Note: All parameters are defined at the beginning of an evapotranspiration period and are assumed to vary linearly from the beginning of one period to the beginning of the next period.
Potential Evaporation (PEVAL): [L]/[T]. May be estimated using, say, the Penman equation.
Surface resistance (SRES): 1/[L]. In absence of a surface crust, the value should be set to the reciprocal of the depth of the top node. If a surface crust is present, the value may be reduced to account for a lower permeability in the crust relative to the soil.
Pressure potential of atmosphere (HA): [L]. May be computed using the Kelvin equation
HA = R * T * ln(ha) / (Mw * g)
where
R = universal gas constant
T = temperature in degrees Kelvin
ha = relative humidity of atmosphere
Mw = molecular weight of water
g = acceleration due to gravity
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