At a water elevation of 6,391 feet, mono lake (see figures below) has a volume of 2,940,000 acre-feet and a surface area of 48, 1 00 acres. annual inputs to the lake include 10 inches of direct precipitation, runoff of gauged streams of 200,000 acre-feet per year, and ungauged runoff and groundwater inflow of 30,000 acre-feet per year. evaporation from the lake surface is 50 inches per year. 5. what is the residence time for water in mono lake when the water surface is at 6,391 ft? *note: the residence time of a body of water is the average amount of time it would take for the volume of water in the lake to be exchanged once.