Section 4: Soil and Water Conservation Engineering
Hydrology
Subtopic: Hydrograph analysis, unit hydrograph theory and application
Question Statement:
Hourly discharge at a watershed outlet, due to 2 cm excess rainfall (0–1h) and 3 cm excess rainfall (1–2h), constant base flow 1 m³/s:
*Table (reproduced from source): Hourly discharge hydrograph*
Options:
(A) 7.56
(B) 8.24
(C) 8.35
(D) 8.86
Official Answer Key:
A
Step-by-Step Solution & Explanation
Direct runoff (total minus base flow of 1 m³/s at each ordinate): 0,6,25,36,26,12,0 (m³/s). Total direct-runoff volume = Σ(ordinates)×Δt = (0+6+25+36+26+12+0)×3600 s = 105×3600 = 378,000 m³. Total excess rainfall depth = 2+3 = 5 cm = 0.05 m. Watershed area = Volume/Depth = 378,000/0.05 = 7,560,000 m² = 7.56 km². Answer: A.
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