2.3 TECHNIQUES USED TO CALCULATE
POTENTIAL EVAPOTRANSPIRATION
There are numerous approaches used to estimate ET and potential
evapotranspiration (ETp). The following methods are frequently used:
mass (water vapor) transfer, energy budget, watershed water budget, soil
water budget, groundwater fluctuations, and empirical formulae. The
different techniques have been developed partly in response to the availa-
bility of data for estimating ET. Each method has certain advantages and
limitations. The availability of data is often the limiting factor in the
choice of calculation technique for practical applications.
The choice of calculation technique also depends on the intended use
(Burman et al., 1981; Doorenbos and Pruitt, 1977; Jensen, 1974; Linsley
et al., 1975; Saxton, 1982) and on the time scale required by the problem.
For example, irrigation management requires daily estimates of ET to
allow producers to make rational decisions concerning the timing and
amount of irrigation. In contrast, basin level planning may require
monthly estimates of ETto project changes in water supplies and require-
ments during the year.
2.3.1 Penman Method
The Penman (1948) equation was derived by rearranging the energy
balance equation (Equation 2) without the small photosynthetic compo-
nent. When applying the Penman formula over a 24-hour period, the net
energy component going into heating the soil is small, because a large
part of the energy absorbed by the soil during the day is lost at night.
Therefore, the soil heat flux density term, G, can be dropped for 24-hour
calculations. The sensible heat flux density term, H, is replaced in the
Penman formulation by mathematical substitutions, using the saturation
vapor pressure vs. temperature relationships. Finally, these procedures
yield the Penman formula for potential evapotranspiration based on four
major climatic factors: net radiation, air temperature, wind speed, and
vapor pressure deficit. The reader is referred to Penman (1948, 1956,
1963), Tanner and Pelton (1960), Monteith (1964), Tanner and Fuchs
(1968), Jensen (1974), Doorenbos and Pruitt (1977) and Burman et al.
(1981) for more thorough discussions of the derivation and its applica-
tion. The potential ET for each day can be expressed as
ET, AR,,/ + E,(3)
EAT = -Y(3)
A+Py
where ETp = daily potential evapotranspiration, mm/day
A = slope of saturated vapor pressure curve of air, mb/C
R, = net radiation, cal/cm2 day
X = latent heat of vaporization of water, 59.59-0.055 Tavg
cal/cm2 mm or about 58 cal/cm2 mm at 29C
Ea = 0.263 (ea ed)(0.5 + 0.0062u2)
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