Here is a general estimation. http://www.grow.arizona.edu/Grow--GrowResources.php?ResourceId=208 That is about 1.5 inches per wk annual average in the southeast. I have read posts about humidity being high, etc. Actually, logically, when humidity is low, as this time of year when fronts come thru and humidity drops and wind howls is the worst time. Probably 1/2 or more inch per day. Also, following this logic, Arizona has over 100 per year. Aeration also causes evaporation(but I love aerations effects).
Also, as Otto has quoted many times...all ponds leak. You need to 1. have sufficient watershed for the area 2. a way of replenishing the water 3. keep the biomass low enough to withstand the drop in water volume.
