You should be able to get similar data for your area of the country to the information posted below. This is from NOAA historical information on rainfall and pan evaporation. I have used this to predict and verify runoff rates vs. various type and amount of rain events ( such as a 1" hard rain vs. a 2" slow rain) and to track pond level change against expected drop due to evaporation.
This might help,
http://hyper20.twdb.state.tx.us/Evaporation/evap2.html

Mike