Digital Climatic Atlas of Texas - Texas Water Development Board

Many evaporation models have been developed over the years to estimate
evaporation from water surfaces based on climatic parameters such as maximum and
minimum temperature, dew point temperature, solar radiation, wind speed and relative
humidity. The simple models such as the Thornthwaite method use only temperature and
the more complex models such as Penman-Monteith require inputs of almost all of the
climatic parameters mentioned previously to estimate evaporation (ASCE 1996).
However, all of these datasets are not readily available for 1890 to 2000. Hence, an
optimal balance had to found by selecting a model that computes evaporation with
reasonable accuracy using limited climatic data.
The aerodynamic method is one of the widely used procedures for estimating
evaporation from lakes and large reservoirs. The model was originally developed by U.S.
Geological Survey in the 1950’s (ASCE 1996):
E = M ( e − e ) u
(6)
s
z
z
where E is the evaporation in mm per day, M is the mass transfer coefficient, e s is the
saturation vapor pressure at the surface water temperature Ts (kPa), e z is the vapor
pressure of the air at height z (kPa) and u z is the wind velocity at level z in m/s. Although
direct measurements of saturated vapor pressure (e s ) and actual vapor pressure (e z ) were
not available, they can be calculated from maximum, minimum, and dew point
temperatures. The saturation vapor pressure and actual vapor pressure were calculated as
follows (Allen et al., 1998):
⎡ 17.27T
⎤
e ( T ) = 0.6108exp⎢
⎣T
+ 237.3⎥
(7)
⎦
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