Unsaturated flow is one of the central processes in most model applications. The unsaturated zone is usually heterogeneous and characterized by cyclic fluctuations in the soil moisture as water is replenished by rainfall and removed by evapotranspiration and exchange to the groundwater table.
Unsaturated flow is primarily vertical since gravity plays the major role during infiltration. Therefore, unsaturated flow in MIKE SHE is calculated only vertically in one-dimension, which is sufficient for most applications. However, this assumption may not be valid, for example, on steep hill slopes.
There are three options in MIKE SHE for calculating vertical flow in the unsaturated zone:
· the full Richards equation, which is the most computationally intensive, but also the most accurate when the unsaturated flow is dynamic;
· a simplified gravity flow procedure, which ignores capillary forces, and is suitable when you are primarily interested in the time varying recharge and not the dynamics in the unsaturated zone; and
· a simple two-layer water balance that is suitable when the water table is shallow and groundwater recharge is primarily influenced by evapotranspiration in the root zone.
More detailed information on the setup and calculation of unsaturated flow is found in the Chapters:
· Working with Evapotranspiration - User Guide (V1 p. 445)
· Working with Freezing and Melting - User Guide (V1 p. 455)
· Working with Unsaturated Flow - User Guide (V1 p. 581)
The Technical Reference manual includes detailed information on the calculation methods - Unsaturated Zone - Technical Reference (V1 p. 549).