GATE AG 2020 • Q59 NAT • 2 Marks

Section 4: Soil and Water Conservation Engineering

Hydrology

Subtopic: Runoff estimation

Question Statement:

Curve Number (CN) of a 40 ha watershed (AMC-II) is 80; initial abstraction = 20% of maximum retention S. For a 40 mm rainfall event, find the direct runoff (mm). (Rounded to 2 decimal places)

Official Answer Key:

8.10 to 8.30

Step-by-Step Solution & Explanation

S = 25400/CN − 254 = 25400/80 − 254 = 63.5 mm. Ia = 0.2S = 12.7 mm. Runoff Q = (P−Ia)²/(P−Ia+S) = (40−12.7)²/(40−12.7+63.5) = 27.3²/90.8 = 745.29/90.8 ≈ 8.21 mm.

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