Given a reservoir watershed of 22 square miles, and assuming a
rainfall of 0.75
inches of rain, with 35% of the rainfall draining overland as
surface runoff and
entering the reservoir, what does this runoff contribution amount
to in acre-feet of
water?

308 acre-ft of water

Explanation:

Given:

Area of the watershed = 22 square miles

Depth of Rainfall = 0.75 in ==0.0625 ft

Percentage rainfall falling in reservoir as runoff = 35%

Now,

1 square mile = 640 acre

Thus,

22 square miles = 22 × 640 = 14,080 acres

Thus,

The total volume of rainfall = Area of watershed × Depth of the rainfall

or

The total volume of rainfall = 14,080 acres × 0.0625 ft = 880 acre-ft

also,

only 35% of the total rainfall is contributing as runoff

thus,

Runoff = 0.35 × 880 acre-ft = 308 acre-ft of water