I am having some trouble with the equations used for calculation of
evapotranspiration in Bonan, G. B. 1989. A computer model of the solar
radiation, soil moisture, and thermal regimes in boreal forests.
Ecological Modeling 45:275-306 ) [Equations in question are on page
It does not make intuitive sense to me that values for Ep should be
negative (which is how my calculations have come out), nor that they
should have the units cal cm^-2 day^-1. Walter Larcher's text uses
measures of mass or volume of water per unit area per unit time for
evaporation and transpiration.
Should saturation vapor pressures be negative?
Does anyone know enough about these kinds of calculations to give me
some advice? The main equation is Ep = a (Ta + b) Rs.