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### Chemical and Process Engineering Resources

 Category: Mass Transfer Question: How do I calculate pond evaporation rates? Keywords: evaporation,rates,ponds Answer: There are many factors involved in the calculation of water evaporation rates from ponds or lakes, and one of the most important factors is the wind velocity. I would suggest that you study the model recommended by ASHRAE (the American Society of Heating, Refrigeration and Air-conditioning Engineers): W = [A + ((V)](Pw - Pa)/Hv where: W = water evaporation rate, (lb/hr) per sq.ft. of pond surface areaA = a constant = 95 (**see note below)B = a constant = 37.4 (**see note below)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. **A study by the Department of Energy concluded that the ASHRAE equation was more accurate if A was taken as 68 (instead of 95) and if B was taken as 32 (instead of 37.4). That study may be read at the website below. Links: US Department of Energy Evaporation Study