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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
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
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
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
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
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
4/29
Hydrology
Unit 4+ 5
Rainfall
RunoffAnalysis
The Surface Runoff Process
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
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
5/29
Hydrology
Unit 4+ 5
Rainfall
RunoffAnalysis
The Surface Runoff Process
Relation between
source area andrainfall depth for a
sample catchment;
Source: Humborg 1989
Runoffis produced on only
parts of a catchment
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
6/29
Hydrology
Unit 4+ 5
Rainfall
RunoffAnalysis
Antecedent soilmoisture
RUNOFF
Soil type
VegetationRainfallcharacteristics
Topography
Factors Affecting Surface Runoff
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
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
Factors Affecting Surface Runoff
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
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
Factors Affecting Surface Runoff
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
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
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
10/29
Hydrology
Unit 4+ 5
Rainfall
RunoffAnalysis
Topography: Slope
High runoff
Low runoff
Factors affecting surface runoff
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
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
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
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
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
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.
Determination of Surface Runoff
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
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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
Determination of Surface Runoff
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
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
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
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
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
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.
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
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Hydrology
Unit 4+ 5
Rainfall
RunoffAnalysis
The SCS - method : Steps
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.
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
19/29
Hydrology
Unit 4+ 5
Rainfall
RunoffAnalysisDetermination of Runoff
The SCS - method : Steps
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
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
20/29
Hydrology
Unit 4+ 5
Rainfall
RunoffAnalysisDetermination of Runoff
The SCS - method : Steps
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)
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
21/29
Hydrology
Unit 4+ 5
Rainfall
RunoffAnalysis
Determination of Runoff
The SCS - method : Steps
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
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
22/29
Hydrology
Unit 4+ 5
Rainfall
RunoffAnalysisDetermination of Runoff
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
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
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
Determination of Runoff
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
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
24/29
Hydrology
Unit 4+ 5
Rainfall
RunoffAnalysisDetermination of Runoff
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
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
25/29
Hydrology
Unit 4+ 5
Rainfall
RunoffAnalysisDetermination of Runoff
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
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
26/29
Hydrology
Unit 4+ 5
Rainfall
RunoffAnalysisDetermination of Runoff
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
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
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
Determination of Runoff
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
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.
7/29/2019 Hydrology RAINFALL - RUNOFF ANALYSIS
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.
http://localhost/var/www/apps/conversion/tmp/scratch_9/hydro-unit06e.ppt