Richards Equation:Modeling the Unsaturated Zone with Richards Equation
Modeling of the unsaturated zone is a key addition of the GSSHA model. Richards’ equation is currently the most complete method to simulate soil water movement in the unsaturated zone. When Richards’ equation is solved in GSSHA to calculate soil moistures and soil water movement, infiltration, actual evapotranspiration, and groundwater recharge are also calculated as part of an integrated solution. Because most soil water movement is in the vertical direction, a 1-D representation of the soil column is employed to simulate the unsaturated zone. The GSSHA representation of the soil column is shown in Figure 21. As shown in this figure, when using Richards’ equation to represent the unsaturated zone, the soil column is subdivided in to three layers: A, B, and C horizons. Parameters must be specified for each of these three layers. Unlike the Green and Ampt based models of infiltration, Richards’ equation is a partial differential equation that must be solved numerically. Therefore, each of the soil layers must be subdivided into cells. The user must specify the cell size of each layer, along with the hydraulic properties of the soil.
To use Richards’ equation, several global and distributed parameters must be set. The global parameters are related to the numerical solution of Richards’ equation. The distributed parameters describe the soil and are similar to the parameters used in the Green and Ampt approaches.
Richards’ equation is selected from the GSSHA Job Control Parameters dialog under the Infiltration options.