Correct Answer - Option 4 :
2.533 cm/hr
Concept:
The ϕ – index is a rate of infiltration in which, the rate of infiltration exceeds the value at which volume of runoff become equals to the volume of rainfall.
\( {ϕ_{\left( {index} \right)}} = \frac{{{Total\; Infiltration}}}{{Total\;time\;of\;the\;storm}}\)
\( {ϕ_{\left( {index} \right)}} = \frac{{{P_{total}} - Q}}{{Total\;time}}\)
where
PTotal = Total precipitation
Q = Runoff
Calculation:
Total depth of precipitation (P) = 3.5 + 4.2 + 2.9 = 10.6 cm
Surface runoff (Q) = 3 cm
ϕ - Index is given by, \( {ϕ_{\left( {index} \right)}} = \frac{{{P_{total}} - Q}}{{Total\;time}}\)
⇒ \(\phi - index = \frac{{10.6 - 3}}{3} = 2.533\ cm/hr\)
