Ask A Scientist© Environmental Earth Science Archive

name         Andy T.
status       other
age          50s

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)
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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
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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?
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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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