Total Water Inflow - Total Water Outflow = Change in Water Volume

Water Inflow from Precipitation = P x A = 0.8 ft x 200 acres = 160 acre-ft

Water Inflow from Streamflow = 40 acre-ft

Total Water Inflow = 160 acre-ft + 40 acre-ft = 200 acre-ft

Total Water Outflow from Evapotranspiration = E x A = 1 ft x 200 acres = 200 acre-ft

Change in Water Volume = 200 acre-ft - 200 acre-ft = 0