Soil Evaporation

Soil evaporation, E_s, occurs from the upper part of the unsaturated zone and consists of a basic amount of evaporation, ET_ref ^.f₃ (q), plus additional evapo­ration from excess soil water as the soil saturation reaches field capacity. This can be described by the following function:

(20.9)         

where ET_ref is the reference evapotranspiration, E_at is the actual transpiration (Eq. (20.4)), f₁ (LAI) is from Eq. (20.5) and the functions f₃(q) and f₄(q) are given by

(20.10)      

In the absence of vegetation f₁(LAI) can be set to zero and E_at in Eq. (20.9) goes to zero. This allows us to see how E_s varies in relation to ET_ref for differ­ent values of q. Thus, Eq. (20.9) can be simplified to

(20.11)   

which is plotted in Figure 20.6.

In the MIKE SHE, soil evaporation is restricted to the upper node in the unsaturated zone, which, generally, should be about 10 centimetres deep, or less.

Figure 20.6        Soil evaporation E_s in relation to ET_ref as a function of q in the top layer when f(LAI) = 0.