Soil and Water Conservation Engineering · Hydrology · 2 marks · Numerical answer
A watershed receives90 mm rain. Initial abstraction=.25S. Forest retention S=136 mm changes to 64 mm after cultivation. Find percent change in daily runoff volume, to one decimal.
Use $Q=(P-.25S)^2/(P+.75S)$ when P>.25S. Forest runoff=56²/192=16.3333 mm;cultivated runoff=74²/138=39.6812 mm. Relative increase=(39.6812−16.3333)/16.3333×100=**142.9%**. Equal catchment area makes the runoff-volume ratio equal the depth ratio.
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