I can't give you an exact number of gallons but I'll share my experience just
to give you a rough idea. I am in Louisville, KY where the summer temps are
high 80's and above with high humidity. My pond is 10' X 16' and 26 inches
deep. There is a waterfall and creekbed. The creekbed measures about 5' wide by
30' long and the whole thing is in full sun.
On those weeks when we have no rain, I will lose maybe 1 inch on water in the
pond. By my caculations, that's about 100 gallons.
Donna