Question

A storm with 14 cm precipitation produced a direct runoff of 8 cm. The time distribution of the storm is as shown in the table below:

$\begin{array}{cc}\text{Time from}& \text{Incremental}\\ \text{start(h)}& \text{Rainfall in cm}\\ 1& 1.0\\ 2& 2.0\\ 3& 2.8\\ 4& 3.3\\ 5& 2.5\\ 6& 1.8\\ 7& 0.6\end{array}$

What is the value of $\varphi -index$ of the storm?

• A. 0.5 cm/h
• B. 0.7 cm/h
• C. 0.8 cm/h
• D. 0.9 cm/h

Answer 1

The correct option is D 0.9 cm/h

$\text{Total rainfall}=14cm$
$\text{Direct runoff}=8cm$
$\text{Infiltration}=14-8=6cm$
$w-index=\frac{6}{7}=0.86cm/hr$

It is greater than 0.6 cm/hr

$\therefore \varphi -index=\frac{(1+2+2.8+3.3+2.5+1.8)-8}{6}$

$=0.9cm/hr$