The rainfall on five successive days on a catchment was 2,6,9,5 and 3 (in cm units). If the ɸ-index for the storm is taken as 3 cm/day, the total direct runoff from the catchment will be:

Correct option is C

The -index represents an infiltration rate where the infiltration rate surpasses the threshold at which the volume of runoff equals the volume of rainfall.
Calculation:
if Rainfall < ��-index; Runoff = 0
if Rainfall ≥�� -index; Runoff = Rainfall - ��-index
The total direct runoff from the catchment
= 0+(6-3) + (9-3) + (5-3) + (3-3) =11 cm