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MODFLOW-NWT model data sets for simulating effects of groundwater withdrawals on streamflows in Northwestern Chippewa County, Wisconsin

March 23, 2018

A new groundwater flow model for western Chippewa County, Wisconsin has been developed by the Wisconsin Geological and Natural History Survey (WGNHS) and the U.S. Geological Survey (USGS). An analytic element GFLOW model was constructed and calibrated to generate hydraulic boundary conditions for the perimeter of the more detailed three-dimensional MODFLOW-NWT model. This three-dimensional model uses the USGS MODFLOW-NWT finite difference code, a standalone version of MODFLOW-2005 that incorporates the Newton (NWT) solver. The model conceptualizes the hydrogeology of western Chippewa County as a six-layer system which includes several hydrostratigraphic units. The model explicitly simulates groundwater-surface-water interaction with streamflow routing. Model input included recent estimates of aquifer hydraulic conductivities and a spatial groundwater recharge distribution developed using a GIS-based soil-water-balance model for the study area. Groundwater withdrawals from pumping were simulated for 269 high-capacity wells across the entire model domain, which includes western Chippewa County and portions of eastern Dunn County and southeastern Barron County. Model calibration used the parameter estimation code PEST, and calibration targets included heads and stream flows. Calibration focused on the period from during 2011 to 2013 when the largest amount of calibration data were available. Following calibration, the model was applied to two distinct scenarios; one evaluating hydraulic impacts of more intensive industrial sand mining and the second evaluating the hydraulic impacts of more intensive agricultural irrigation practices. Each scenario was developed with input by Chippewa County and a stakeholder group established for this study, and designed to represent reasonable future build-out conditions for both mining and irrigated agriculture. The mining scenario underscores the potential hydraulic impacts related to changing land-use practices (i.e., hilltops and farm land becoming sand mines), while the irrigated agriculture scenario illustrates the potential hydraulic impacts of intensifying existing land-use practices (i.e., installing new wells to irrigate farm fields).