I came up with 310 acres for the watershed. I don't think a 30 acre lake would work, more like 10-15 acres. With only 22.5 inches of rain and 1/2 of that for runoff it would take at least 2-3 years to fill up a 30 acre lake. I am guessing but if you built the 30 acre lake you would have an average depth of 10-15 feet. That would equal 450 acre feet of water. Your watershed will provide about 290 acre feet per year. You will also lose at least 6-7 feet of water to evaporation every year.
