The unit hydrograph due to a storm may be obtained by dividing the ordinates of the direct runoff hydrograph by

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