Allied & ICAR Exam Bank • ALLIED_1191NAT • 2 Marks
Section 4: Soil and Water Conservation Engineering
Hydrology and Runoff
Subtopic: Phi-Index
Question Statement:
A 3-hour storm produces rainfall increments of $3.0\text{ cm}$, $5.0\text{ cm}$, and $2.0\text{ cm}$ in successive 1-hour intervals (total rainfall = 10.0 cm). If the total direct runoff measured is $4.0\text{ cm}$ and rainfall occurs at all hours above the index, the $\phi$-index of the storm in cm/h is:
Answer Key:
2.0
Step-by-Step Solution & Explanation
Total infiltration $F = P - R = 10.0 - 4.0 = 6.0\text{ cm}$. Storm duration with effective rainfall $t_e = 3\text{ h}$. $\phi = \frac{F}{t_e} = \frac{6.0}{3} = 2.0\text{ cm/h}$. (Since all hourly intensities $\ge 2.0\text{ cm/h}$, this is consistent).