Section 4: Soil and Water Conservation Engineering
Hydrology
Subtopic: Hydrological cycle and measurement of its components
Question Statement:
For a closed hydrological river basin over a multi-year balance period where net subsurface and surface storage change is negligible ($\Delta S \approx 0$), the long-term water balance is expressed as:
Options:
(A) $P = R + ET$
(B) $P = R - ET$
(C) $P + R = ET$
(D) $ET = P \times R$
Answer Key:
A
Step-by-Step Solution & Explanation
The general hydrologic water balance equation for a catchment is:
$P - R - ET - G_{out} = \Delta S$
For a closed basin over a long period, net groundwater outflow $G_{out} \approx 0$ and storage change $\Delta S = 0$, reducing to $P = R + ET$ (Precipitation equals Runoff plus Evapotranspiration). Option A is correct.