CLIMATE VARIABILITY: GROUNDWATER FLOODING AND MITIGATION IN WISCONSIN

Heavy rain caused widespread flooding across much of the Midwest in June 2008. Floodwaters inundated over 4,380 acres in the town of Spring Green, Wisconsin, on a terrace a mile north of the Wisconsin River floodplain. Antecedent conditions included the wettest month on record in August 2007 followed by a winter with twice the normal amount of precipitation. In the sand and gravel aquifer in Spring Green, the water table elevation rose by 5.0 feet in response to spring recharge. An additional 3.4 feet of water table rise was recorded after the June storms. Field observations and results of a groundwater flow model support the conclusion that the water table rise above ground surface caused extensive and long-lasting (more than 5 months) flooding.

Groundwater flooding can occur in areas with poorly developed surface drainage when climatologic and hydrogeologic conditions result in water table rise above land surface. Where high water table conditions persist for several years or are a frequent occurrence, the area may be regarded as an isolated wetland or pond. In Spring Green, where groundwater flooding is relatively rare, extensive property and crop loss occurred. The Federal Emergency Management Agency awarded over $4 million in mitigation funds towards purchase of 28 properties in Spring Green that will be returned to open space. The buyouts were based in part on calculation of a flood recurrence interval, which is directly related to the probability of groundwater flooding. Data available for this analysis included a 26-year record of groundwater elevations in the region.

Climate projections for later this century suggest a shift towards increased frequency and magnitude of precipitation in the Midwest. The Spring Green flooding demonstrates that even high-permeability aquifers can be vulnerable to rapid water table rise and groundwater flooding under some climatologic conditions. Based on recent experience across southern Wisconsin, areas susceptible to groundwater inundation could be identified based on physical characteristics, such as drainage density, soil type, aquifer porosity, and depth to the water table. The 2008 flooding in Spring Green suggests a need for such analysis so that water table rise can be considered in land use planning and infrastructure design.