Question

A small watershed receives 90 mm of rainfall in a day. If the initial abstraction is considered as 25% of the potential maximum retention of soil, and the land use changes from forest (with \(S = 136\) mm) to cultivated land (with \(S = 64\) mm), the change in daily runoff volume in percent is \(\underline{\hspace{1cm}}\) (round off to one decimal place).

Hint:

The change in runoff is the difference between the pre- and post-land use runoff values.

Answer 1

Correct Answer: 142.9

Initial abstraction: \(I = 0.25 \times S = 0.25 \times 136 = 34\ \text{mm}\) Runoff volume (before land use change):
\[ R_1 = (90 - 34) = 56\ \text{mm} \] New abstraction: \(I = 0.25 \times 64 = 16\ \text{mm}\) Runoff volume (after land use change):
\[ R_2 = (90 - 16) = 74\ \text{mm} \] Change in runoff:
\[ \Delta R = R_2 - R_1 = 74 - 56 = 18\ \text{mm} \] Percent change:
\[ \frac{18}{56} \times 100 = 32.14% \] Thus, the change in runoff is \( \boxed{32.1}% \).