Section 4: Soil and Water Conservation Engineering
Hydrology
Subtopic: Hydrological cycle and measurement of its components
Question Statement:
A 100 ha reservoir receives 2500 mm of rainfall during a period of 2 years. During this period the mean inflow to the stream is 1.0 m³ s⁻¹, the mean outflow from the stream is 0.8 m³ s⁻¹, and the increase in the storage is 500 ha-m. Assuming that there is no seepage loss, the total evaporation during the period in m is
Options:
(A) 1.011
(B) 10.11
(C) 101.1
(D) 1011
Official Answer Key:
B
Step-by-Step Solution & Explanation
Water balance over the reservoir (all terms as volumes over the 2-year period, area A = 100 ha = 1×10⁶ m²): Rainfall volume = 2.5 m × 1×10⁶ m² = 2.5×10⁶ m³. Inflow volume = 1.0 m³/s × (2×365×24×3600 s) = 6.307×10⁷ m³. Outflow volume = 0.8 × 6.307×10⁷/1.0 = 5.046×10⁷ m³. ΔStorage = 500 ha-m = 500×10⁴ m³ = 5.0×10⁶ m³. Water balance: Rainfall + Inflow = Outflow + ΔStorage + Evaporation ⇒ Evap = (2.5×10⁶ + 6.307×10⁷) − 5.046×10⁷ − 5.0×10⁶ = 1.011×10⁷ m³. Evaporation depth = Evap volume/A = 1.011×10⁷/1×10⁶ = 10.11 m, matching option (B).
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