Runoff from a 250-square mile catchment drains into a reservoir that has an aver

Question

Runoff from a 250-square mile catchment drains into a reservoir that has an average surface area of 4500 acres. The reservoir is to be used for a municipal water supply. Annual rainfall over the watershed is 36 inches and the annual runoff from the catchment is 12 inches. Evaporation from the reservoir averages 40 inches per year. The municipal water demand equals 100 million gallons per day. - Compute the change in volume (acre-ft) then the change in depth (ft) of water stored in the reservoir over one year

Explanation / Answer

Surface area of reservoir = 4500 acres

Increase in its volume due to rainfall = 36 * 4500 =16652004.8 m^3 = 4.4 x 10^9 gallons

Increase in its volume due to runoff from catchment = 12 * 250 =197357094 m^3 = 5.22 x 10^10 gallons

Decrease in its volume due to evaporation = 40 * 4500=18502227.6 m^3 = 4.89 x 10^9

Decrease in its volume due to consuption = 100 million gallons * 365 =138167530 m^3

Therefore, total change in volume =16652004.8 + 197357094 - 18502227.6 - 138167530 = 57339341.2 m^3

= 2.025 x 10^9 cubic ft

Change in depth =2.02500 x (10^9) (cubic ft) / (4500 acres) = 3.15 meters =10.3346 ft

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