The physically based approach, for calculating soil
evaporation, originates from the idea of solving an energy balance equation for
the soil surface. According to the law of conservation of energy the net
radiation at the soil surface, *R _{ns}*, is assumed to be
equal to the sum of latent heat flux,

The three different heat fluxes are estimated by an
iterative procedure where the soil surface temperature, *T _{s}*, is varied according to a given
scheme until eq. (4.2) is
balanced:

(4.3)

where *r _{as}* is the aerodynamic
resistance calculated as a function of wind and temperature gradients (Eq. (4.12) - (4.24)),

where *d _{vapb}* is the tortuosity given as an
empirical parameter,

A plastic sheet can optionally be included in the
simulation (see switch Surface cover). This sheet prevents soil
evaporation from the fraction, *i _{scovevap}*.

More:

Vapour pressure at the soil surface

Resistance approach for soil heat flow

Aerodynamic resistance with stability correction below vegetation canopy