Example of the $-$index method: Constant Infiltration Model

Given a precipitation hyetograph for a particular storm over a $100$ acre watershed

Part $1$: Assume the infiltration can be accounted for with a $$ index constant:

Let $-$index $=2$ inches$/$hour as shown in the figure above

a$)$ Find the runoff volume:

b$)$ Find the fraction of the total precipitation in this storm that contributes to runoff.

Part $2$: An observer measured a volume of runoff due to this rainfall event falling on the watershed equal to $25$ acre$-$feet. What was the average infiltration rate, $?$