Presentations 2016

Irrigation Impacts in the Northern Great Lake States

University of Wisconsin - Stevens Point

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Irrigated agriculture is expanding rapidly into the humid and traditionally nonirrigated eastern US, bringing fresh environmental challenges that have yet to be assessed and needs for management frameworks that have yet to be constructed. The northern great lake states region of Minnesota, Wisconsin, and Michigan is one part of the eastern US where irrigation expansion is rapid, rising from negligibility in the 1950s to 290,000 ha by 1978 and to 639,000 ha in 2013. Irrigation in the northern great lake states is mostly groundwater-sourced and commonly occurs in areas containing coarse-grained glacial sediments, droughty soils, and productive aquifers. Unlike much of the arid US west, irrigation in the northern lake states is “supplemental.” By augmenting rainfall, periods when soil moisture might be limiting are bridged, allowing high water demand agriculture and greater profitability. In contrast with the west, where irrigation water is often supplied by surface water, aquifers poorly connected to local hydrologic processes, or the valley aquifers of large rivers, lake states irrigation source water is predominantly locally-recharged groundwater with strong connections to local lakes and streams.We characterized the impacts of groundwater irrigation in the northern lake states in the Wisconsin central sands, a 6500 km2 region composed of 30-60 m of coarse glacial sediments overlying less permeable bedrock. The area receives about 800 mm y-1 of precipitation of which about 250 mm y-1 recharges groundwater. Groundwater feeds numerous lakes, wetlands, and nearly 1000 km of headwater streams that support coldwater ecosystems. Over 2000 high capacity wells tap the same groundwater to irrigate field corn, potato, sweet corn, and other processing vegetables. Irrigation effects on central sands surface waters have been substantial and include complete drying of some lakes and streams. Commonly, baseflows are reduced by a third or more in stream headwaters and water levels are reduced in aquifers and lakes by more than a meter. Observed flow and stage reductions can be explained by an increase of 45 to 142 mm y-1 in evapotranspiration on irrigated land compared with pre-irrigated land cover.We conclude that irrigation water availability in the northern lake states and other regions with strong groundwater-surface water connections is tied to concerns for surface water health. Thus a management framework will require a focus on the upper few meters of aquifers on which surface waters depend rather than the depletability of the aquifer.