For a stable water system, water evaporation ([E.sub.a]) can be described as follows (Lemon 1956):
(1) [E.sub.a] = [[rho.sub.c]/Lv ([e.sub.0] – [e.sub.a]/[r.sub.a]
where [e.sub.0] is the vapor pressure above the water surface, [e.sub.a] is the vapor pressure on the water surface, [r.sub.a] is the rate of water passed through the air, [[rho].sub.c] is the thermal capacity of dry air, v is the constant of psychrometer, and L is the parameter of thermal exchange of water evaporation. Under stable environmental conditions, [r.sub.a] is a constant determined by the climatic condition.
When surface film is added, the water evaporation rate ([E.sub.s]) can be expressed as:
(2) [E.sub.s] = [[rho].sub.c]/Lv ([e.sub.0] – [e.sub.a]/(r.sub.s] + [r.sub.a]
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