Groundwater:Coupling of the Saturated Zone Model to the GAR Infiltration Model

The GAR routine described in Section 7.4 calculates infiltration and soil moisture based on the movement of multiple sharp wetting fronts that occur with rainfall burst followed by rainfall hiatus. The multi-layer GA model described in Section 7.5 calculates infiltration and soil moisture in a user specified distributed three layer soil model. The infiltration calculated by GAR or multi-layer GA can be used to provide a rough estimate of groundwater recharge to the 2-D saturated groundwater flow simulation. The groundwater recharge for each time step is equal to the infiltration computed by GAR or multi-layer GA. In the model the groundwater recharge is computed each time step as

(52)

where: R is equal to the recharge m³, F is the total infiltration (m), and A is the area of the overland flow cell (m²). The simple two layer soil moisture accounting routine is used to calculate soil moisture. When the groundwater elevations exceed the ground surface elevation, infiltration calculations for the cell cease, and the groundwater surface exchange is calculated as described above. Anytime exfiltration occurs, the infiltration and overland flow processes are initiated if they are not already active. These processes remain active as long as exfiltration occurs and until all water on the overland flow plane stops flowing and infiltrating. Infiltration is not calculated for cells in which exfiltration is occurring.

When using the GA models to couple the surface soils to the groundwater table the properties of the unsaturated groundwater media existing between the specified soil column, SOIL_DEPTH for GAR, and water table are assigned the groundwater porosity. The soil saturation (fraction) for this unsaturated soil may be specified by the user with the SINGLE_UNSAT_SAT card. The default value is 0.75, meaning the soil is 75% saturated. This corresponds, roughly, to field capacity, the value at which the soil can no longer gravity drain. For comparison, the equivalent value in MODFLOW, would be 0.0. To obtain a MODFLOW like simulation the user would specify:

SINGLE_UNSAT_SAT 0.0

When using the RE solution to couple to saturated groundwater, soil moistures are calculated along the entire soil column to the free groundwater surface, regardless of the depth of the groundwater. This provides a variable soil moisture along the entire soil column and represents an advantage of the RE method.

More detail on the coupling of GAR and the saturated groundwater model can be found in these documents: