The Effects of Increased Irrigation on Streamflow, Lake Levels, and Groundwater Levels in the Central Wisconsin Sand Plains

The Central Wisconsin Sand Plains makes up 1.5% of Wisconsin and represents a major groundwater discharge area. Agriculture is highly productive in this region because of irrigation development. Central Wisconsin has the greatest density of high capacity wells in the state, most of which are used for irrigation. Rapid expansion of irrigated lands since the 1960’s has led to concerns about continued availability of groundwater to support surface water systems and existing domestic uses. The objective of this study is to compare historic trends in groundwater and surface water data for the Central Wisconsin Sand Plains to quantify any losses of groundwater. Our hypothesis is that more heavily irrigated areas will show a decline in surface and groundwater levels. In order to address these issues, statistical comparisons of groundwater and surface water records were made for a time prior to heavy irrigation (before 1970) and for the current time period when irrigated lands have greatly expanded. We compared well, river, and lake data to determine aquifer decline. Mean annual and mean monthly water levels from eight USGS monitoring wells (four in heavily irrigated areas and four in minimally irrigated areas) show statistically significant trends. Wells located in areas with many high capacity wells show a decline in water levels of up to four feet while wells located around fewer high capacity wells show no change or an increase in water levels. Overall decreasing trends were found in the baseflow of rivers in highly irrigated areas with the greatest decline occurring close to the local groundwater divide. Comparisons of lake levels using regression analysis from thirteen lakes with yearly measurements show a drop from one to four feet in highly irrigated regions compared to a gain of zero to 2 feet in un-irrigated regions. This research provides information to regulators to help balance the water needs of ecology, irrigators, and recreators.