Soil evaporation can be calculated separately for two different types of surfaces if the surfaces differ such as in the case of drip irrigation (see switch SoilPartitioningArea). This approach is only applicable when soil evaporation is calculated with the surface energy balance approach. The division of the soil surface into two sections is defined by the parameter sfrac1, which determines the fraction of the surface belonging to area one. In the case of drip irrigation sfrac1 equals icover. Soil evaporation is thus calculated with Eqs. (4.2)-(4.9) for each section of the soil, and including the irrigation rate, idrip, in Eq. (4.9).
Plants may shadow the two sections of the soil differently, which can optionally be included in the simulation (see switch SoilPartitioningArea third option). In order to calculate the different amounts of radiation to each soil section, the position of the centre point in section one has to be known. In the case of drip irrigation this position is determined by the parameter ipos. Radiation is distributed through the canopy as explained in the section Radiation processes. Different values of net and long wave radiation to the ground, as well as the fraction of radiation absorbed by the canopy are calculated for each section and used separately in eqs. (4.2)-(4.9) to calculate soil evaporation (as explained above).
If the partitioned soil evaporation is combined with a plastic sheet, the uncovered soil fraction is assumed to be wet. Only if the uncovered soil fraction is larger than the wetted surface will soil evaporation also take place from the dry area.