Question

Consider a watershed and isohyets as shown in the figure. The average rainfall in the watershed is __________ mm (an integer value).

Answer 1

Correct Answer: 20

Step 1: Accurately define the areas for each isohyet based on visual estimates from the diagram. \[ \text{30 mm area} = 1 \text{ km}^2, \quad \text{20 mm area} = 8 \text{ km}^2, \quad \text{10 mm area} = 6 \text{ km}^2 \] Step 2: Calculate the weighted average of rainfall. \[ \text{Average Rainfall} = \frac{(30 \text{ mm} \times 1 \text{ km}^2) + (20 \text{ mm} \times 8 \text{ km}^2) + (10 \text{ mm} \times 6 \text{ km}^2)}{15 \text{ km}^2} = \frac{250}{15} \text{ mm} = 16.67 \text{ mm} \]