Correct Answer - Option 2 : 7
Explanation
We know
Unit of DRH = Unit of UH × Rainfall excess
Rainfall excess or runoff is same
Given,
Direct run off = 7 units
To get Ordinate of Unit Hydrograph
∴ Unit of Hydrograph = Unit of DRH/Rainfall excess
→ Unit of Hydrograph = (Unit of DRH)/7
Unit hydrograph (UH):
A unit hydrograph can be defined as the hydrograph of direct runoff resulting from one unit depth (1 cm) of rainfall excess occurring uniformly over the basin for a specified duration.
The unit hydrograph is the unit pulse response function of a linear hydrologic system.
The unit hydrograph is a simple linear model that can be used to derive the hydrograph of any duration resulting from any amount of excess rainfall for the given area.
Steps to derive hydrograph of duration (D) from unit hydrograph is as follows:
1. Analyze the hydrograph and separate base flow.
2. Measure the total volume of DRO under the hydrograph and convert them to inches or mm over the watershed.
3. Convert total rainfall to rainfall excess through infiltration methods, such that rainfall excess = DRO, and evaluate duration D of the rainfall excess that produced the DRO hydrograph.
4. Divide the ordinates of the DRO hydrograph by the volume in inches or mm and plot these results as the UH for the basin. Timebase is assumed constant for storms of equal duration and thus it will not change.
5. Check the volume of the UH to make sure it is 1.0 inch or 1.0 mm, and graphically adjust ordinates as required.
Unit of DRH = Unit of UH × Rainfall excess
