To Eric in Tn and others,
I may have found a page that explains the difference between the two evaporation rates that Eric in Tn reported to have found. Take a look at:
http://www.grow.arizona.edu/Grow--GrowResources.php?ResourceId=208 It has a US Class A Pan Annual Evaporation Map that you can click on for a little better view. In Tennessee, the 50 inch line runs right through the state. However, if you read further down the page it states that you need to multiply the Pan Evaporation by .7 to get the rate of evaporation for larger impoundments like ponds and lakes. That might explain the variation in the two numbers. I'm not sure of the science behind it, but 50 inches would be the Pan Evaporation, and about 35 inches would be the actual loss from a larger body of water. Hope this link might help others view the evaporation rates for their location.
