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Water ponded on overland flow plane cells will infiltrate into the soil as conditions permit. Infiltration is dependent upon soil hydraulic properties and antecedent moisture conditions, which may be affected by previous rainfall, run on, ET, and the location of the water table. In GSSHA, the unsaturated zone that controls infiltration may be simulated with a 1-D formulation of Richards’ equation (RE), which simulates infiltration, ET, and soil moisture movement in an integrated fashion. Infiltration may also be simulated using traditional Hortonian Green and Ampt (GA) (Green and Ampt, 1911) approaches which are simplifications of RE. There are three optional GA based methods to calculate infiltration for Hortonian basins: 1) traditional GA infiltration, 2) multi-layer GA, and 3) Green & Ampt infiltration with redistribution (GAR) (Ogden & Saghafian, 1997). The traditional GA and multi-layer GA approaches are used for single event rainfall when there are no significant periods of rainfall hiatus. The GAR approach is used when there are significant breaks in the rainfall, or for continuous simulations.

RE is a general equation and can be applied in any type of watershed or conditions. However, the simpler methods based on the GA equation are preferred when runoff is Hortonian, i.e. occurs due to infiltration excess, where the rainfall/run-on of water is greater than the possible infiltration rate. For fine textured soils the GAR method has been shown to closely mimic the RE solution (Ogden and Saghafian, 1997) and when applied in basins identified as Hortonian, the GAR method has been shown to produce results comparable with the RE (Downer and Ogden, 2003a).

However, when Hortonian flow is not the predominant stream flow producing mechanism, application of GA type models is ill advised and can result in erroneous results (Downer et al., 2002a). For cases where Hortonian flow is not the predominate process generating stream flow the RE should be used, and coupled with the saturated groundwater solution as appropriate. The saturated groundwater model can also be coupled with the GAR model. This represents a rather crude approximation of the actual processes, but has proven useful for numerous studies Eau Galle TN , JD31 TN. The advantage of this approach is a savings in computation time.

Representation of the soil column below each cell with Richards’ equation is presented. Formulation and application of the GA model is well described in other sources (i.e. Maidment, 1993) as well as the GAR method (Ogden and Saghafian, 1997). Formulation, solution, and application of the multi-layered GA model as applied in the GSSHA model is presented in Section 7.3.

GSSHA User's Manual

7 Infiltration
7.1     Richards’ Equation
7.2     Green and Ampt (GA)
7.3     Multi-layer Green and Ampt
7.4     Green and Ampt with Redistribution (GAR)
7.5     Parameter Estimates