hydrology rainfall - runoff analysis

7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS

1/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis

RAINFALL - RUNOFF ANALYSIS

Types of runoff

Surface runoff process

Factors affecting runoff

Determination of runoff

Measurement

Assessment


2/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis

Surface runoff(rill or sheet) Stream flow

(discharge)

Types of Runoff

Surface runoff: runoff as sheet or rill flow at the surface before itreaches a stream

Stream flow (discharge): runoff in seasonal streams or rivers

Most of the water harvesting methods are related to the harvest ofsurface runoff


3/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis

Rainfall Intensity (RI)

Infiltration capacity of soil(ICS)

Surface runoff

RI > ICS Production ofsurface runoff

The Surface Runoff Process

Rainfall Intensity (RI)

Infiltration capacity of soil(ICS)

RI < ICS No surface runoff


4/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis


Infiltration Capacity depends on:

Soil type

Organic matter/ mulch layer

Antecedent soil moisture

Soil depth

Compacted soil layers

Type of soil tillage

Source: Critchley & Siegert 1991

Rainfall-Infiltration-Runoff relationship


5/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis


Relation between

source area andrainfall depth for a

sample catchment;

Source: Humborg 1989

Runoffis produced on only

parts of a catchment


6/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis

Antecedent soilmoisture

RUNOFF

Soil type

VegetationRainfallcharacteristics

Topography

Factors Affecting Surface Runoff


7/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis

Soil type

V

Solid

Air

Water

Vv

Porosity = Vv x 100

VHigh porosity High infiltration Low runoff(macropores)

Infiltration capacity curves for differentsoils; Source: Critchley & Siegert 1991



8/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis

Intensity of Rainfall

Runoff

Crusting, Capping or Sealing

Bigger Raindrops

High runoffLess infiltrationCrusting, capping or sealing

Filling of pores with fine particles

Breakdown of soil aggregates

High kinetic energy

Bigger raindrops

High intensity of rainfall



9/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis

Vegetation: Retards surface flow

Reduces crusting

Induces interception storage Increases porosity due to the root system

Densely coveredBare land

High Runoff Low Runoff

Factors affecting surface runoff


10/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis

Topography: Slope

High runoff

Low runoff

Factors affecting surface runoff


11/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis

Runoff efficiency

increases withdecreasing size ofcatchment

Runoff efficiencyis volume of runoffper unit of area

Area in ha

Runoff efficiency as a function of catchment size: from a study in Israel;Source: Critchley & Siegert 1991

Factors affecting runoff : Catchment size

Runoffefficiencyin%

80

70

60

50

40

30

20

10

80

0

0

50

0

30

20

10

10-2 10-1 10 102 103

Catchment size in ha


12/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis

Runoff Plots

Steps:

Step 1: Select a plot from which

runoff is to be collected under

controlled conditions.

similar physical characteristicswith the WH scheme planned

minimum sizerecommended is

3-4 m in width & 10-12 m in length

Measurement

Determination of Surface Runoff


13/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis

>15cm

Sheet metal, woodenplank or earth bundStep 2: Around the plots metal sheets or

wooden planks must be driven into the

soil with at least 15 cmof height abovethe ground. Earth bunds of the sameheight may be constructed.



14/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis

Step 3: Install a rain gauge

near the plot, a gutter at thelower end of the plot, and ameasuring tank at the lowerend of the gutter.

Step 4: Measure the volumeof water collected in the raingauge and in the runoff tankafter every storm (or every dayat a specific time).

Standard layout of a runoff plot;

Source: Critchley & Siegert 1991



15/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysisDetermination of Runoff from Large Catchments

The SCS - method ( U.S. Soil Conservation Service)

The SCS - method: Enables determination of runoff coefficient

without direct runoff measurement

Makes use of rainfall depth (P) and curve

number or hydrologic soil- cover complex

number (CN)

Curve number

(CN) is a function of: Mainly useful for large catchments. Hydrologic soil group

Land use

Hydrologic condition

Antecedent moisture conditions


16/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysisDetermination of Runoff

The SCS - method : Steps

Antecedent soil moisture categories for SCS antecedent preciptation index, Source: DVWK 1984

Step 1: Determine the soil moisture class having the rainfallheight during the previous 5 days in vegetation period and othertime. Refer to the following table.

Soil moisture classRainfall depth of the previous 5 days in mm

Vegetation period Remaining time

I < 30 < 15

II 30 - 50 15 - 30

III > 50 > 30


17/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis

Determination of RunoffThe SCS - method : Steps

Step 2: Choose the soil group from the following table

Group A:(low runoff potential): Soils having high infiltration rateseven when thoroughly wetted and consisting chiefly ofdeep, well to excessively drained sand or gravels. Thesesoils have a high rate of water transmission.

Group B: Soils having moderate infiltration rates when thoroughly

wetted and consisting chiefly of soils with a layer thatimpedes downward movement of water, or soils withmoderately fine to fine textures. These soils have amoderate rate of water transmission.


18/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis


Determination of Runoff

SCS - soil grouping; Source: DVWK 1984

Group C: Soils having slow infiltration rates when thoroughlywetted and consisting chiefly of soils with a layer thatimpedes downward movement of water, or soils with

moderately fine to fine textures. These soils have a lowrate of water transmission.

Group D: (high runoff potential): Soils having very slow infiltrationrates when thoroughly wetted and consisting chiefly ofclay soils with a high swelling potential, soils with a

permanent high water table, soils with a clay pan or claylayer at or near the surface, and shallow soils overnearly impervious material. These soils have a very lowrate of water transmission.


19/29

Hydrology

Unit 4+ 5

Rainfall



Step 3: From the following table look for the curve number(CN) corresponding to the appropriate land use and soil group.

CN values forvarious conditionsof land use andsoil group for soilmoisture class II;Source: DVWK1984

Land use

CN for soil group

A B C D

Waste land(without appreciable vegetative growth)

77 86 94 94

Tuber crops 70 80 90 90

Grain, fodder plants 64 76 88 88

Pasture (normal) 49 69 84 84

Pasture (sparse) 68 79 89 89

Forest (patchy) 45 66 83 83

Forest (medium) 36 60 79 79

Impermeable areas

(sealed surfaces in urban areas, roads etc.)

100 100 100 100


20/29

Hydrology

Unit 4+ 5

Rainfall



Step 4: For moisture class other than class II, correct the obtained CNusing the following graph.

CN values for soilmoisture class I and IIIas a function of CNvalues for soil moistureclass II; Source: DVWK1984

Old curve number (Cn for soil class II)

New

curvenumber

(N

forsoilc

lassI&III)


21/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis



Surface runoff as a function of rainfall and curve number;Source: DVWK 1984

Rainfall (P) in mm

Curve

number(CN)

240

220

200

180

160

140

120

100

80

60

40

20

0

0 20 40 60 80 100 120 140 160 180 200 220 240

10 0

95

90

85

80

75

70

65

60

55

50

45

35

30

25

20

P in mm

Peff

in mm

Surfacerunoff(R)inmm

Step 5: Having the curve number (CN) and the rainfall depth (P), look thecorresponding surface runoff amount from the following graph or compute the surfacerunoff using the following formula and then calculate the runoff coefficient (K)

KR

P

K: runoff coefficient

R: Surface runoff in mm

P: Rainfall in mmCN: Curve number

4.25

8800

4.25

2

200

4.25

2

CN

p

CN

p

R


22/29

Hydrology

Unit 4+ 5

Rainfall


Assessment

Use of runoff coefficient

Runoff = Runoff Coefficient X Rainfall

Graphical correlation between rainfall & runoff

Analytical correlation between rainfall & runoff

Runoff = a + k X Rainfall


23/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis

Runoff coefficient K

K = Yearly (seasonal) Total Runoff

Yearly(seasonal) Total Rainfall

Yearly or Seasonal Runoff Coefficient


Runoff coefficient need to be based on actual,simultaneous measurements of both rainfall & runoff

Avoid use of runoff coefficients derived for watersheds

other than the project area

At least 2 years of rainfall & runoff measurements arerequired to arrive at a representative figure.

Ranges usually between 0.1 and 0.5


24/29

Hydrology

Unit 4+ 5

Rainfall


Graphical Correlation between Rainfall & Runoff

1

k

Rainfall (mm)

Runoff(mm)

a

Runoff = a+ k x Rainfall

a : Y-intercept

k : Slope of the fitted line


25/29

Hydrology

Unit 4+ 5

Rainfall


22

PPN

RPRPNk

a R K P

N

2222 *

RRNPPN

RPPRNr

R a k P R= Runoff (mm)

P = Rainfall (mm)

N = number of sets of P and R

r = Coefficient of correlation

( 0 < r < 1)

a & K= Constants

Analytical Correlation between Rainfall & Runoff


26/29

Hydrology

Unit 4+ 5

Rainfall


k= (18 x 3083.3 - 348 x 104.3)/(18 x 9534 - 3482) = 0.38

a = (104.3 - 0.38 x 348)/18 = -1.55

R= 0.38 x P - 1.55

For P = 35mm, R= 0.38 x 35 - 1.55 = 11.75mm

Analytical MethodMonths Rainfall (P) Runoff (R) P x R P^2 R^2

1 5.00 0.50 2.50 25.00 0.25

2 35.00 10.00 350.00 1225.00 100.00

3 40.00 13.80 552.00 1600.00 190.44

4 30.00 8.20 246.00 900.00 67.24

5 15.00 3.10 46.50 225.00 9.61

6 10.00 3.20 32.00 100.00 10.24

7 5.00 0.10 0.50 25.00 0.01

8 31.00 12.00 372.00 961.00 144.00

9 36.00 16.00 576.00 1296.00 256.00

10 30.00 8.00 240.00 900.00 64.00

11 10.00 2.30 23.00 100.00 5.29

12 8.00 1.60 12.80 64.00 2.56

13 2.00 0.00 0.00 4.00 0.00

14 22.00 6.50 143.00 484.00 42.25

15 30.00 9.40 282.00 900.00 88.36

16 25.00 7.60 190.00 625.00 57.76

17 8.00 1.50 12.00 64.00 2.25

18 6.00 0.50 3.00 36.00 0.25Total 348.00 104.30 3083.30 9534.00 1040.51 All figures are in mm

Example:

Develop correlationbetween rainfall and

runoff graphically andanalytically for themonthly data given inthe table and computethe runoff correspondingto 35mm of rainfall


27/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis

Rainfall - Runoff Relation

-20

2

4

6

8

10

12

14

16

18

0 10 20 30 40 50

Rainfall in mm

Runoffinmm

Runoff

Linear

k= slope = 0,38a = y-intercept = -1.56

The runoff corresponding to 35 mm of rainfall is 11.75 mm

Graphical Method



28/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysisQuantity of Water to be Harvested

1 mm of runoff results in 1 liter of water harvest per square meter( 1 l/m2)

If catchment area is 100m2, and

The runoff with 67% of probability is 11.75 mm(as in the previous example),

Expected quantity of water to be harvested (in liter)

= Runoff (mm) x catchment area (m2)

= 11.75mm x 100m2

= 1175 liters

The expected quantiy of water harvested from thiscatchment area is 1175 liters.


29/29

Hydrology

Unit 4+ 5

Rainfall

RunoffAnalysis

Summary

Surface runoff is produced when the rainfall intensity exceeds the infiltration

capacity

Rainfall characteristics, antecedent soil moisture, vegetation and topography

affect runoff volume Runoff plots measure runoff from small catchments and yield good results for

microcatchment systems.

Runoff is computed from rainfall either employing suitable runoff coefficient or

through graphical or analytical correlation between rainfall and runoff.

For unmonitored catchments the SCS - method can be employed to compute therunoff

Runoff coefficient usually ranges between 0.1 and 0.5.
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hydrology rainfall - runoff analysis

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