The empirical approach for soil evaporation is based on the Penman combination equation as suggested by Monteith (1965). It uses the available energy at the soil surface, Rns-qh, to calculate latent heat flux from the soil surface, LvEs, from which the soil surface evaporation, Es, can be derived:
where Rns is the net radiation at the soil surface, qh is the soil surface heat flux from the previous time step, ras is the aerodynamic resistance, rss is the surface resistance at the soil surface, es is the vapour pressure at saturation in the air, ea is the actual vapour pressure in the air, and ∆ is the slope of saturated vapour pressure versus temperature curve. The density, ρa, and heat capacity, cp, of air, the latent heat of vaporisation, Lv, as well as the psychrometer constant, γ, are all considered as physical constants.
The surface resistance at the soil surface, rss, can be estimated by two different empirical functions accounting for moisture conditions at the soil surface and the water tension in the uppermost soil layer. The first approach (“PM-eq, Rs(1Par)”) is based on only one governing parameter:
where rψ is an empirical coefficient and ψs is the water tension in the uppermost layer (see viewing function Surface Resistance, Penman eq. 1 par). The δsurf is the mass balance at the soil surface in units mm of water (see eq. 4.9).
The second approach (“PM-eq, Rs(3Par)”) is based on three governing parameters:
Optionally, (“K-function”) the soil evaporation can be estimated as the minimum value of the flow rate that could be supplied from the middle point of the uppermost soil layer and the potential rate according to Eq. (4.26) taking rss=0.
The soil surface temperature will also be estimated (for all
of the three approaches described above) if the switch
Surface cover Value Meaning No No surface cover Plastic sheet A plastic sheet covering parts of the soil and thus
preventing soil evaporation. Surface Temperature
No surface cover
A plastic sheet covering parts of the soil and thus preventing soil evaporation.
is put to “f(PM-equation)”. This is done by first solving the heat balance equation for the sensible heat flow to the air as:
where the soil surface heat flux, qh, is taken from the preceding time steps. The soil surface temperature is finally given as:
Alternatively the soil surface temperature can be set equal to the air temperature except when snow covers the surface (option “Air temperature”).