I've heard that rule of thumb before, but really question it. Doesn't the real answer depend on where you live, the amount of annual rainfall you get, and the surrounding soil composition?
I had a NRCS "expert" tell me 4 years ago that my planned 3.5 acre pond would never fill up or stay full because it has only about 2 acres or less of watershed....it filled within one month of completion and has been full ever since (for the last 4 years). Now if that same pond was in West Texas, no amount of watershed could keep it full.
Seems to me that the rule of thumb needs a little common sense applied to it.
