Correct Answer - Option 3 : 3 cm
Concept:
(i) Infiltration is the movement of water through soil. It is the process by which water enters into surface strata of soil to meet initial soil moisture deficiency. Infiltration Index is a method used for measurement of Infiltration. It represents average rate of infiltration. There are two types of infiltration index:-
(ii) φ- Index – It represents average infiltration rate during the period of rainfall excess. In the calculation of φ- Index initial losses is also considered as infiltration quantity.
(iii) W-Index- This is the average infiltration rate during the entire/whole period of storm. In the calculation of W-Index initial loss is separated from the total infiltration.
φ- Index is greater than or equal to W-index.
If in a certain duration T, the rainfall is P, Runoff is Q, initial losses is l, then W-index is given by
W \(=\frac{{P - Q - L}}{T}\)
φ- Index = \(\frac{{P - R}}{T}\), here T=Effective time duration in which rainfall intensity is greater than φ- Index
When intensity of rainfall is less than or equal to φ- Index then there will be no runoff.
Calculation:
A rain gauge recorded hourly rainfall as 5 cm, 2 cm, 4 cm and 3 cm for a four hour storm respectively.
φ- index =3 cm/hour
Intensity of rainfall for 2nd hour and 3rd hour is 2 cm/hour and 3 cm/hour which is less than and equal to φ- index respectively, so it is not required to consider those amount of rainfall during φ- index calculation.
So Total Precipitation = 5 + 4 = 9 cm
Te= Effective time duration in which rainfall intensity is greater than φ- Index=2 hour
So 3 \(=\frac{{9 - R}}{2}\)
R = 9 – 6 = 3 cm
