Hydrology of surface water, homework # 6
Due: 11/05
- Obtain a Unit Hydrograph for a basin of 315 km2 of area using the rainfall and streamflow data tabulated below.
| Time | Observed Hydrograph (m3/s) |
| (h) | |
| 0 | 100 |
| 1 | 100 |
| 2 | 300 |
| 3 | 700 |
| 4 | 1000 |
| 5 | 800 |
| 6 | 600 |
| 7 | 400 |
| 8 | 300 |
| 9 | 200 |
| 10 | 100 |
| 11 | 100 |
| Time | Gross Precipitation (GRH) |
| (h) | (cm/h) |
| 0 – 1 | 0.5 |
| 1-2 | 2.5 |
| 2-3 | 2.5 |
| 3-4 | 0.5 |
Step 1: Empirical Unit Hydrograph Derivation
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.
Step 2: Compute the volume of Direct Runoff. This volume must be equal to the volume of the Effective Rainfall Hyetograph (ERH).
Step 3: Express VDRH in equivalent units of depth:
Step 4: Obtain a Unit Hydrograph by normalizing the DRH. Normalizing implies dividing the ordinates of the DRH by the VDRH in equivalent units of depth.
Step 5: Determine the duration D of the ERH associated with the UH obtained in 4. In order to do this:
A: Determine the volume of losses, VLosses which is equal to the difference between the volume of gross rainfall, VGRH, and the volume of the direct runoff hydrograph, VDRH .
- Compute the f-index equal to the ratio of the volume of losses to the rainfall duration, tr.
- Determine the ERH by subtracting the infiltration (e.g., f-index) from the GRH:
