Evapotranspiration - Technical Reference

In MIKE SHE, the ET processes are split up and modelled in the following order:

1.       A proportion of the rainfall is intercepted by the vegetation canopy, from which part of the water evaporates.

2.       The remaining water reaches the soil surface, producing either surface water runoff or percolating to the unsaturated zone.

3.       Part of the infiltrating water is evaporated from the upper part of the root zone or transpired by the plant roots.

4.       The remainder of the infiltrating water recharges the groundwater in the saturated zone where it will be extracted directly if the roots reach the water table, or indirectly if capillarity draws groundwater upwards to replace water removed from the unsaturated zone by the roots.

The primary ET model is based on empirically derived equations that follow the work of Kristensen and Jensen (1975), which was carried out at the Royal Veterinary and Agricultural University (KVL) in Denmark.

MIKE SHE also includes a simplified ET model that is used in the Two-Layer UZ/ET model. The Two-Layer UZ/ET model divides the unsaturated zone into a root zone, from which ET can occur and a zone below the root zone, where ET does not occur. The Two-Layer UZ/ET module is based on a formulation presented in Yan and Smith (1994). Its main purpose is to provide an esti­mate of the actual evapotranspiration and the amount of water that recharges the saturated zone. It is primarily suited for areas where the water table is shallow, such as in wetland areas.