Question

A drainage basin has an area of 210 km$^2$. The average depth of rainfall received by it during a monsoon period is computed as 65 cm, while the runoff measured at its outlet during the same period is estimated to be $5.68 \times 10^7$ m$^3$. What percentage of rainfall has become runoff?

• A. 50.5%
• B. 41.62%
• C. 61.42%
• D. 38%

Hint:

Always convert area and depth into consistent units (e.g., m$^2$ and m) before calculating volume for runoff and rainfall.

Answer 1

The Correct Option is B

We are given:
- Area of the drainage basin = 210 km$^2$
- Average depth of rainfall = 65 cm
- Runoff volume = $5.68 \times 10^7$ m$^3$
Step 1: Convert units to SI.
Area = 210 km$^2$ = $210 \times 10^6$ m$^2$
Rainfall depth = 65 cm = 0.65 m
Step 2: Calculate total volume of rainfall.
Total rainfall volume = Area $\times$ Rainfall depth \[ = 210 \times 10^6 \times 0.65 = 1.365 \times 10^8 \text{ m}^3 \] Step 3: Use given runoff volume.
Runoff volume = $5.68 \times 10^7$ m$^3$
Step 4: Calculate percentage of rainfall that became runoff.
\[ \text{Percentage runoff} = \left( \frac{5.68 \times 10^7}{1.365 \times 10^8} \right) \times 100 = 41.62% \] \[ \boxed{41.62%} \]