The surface temperature, Ts, is the upper boundary condition for the soil and can be specified in different ways (see switch “Surface temperature” in the section on soil evaporation). If soil surface temperature, Ts, is not measured, the simplest way is to assume for snow free periods that the surface temperature equals the air temperature. If soil evaporation is not accounted for, this approach has to be used.
If the interaction between aerodynamic properties, plant cover and surface evaporation is of interest, the surface temperature may also be calculated by solving the heat flow equation at the soil surface. This physical approach is described in the section on soil evaporation, and is also relevant for the boundary condition for the water flow equations.