Building a Model:Saturated Groundwater Modeling

Whether to simulate saturated groundwater depends on the properties of the watershed to be simulated and the availability of sub-surface information. For intermittent streams, lateral groundwater flow is not likely an important consideration. For streams with significant baseflow, and for streams where there is an obvious contribution of groundwater flow or known or suspected saturated source areas during rainfall events, groundwater simulations will likely be required to capture the shape and volume of discharge hydrographs (Downer et. al., 2002a, Downer et al. 2002c).

When saturated groundwater is to be simulated the information on the bedrock elevations and properties of the saturated groundwater media are required. In addition, boundaries must be supplied along the watershed boundary. Unlike in surface water flow, where the boundary condition is obvious and simple, assignment of groundwater boundaries can be difficult and the boundary conditions can dominate the groundwater flow solution. If there is a known groundwater divide along all (unlikely) or part (more likely) of the watershed boundary then a no flow boundary can be used on this part of the watershed boundary. Otherwise, head boundaries must be imposed. Imposition of improper head boundaries will lead to bad simulations despite an otherwise good model. Head boundaries along the edge of the watershed should come from measured well data or from a good regional groundwater model that includes the watershed of interest.

If channel routing is being performed, the streams should be defined as RIVER_FLUX boundaries and the flux between the stream and the groundwater will be computed every stream routing time step. For this calculation to be meaningful, the thalweg elevation and the grid land surface elevation must be correct, or at least the difference in thalweg elevation and land surface elevation must be correct. For this reason, land surface elevations in the cells corresponding to stream nodes may need to be adjusted. This is almost a requirement if smoothing of the thalweg elevations has occurred. If groundwater/stream interactions are to be simulated, it is best to begin by editing the land surface grid elevations to produce a smooth channel profile, and then subtract the incision of the channel in the grid cell to determine the thalweg elevations to be used in the channel input file.

Initial water surface elevations may be interpolated from well data or may be some assumed elevations. In either case it is usually necessary to run the groundwater simulations for an extended period to produce realistic initial values of groundwater elevation to be used in simulations. For instance, if the simulation period of interest is May though September, the model should be run from January to May to produce a starting WATER_TABLE file. Depending on the outcome, this procedure may have to be repeated multiple times, where the ending water surface elevation becomes the starting water surface elevation for the next attempt. A proper initial water surface has been established once the baseflow is on the correct order of magnitude, and maps of groundwater elevation are both smooth and agree reasonably well with well observations.