Section 4: Soil and Water Conservation Engineering
Watershed Management
Subtopic: Water budgeting in watershed
Question Statement:
In the Thornthwaite monthly hydrologic water balance model, when monthly precipitation ($P$) exceeds monthly potential evapotranspiration ($PET$), the excess water ($P - PET$) first goes to:
Options:
(A) Recharge soil moisture storage up to the Available Water Capacity (AWC)
(B) Direct surface runoff into streams, completely bypassing the soil profile
(C) Deep non-recoverable geological loss to the Earth's mantle
(D) Immediate evaporation back to the upper atmosphere
Answer Key:
A
Step-by-Step Solution & Explanation
In the bookkeeping water balance procedure of Thornthwaite and Mather:
1. When $P > PET$, the surplus precipitation is first utilized to replenish root-zone soil moisture storage up to field capacity (soil Available Water Capacity, $AWC$).
2. Only after soil moisture storage reaches field capacity does any remaining excess generate water surplus ($S$) contributing to groundwater recharge and stream runoff. Option A is correct.