If you search for evaporation and precipation for your state you will likely find a website with relevant data. The average amount will depend on your location and in some states the data is quite detailed.

The monthly data varies from year to year but in general the evaporation and precipitation are inversely correlated, that is, in months where the precipitation is above normal .... the evaporation will be below normal.

Last edited by jpsdad; 09/19/18 08:00 AM.
It isn't what we don't know that gives us trouble, it's what we know that ain't so - Will Rogers