GFLOW groundwater flow model for the southern part of the Nicolet Unit of the Chequamegon-Nicolet National Forest, Wisconsin

The model simulates two-dimensional groundwater flow and base flow in streams in the southern Nicolet Unit of the Chequamegon-Nicolet National Forest using the analytic element program GFLOW (Haitjema, 1995). Significant streams and lakes in the model domain are represented at varying levels of detail as linesink elements fully connected to the groundwater system. The highest level of detail is given to surface water features within the Forest Unit, while greatly simplified features around the perimeter of the model provide a boundary condition for the groundwater flow system. Groundwater discharge to the linesinks is computed, and routed through the surface water network as base flow. Recharge to the groundwater system was obtained from a separate soil-water-balance (SWB) model, which uses topography, soils, land use, and climatic data to estimate recharge as deep drainage from the soil zone. Estimated average annual recharge rates were supplied to the GFLOW model on a regular 1-km grid covering the model domain, using the Hybrid GFLOW-MODFLOW feature of GFLOW.

The model was calibrated to groundwater levels, interpreted lake stages, and estimated annual base flows in streams, \with an emphasis on data collected since 1970. Parameters representing hydraulic conductivity, groundwater recharge, and surface water bed-sediment resistance were adjusted using the parameter estimation program PEST, to produce simulated groundwater levels and base flow estimates that agree with field measurements, and input parameter values that are consistent with previous studies and the conceptual understanding of the area. This USGS data release contains all of the input and output for the simulations described in the published report, which can be found in the main directory of this data release.