Section 4: Soil and Water Conservation Engineering
Hydrology
Subtopic: Runoff estimation
Question Statement:
A watershed with various land uses (as specified in the table below) receives a rainfall of 152.4 mm. If an initial abstraction (Ia) is 0.2 times the potential maximum retention (S), and the antecedent moisture content (AMC) of averaged condition is assumed, the depth of runoff volume in mm is ____________. [round off to two decimal places]
Official Answer Key:
111.80 to 112.00
Step-by-Step Solution & Explanation
• Composite CN = 0.40(83)+0.25(80)+0.20(94)+0.15(93) = 33.2+20+18.8+13.95 = 85.95
• S = 25400/CN − 254 = 295.52−254 = 41.52 mm; Ia = 0.2S = 8.304 mm
• Runoff Q = (P−Ia)²/(P−Ia+S) = (152.4−8.304)²/(152.4−8.304+41.52) = (144.096)²/185.616 = 111.85 mm
Computed value: 111.85 mm (within official range 111.80–112.00)
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