Surface-Groundwater Interactions
In the analytic element method surface water features, which form boundary conditions to the groundwater flow regime, are represented by line-sinks. In the far-field (outside the domain of interest in the model) the heads in the aquifer are set equal to the water levels in the streams and lakes, constant head or Dirichlet boundaries. In the near-field, however, it may be desirable to include resistance to flow between the aquifer and some of the surface water features, head dependent flux or Cauchy boundaries. In GFLOW the user can specify a resistance, width, and depth parameter for line-sink strings. The document "Dealing with Resistance to Flow into Surface Waters" provides guidelines for selecting these parameters.

GFLOWhas special features to model lakes as part of a stream network without specifying the lake stage in advance. The lake stage is determined based on a water balance calculation, which includes both surface water and groundwater inputs and outputs, as well as precipitation and evapotranspiration. Outlet streams may be given a "stage-outflow" table and the lake may be given a "stage-lake area" table. GFLOW provides a detailed water balance report for the lake. Details on the working and use of these lake-features are found in the document "Modeling Lake-Groundwater Interactions in GFLOW."

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