15
temperature. It was also assumed that the net soil heat flux is zero
over a 24 h period. The resulting relationship
Pacpahfg (ea eas) + s(Rn G)
Ep = (- + s) (2-15)
where:
Ep potential evaporation [m3.m-2]
ea = vapor pressure of atmosphere [kPa]
eas = saturated vapor pressure at the air temperature [kPa]
h = surface vapor transfer coefficient [m's-1]
G = sensible heat flux in the atmosphere [W-m-2]
Rn = net radiation upon the soil surface [W-m-2]
s = slope of saturated vapor pressure line [kPa.'K-1]
7 = psychrometric constant [kPaOK-1]
provides an expression for the potential evaporation from the surface.
The boundary layer resistances account for advective conditions. The
Penman equation is the only method for estimating evaporation based
upon theoretical ideas and requires no highly specialized equipment.
Estimates of the evaporation have an accuracy of 5 to 10 percent on a
daily basis (Van Bavel, 1966). The disadvantage to the combination
method, as implemented by Penman, is that only daily estimates are able
to be determined and crop coefficients are necessary to estimate actual
evapotranspiration from various crops (Jones et al., 1984). The values
of the crop coefficients typically exhibit regional variation as well
as variation due to stage of crop growth.
Staple (1974) modified the Penman model to provide the upper
boundary condition for the isothermal diffusion of water in the soil.
The Penman model was modified by multiplying the saturated vapor