Determine the runoff
coefficient using following rainfall and streamflow data :
1. Time (h)
0 , 0.5 , 1.0 , 1.5 , 2.0 , 2.5 , 3.0 , 3.5 , 4.0 , 4.5 , 5.0 .
2. Stream flow (m^3 / s )
200 , 250 , 275 , 500 , 1500 , 2000, 1000 , 750 , 500 , 450 , 200 .
3. Rainfall (cm)
nil , 0.40 , 5.70 , 8.50 , 6.60 , 0.30 , nil , nil , nil , nil , nil .
Assume base flow as 200 m^3/s and C.A = 50 km^2 .
Answers
Explanation:
Obtain a Unit Hydrograph for a basin of 282.6 km2 of area using the rainfall and streamflow data tabulated below.
Time
(h)
Observed Hydrograph (m3/s)
0
160
1
150
2
350
3
800
4
1200
5
900
6
750
7
550
8
350
9
225
10
150
11
140
Time
(h)
Gross Precipitation (GRH)
(cm/h)
0 - 1
0.25
1 - 2
2.75
2 - 3
2.75
3 - 4
0.25
In this process: use the horizontal straight-line method to separate baseflow.
Empirical Unit Hydrograph Derivation
1. Separate the baseflow from the observed streamflow hydrograph in order to obtain the Direct Runoff Hydrograph (DRH).
For this example, use the horizontal line method to separate the baseflow. From observation of the hydrograph data, the streamflow at the start of the rising limb of the hydrograph is 150 m3/s. Thus, use 150 m3/s as the baseflow.
2. Compute the volume of Direct Runoff. This volume must be equal to the volume of the Effective Rainfall Hyetograph (ERH).