Here are two links to evaporation calculations for small lakes / ponds. This information is a study of the evaporation rates of a specific pond and gives information on how to calculate losses. The other gives the PER for Texas. This same information is available for all states and is available from the National Weather Service or from your USDA.
All you need to do then is find the annual rainfall ( seasonally / monthly) and know waht the run off coeficient is.

http://hyper20.twdb.state.tx.us/Evaporation/pancoef.txt
http://snow.ag.uidaho.edu/Publications/pond_evap/POND.html


Mike