Answer & Solution
Answer: Option
B
Solution:
Understanding Unit Hydrographs:
Imagine a unit hydrograph as the standard response of a watershed (an area of land where all water drains to a common outlet) to a unit depth of rainfall. It is a tool to predict how a river will react to rainfall.
How is the unit hydrograph derived?
To derive a unit hydrograph, we isolate the effect of rainfall from the direct runoff hydrograph (DRH), which shows the flow in a river directly caused by rainfall.
What do we divide by?
We scale down the DRH to represent the response to a unit depth of rainfall. The steps are:
1. Divide the ordinates of the DRH (values at different times) by the period of the storm.
2. This ensures that the hydrograph ordinates correspond to a unit depth of rainfall excess for the specified duration.
Why not the other options?
- Direct Runoff Volume: While this shows the total runoff, dividing the DRH by volume does not yield a unit hydrograph.
- Total Rainfall: Not all rainfall contributes to runoff; some is lost to infiltration and evaporation.
- None of the above: This is incorrect because the correct method is dividing by the storm period.
Correct Answer: Option B: Period of Storm
