Question Bank • QB_EXP_SWCE_RNF_011 NAT • 1 Mark

Section 4: Soil and Water Conservation Engineering

Runoff Estimation: Rational & SCS-CN Methods

Subtopic: SCS-CN potential maximum retention

Question Statement:

In the USDA-SCS runoff curve number method, an agricultural watershed has a composite curve number $CN = 77$. The potential maximum soil water retention $S$ is ________ $\text{mm}$ (round off to one decimal place).

Answer Key:

75.9

Step-by-Step Solution & Explanation

The relationship between potential maximum retention $S$ (in $\text{mm}$) and curve number $CN$ is: $S = \frac{25400}{CN} - 254$ Given $CN = 77$: $S = \frac{25400}{77} - 254 = 329.87 - 254 = 75.9\text{ mm}$
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