# How do I calculate pond evaporation rates?

How do I calculate pond evaporation rates?

There are a few factors when calculating the water evaporation rates from ponds or lakes, among the more important ones are wind velocity. I recommend you study the ASHRAE model: W = [A + (B)(V)](Pw – Pa)/Hv where: W = water evaporation rate, (lb/hr) per sq.ft. of pond surface area A = a constant = 95 B = a constant = 37.4 V = air velocity over the pond surface, miles/hr Pw = vapor pressure of water at the pond water temperature, inches of Hg Pa = vapor pressure of water at the air dewpoint temperature, inches of Hg Hv = heat of vaporization of water at the pond water temperature, Btu/lb A good set of Steam Tables will provide you with the latent heat of vaporization and the vapor pressure of water at the temperatures involved. The vapor pressures may be provided in units other than inches of Hg, and you will need to make the conversion.

**There was a study by the Department of Energy that concluded ASHRAE equation was more accurate if A was taken as 68 (instead of 95) a