Richards Equation:Soil depth and discretization
The soils in the unsaturated zone need to be defined down to a depth that includes the active region of the soil, where the water content experiences large changes in value. This should include the root zone. Typically, soil surveys describe the A, B, and C soil horizons of soils in the study region. In lieu of field measurements, these surveys provide information on the appropriate thicknesses of each of the soil layers for the Richards’ equation model. While Downer (2002) showed that the GSSHA model is not sensitive to the depth of the soil layer, it is best to err on the conservative side and make the soil columns deeper than necessary. As this will increase the computation time, it may be useful to experiment with the soil layer depths to reduce the amount of soil being simulated, if possible. When the saturated zone is the lower boundary, the area between the defined soil column and the water table will be assigned the parameters used in the third, C, layer, and the cells will be the size specified in the WMS subsurface parameters dialog found in the job control options.
The other big consideration in application of Richards’ equation is the cell discretization. Essentially, the smaller the cells the better the answer, but the longer the simulation time. Very small cells, 1 cm or less, will typically be required in the top 10 cm of the soil column (van Dam and Feddes 2000; Downer 2002). Much larger cells may be used at depth in the soil column. Increases in the cell size between layers should be gradual.