Question

By depth-area-duration (DAD) analysis, the maximum average depth of rainfall over a \(10^{3}\) \(\text{km}^2\) catchment due to a 6-hour storm is 80 mm. For the same storm, the maximum average depth of rainfall for a \(10^{4}\) \(\text{km}^2\) catchment, in mm, is

• A. greater than 88
• B. lesser than 80
• C. equal to 80
• D. between 81 and 88

Hint:

Think about how DAD curves behave as the area around a storm centre grows for a fixed duration.

Answer 1

The Correct Option is B

This question checks whether you know that DAD curves always slope downward as area increases, for a fixed storm duration.

1. greater than 88: wrong direction, the average depth cannot rise when the area around the same storm is enlarged.
2. lesser than 80: matches the DAD trend. A $10^{4}$ km$^2$ ring around the storm centre pulls in drier outer rainfall, so the mean drops below 80 mm.
3. equal to 80: would only hold if rainfall were perfectly uniform over both areas, which storms never are.
4. between 81 and 88: this range sits above 80 mm, again the wrong direction for a growing area.

The maximum average depth for the bigger catchment is lesser than 80 mm, so option B is correct.