Question Bank • QB_SWCE_ADV_125
MSQ • 2 Marks
Section 4: Soil and Water Conservation Engineering
Hydrology
Subtopic: Runoff estimation
Question Statement:
Which of the following statements regarding the USDA Soil Conservation Service (SCS) Curve Number ($CN$) method for runoff estimation are correct?
Options:
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(A) The potential maximum soil moisture retention $S$ in millimeters is given by $S = \frac{25400}{CN} - 254$
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(B) The standard initial abstraction is parameterized as $I_a = 0.20 S$
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(C) Direct storm runoff occurs only when total storm rainfall $P$ exceeds the initial abstraction $I_a$ ($P > I_a$)
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(D) A Curve Number of $CN = 100$ represents a completely impervious surface where potential retention $S = 0$ and $Q = P$
Step-by-Step Solution & Explanation
All four statements are foundational principles of the SCS-CN method:
- A: $S = \frac{25400}{CN} - 254$ converts dimensionless $CN$ to depth in $\text{mm}$.
- B: Empirical relation $I_a = 0.2 S$ accounts for interception, surface depression storage, and initial infiltration.
- C: If $P \le I_a$, direct runoff $Q = 0$.
- D: At $CN = 100$, $S = 0$, giving $I_a = 0$ and $Q = P$ (complete runoff conversion).
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