Vapour pressure at the soil surface

Vapour pressure at the soil surface is given by the surface temperature, Ts, the water tension of the uppermost layer, Ψ1, and an empirical correction factor, ecorr, accounting for steep gradients in moisture between the uppermost layer and the soil surface (Alvenäs & Jansson, 1997):


where R is the gas constant, Mwater is the molar mass of water, g is the gravity constant and es is the vapour pressure at saturation (see viewing function Vapour pressure at the soil surface).

The empirical correction factor, ecorr, depends on an empirical parameter ψeg and a calculated mass balance at the soil surface, δsurf, which is allowed to vary between the parameters sdef and sexcess given in mm of water.



where Wpool is the surface water pool, qin is the infiltration rate, Es is the evaporation rate, qv,s, is the vapour flow from soil surface to the central point of the uppermost soil layer, and the irrigation rate to the uppermost soil layer.