PM

d) 8 PM


Que 10. A recording type rain gauge

a) produces a mass curve of rain fall

b) records the cumulative rain

c) is sometimes called integrating rain gauge or continuous rain gauge

d) all the above.


Que 11. In India the recording type rain gauge generally used, is

a) weighing type

b) tipping type

c) float recording type

d) NOTA


Que 12. For determination of average annual precipitation in a catchment basin, the best method is

a) Arithmetical method

b) Thiessen's mean method

c) Isohyetal method

d) None of these


Que 12. For determination of average annual precipitation in a catchment basin, the best method is



c) Isohyetal method

d) None of these


Que 13. While calculating the average depth of annual precipitation in a catchment basin, importance to individual raingauge station is given in



c) Isohyetal method

d) both (b) and (c).


Que 14. The quantity of water retained by the sub-soil against gravity, is known

a) Yield

b) Porosity

c) specific yield

d) Specific retention


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