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Simulation of saturated 2-D groundwater flow is selected by placing the GW_SIMULATION card in the project file. When groundwater simulations are performed, the starting water surface elevations (m) must be provided in a file specified with the WATER_TABLE card; the file containing bedrock elevations (m) is specified with the AQUIFER_BOTTOM card. The groundwater time step must also be specified (s) using the GW_TIMESTEP card, typically 1 hour, 3600 s. A variety of boundary conditions can be specified with the GW_BOUNDFILE card. Saturated groundwater may be simulated using either the GAR or RE methods to calculate infiltration. In GSSHA v5 and above, multi-layer GA can also be used to simulate infiltration for groundwater models. The saturated groundwater solution was developed in conjunction with the RE unsaturated groundwater solution, and is intended to be used with RE. The GAR method was linked to the saturated groundwater model to provide a simple, quick, and approximate groundwater recharge estimate. This method is a useful screening tool, and can be used to help define parameter values before applying the more rigorous RE method. The multi-layer GA model was extended for use with continuous simulations and as a result, was also extended for use with the saturated groundwater model. Whether GAR or RE or multi-layer GA is used affects how parameter values are assigned, as described below.

GSSHA User's Manual

8 Groundwater
8.1     General
8.2     Formulation
8.3     Solution
8.4     Assignment of Parameter Values
8.5     Boundary Conditions
8.6     Coupling of the Saturated Zone Model with the Richards’ Equation Model of the Unsaturated Zone
8.7     Coupling of the Saturated Zone Model to the GA Type Infiltration Models