High-capacity wells and baseflow decline in the Wolf River Basin, northeastern Wisconsin (USA)

The baseflow of the Wolf River (drainage area of 1200 km²) in northeastern Wisconsin (USA) has declined by over 30 % during the last 30 years, whereas climatic, land cover, and soil characteristics of the basin have remained unchanged. Because groundwater basins do not always coincide with surface water basins, estimating groundwater discharge to streams using variables only pertinent to the surface water basin can be ineffective. The purpose of this study is to explain the decline in the baseflow of the Wolf River by developing a multiple regression model. To take into account variables pertaining to the groundwater basin, withdrawal rates from high-capacity wells both inside the Wolf River basin and in two adjacent basins were included in the regression model. The other explanatory variables include annual precipitation and growing degree days. Groundwater discharge to the river was calculated using streamflow records with the computer program Groundwater Toolbox from the United States Geological Survey. Without the high-capacity wells data, the model only explained 29.6 % of the variability in the groundwater discharge. When the high-capacity wells data within the Wolf River basin were included, r² improved to be 0.512. With the high-capacity wells data in adjacent basins, r² improved to be 0.700. The study suggests that human activity taking place outside of the basin has had an effect on the baseflow and should be taken into account when examining baseflow changes.