Overview of Numerical Methods

There are three options in MIKE SHE for calculating vertical flow in the unsaturated zone:

Richards Equation

The full Richards equation requires a tabular or functional relationship for both the moisture-retention curve and the effective conductivity.

The full Richards equation is the most computationally intensive, but also the most accurate when the unsaturated flow is dynamic.

Gravity Flow

The simplified gravity flow procedure assumes a uniform vertical gradient and ignores capillary forces.

The simplified gravity flow procedure provides a suitable solution when you are primarily interested in the time varying recharge to the groundwater table based on actual precipitation and evapotranspiration and not the dynamics in the unsaturated zone.

Two Layer Water Balance

The simple two-layer water balance method divides the unsaturated zone into two zones: the root zone and the zone between the roots and the water table.

The simple two-layer water balance method is suitable when the water table is shallow and groundwater recharge is primarily influenced by evapotranspi­ration in the root zone.