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Name: Andy T.
Status: Other
Age: 50s
Location: N/A
Country: N/A
Date: September 2002


Question:
Is there an approximate formula for the evaporation rate of a body of water(such as a lake), taking into account water temperature, surface area, and possibly relative humidity? Rate(in vol^3/sec) = f(area,temperature,humidity)



Replies:
Rates of evaporation are difficult to determine accurately because so many factors enter in -- wind speed, presence of impurities in the water, cooling of the surface by the evaporation itself, and so on, but an approximate equation taking temperature and relative humidity into account is: Rate = P*( 1- R.H./100 ), where P is the vapor pressure of water at the given lake temperature and R.H. is the relative humidity. As you can see the rate of evaporation increases rapidly with increasing temperature because the vapor pressure of water does so, but the rate of evaporation approaches zero as the R.H. approaches 100% regardless of the temperature.

Vince Calder


Thanks, but how can Rate = P*( 1- R.H./100 ) be correct if P is in units of pressure (Force/Area) and Rate is in units of Volume/Time (assuming ( 1-R.H./100 ) is dimensionless). Is there a constant missing somewhere?

The equation does indeed contain several unstated parameters. The only ones that I included are strictly the effects of the increase in vapor pressure with temperature, and the effect of relative humidity, so a "proportional to" should be used to be more accurate. The effects of wind speed, air temperature (which in general will be different that the water temperature), temperature gradients in the water, turbulence, and a bunch of other factors are not included. Actually calculating the evaporation rates that would be quantitatively pertinent requires a rather complicated model to incorporate all those (and probably other) factors that influence the evaporation of water. Sorry about the misunderstanding.

Vince Calder



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