Section 4: Soil and Water Conservation Engineering
Hydrology
Subtopic: Hydrological cycle and measurement of its components
Question Statement:
A river basin having a catchment area of $120\text{ km}^2$ received an annual rainfall of $1000\text{ mm}$. The annual runoff measured at the river gauging station was $36 \times 10^6\text{ m}^3$. Assuming negligible deep percolation and zero net change in basin water storage ($\Delta S = 0$), the annual evapotranspiration loss from the basin in $\text{mm}$ is ________ (answer in integer).
Answer Key:
700
Step-by-Step Solution & Explanation
From the catchment water budget equation:
$P = Q + ET + \Delta S$
Given $\Delta S = 0$, $ET = P - Q$.
1. Precipitation depth $P = 1000\text{ mm}$.
2. Runoff depth:
$Q = \frac{36 \times 10^6\text{ m}^3}{120 \times 10^6\text{ m}^2} = 0.30\text{ m} = 300\text{ mm}$
3. Evapotranspiration:
$ET = 1000 - 300 = 700\text{ mm}$