FWIW, my pond will go from 6' low to overflowing in the Spring due to snow melt and Spring rains. I haven't calculated my watershed, but in a normal year, it's enough to fill my pond.

We're talking somewhere around 2.5 million gallons of water to do that.

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).