Vapour pressure at the soil surface

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

                                               (4.7)

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

The empirical correction factor, e_corr, depends on an empirical parameter ψ_eg and a calculated mass balance at the soil surface, δ_surf, which is allowed to vary between the parameters s_def and s_excess given in mm of water.

                                                           (4.8)

            (4.9)

where W_pool is the surface water pool, q_in is the infiltration rate, E_s is the evaporation rate, q_v,s, is the vapour flow from soil surface to the central point of the uppermost soil layer, and i_drip.is the irrigation rate to the uppermost soil layer.