Section 4: Soil and Water Conservation Engineering
Hydrology
Subtopic: Runoff estimation
Question Statement:
In watershed hydrology and runoff estimation, which of the following statements regarding the USDA Soil Conservation Service (SCS) Curve Number (CN) method are TRUE?
Options:
(A) Curve Number (CN) values range between 0 and 100, where 100 represents a completely impervious surface
(B) The potential maximum soil retention $S$ in mm is related to CN by $S = \frac{25400}{\text{CN}} - 254$
(C) The initial abstraction $I_a$ is traditionally taken as $0.2 S$
(D) For rainfall $P < I_a$, the direct surface runoff is negative
Answer Key:
A, B, C
Step-by-Step Solution & Explanation
1. CN ranges from 0 to 100 (impervious surfaces like water bodies or concrete have $CN = 100$). (Statement A is True)
2. In metric units, potential maximum retention is $S = \frac{25400}{CN} - 254\text{ mm}$. (Statement B is True)
3. Initial abstraction (interception, initial infiltration, surface depression storage) is standardly approximated as $I_a = 0.2 S$. (Statement C is True)
4. When rainfall $P \le I_a$, no runoff occurs, meaning runoff is strictly ZERO, never negative. (Statement D is False)