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Lakes and Evaporation
Name: Donald P. G.
Status: educator
Age: 60s
Location: N/A
Country: N/A
Date: 2001
Question:
Given two lakes having equal water temperatures, equal
ambient temperature and relative humidity, and equal daytime radiational
heating and nighttime radiational cooling with one lake at sea level and
the other at an elevation of say 5000 feet above sea level will the lakes
experience equal evaporation rates or will the 5000 foot elevation lake
experience a higher evaporation rate and if so what are the governing
equations?
This question presumes that the lake temperature is such that evaporation
rather than condensation will occur.
Rephrased the question is: With all other factors being equal does a
significant change in barometric pressure influence the evaporation of
water from a water surface?
Replies:
Donald,
The condition that you list, relative humidity
the same, implies that there is approximately the
same density of water vapor in the air at both locations,
even though the atmospheric pressures are different.
Since vapor pressure is proportional to atmospheric
pressure (times .001) to a greater degree than
saturation vapor pressure is (.0000035), your
scenario is unlikely.
Other conditions needed for this scenario are: the same
wind speed, lake dimensions, local geography (affecting
turbulence and thus evaporation rate), and water composition.
Assuming that the scenario exists, there would be
slightly less water vapor in the air at the higher elevation.
Therefore, water can evaporate very slightly faster
from the lake at the higher elevation.
Evaporation under these tightly controlled conditions would be completely
proportional to the difference between the saturation vapor
pressure at the lake surface and the vapor pressure of the air.
If this difference increased 10% (drier air in the absolute sense)
then evaporation would increase 10%. If the air had more moisture,
evaporation would decrease.
Because of the complicated dependence upon turbulence, temperature,
radiation, water composition, and atmospheric pressure, I would prefer
not to try to present the governing equations for evaporation.
These equations are many and complicated and are not normally
handled except in atmospheric computer models. There are many
good technical textbooks that cover this subject, including ones
by a leading expert that I know, Wilfried Brutsaert. If you search
for his name on Amazon.com or a scientific textbook site on the
internet, you will probably find his books listed.
David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
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Update: June 2012
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