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resistance and aerodynamic resistance are an important factor in determining evapo-
transpiration from different surfaces (rough vs. smooth and dry vs. wet).
Whereas the radiation term of the Penman equation is determined mainly by the
energy input at the surface, the aerodynamic term is strongly coupled to the atmo-
spheric conditions (in particular vapour pressure deicit). Through various feedback
mechanisms, the magnitude of the aerodynamic term relative to the radiation term
has a limited range. This fact is employed in Priestley
-
Taylor method, which uses the
radiation term of the Penman equation (also called equilibrium evaporation) as the
starting point and adds the aerodynamic term as a ixed fraction. The Makkink method
is a further simpliication that replaces the available energy by global radiation.
As dewfall is the inverse process of evaporation from a wet surface, it can be well
described with the Penman method.
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