Re: What is a good rule of thumb for the rate of water evaporation?

I'm not a meteorologist, but I did manage to find some information on 
evaporation. The National Weather Service has developed a nomograph for 
calculating lake evaporation. A nomograph is a diagram used for graphically 
calculating one variable using one or more others. This one requires the 
following input data: mean daily temp., solar radiation in langleys 
(cal/cmx2) per day, mean daily dew point temp., and wind movement in miles 
per day. Here is the reference:

Kohler, M. A., T. J. Nordenson, and W. E. Fox, 1955, Evaporation from ponds 
and lakes, US Weather Bureau Research Paper 38.

You can also apparently find the nomograph in 

Roberts, W. J., and J. B. Stall, 1967, Lake evaporation in Illinois. 
Illinois State Water Survey Report of Investigation 57.

Other empirical methods of estimating evaporation can be found in

Harbeck, G. E., 1962, A practical field technique for measuring reservoir 
evaporation utilizing mass-transfer theory. US Geological Survey 
Professional Paper 272-E, 101-5.

and

Kohler, M. A., and L. H. Parmele, 1967, Generalized estimates of free-water 
evaporation. Water Resources Research, v. 3, p. 997-1005.

I hope you find these sources helpful.

David Kopaska-Merkel
Geological Survey of Alabama