In recent years, especially the past 10 years, there has been an alarming decline in groundwater and lake levels and reduced stream flows in the Wisconsin Sand Plains (WSP). This greatly impacts aquatic ecosystems, recreational uses of aquatic resources, and property values of riparian lands. It is clear that reduced stream flows are associated with reduced groundwater elevations. What is not clear is the cause of the lower groundwater level. However, there is a popular belief that the reduction in groundwater table elevations is associated with irrigation of agricultural land from high capacity irrigation wells. Wisconsin common law related to groundwater makes use of a “reasonable use standard” (Kent and Dudiak, 2001), so potential conflicts between lake riparian owners and groundwater-based irrigation indicates the urgency of developing an improved understanding of irrigation’s impacts on groundwater quantity. There is ample evidence that groundwater fluctuations occur naturally because of drought and high rainfall periods (Heath, 1983), but accompanying this natural fluctuation in precipitation has been a tremendous growth in irrigated cropping in the humid parts of the U.S. in general, and particularly in the WSP (WDNR, 1970; Bajwa et al., 1992; Ellefson et al., 2002). At a broader scientific level, there is a need for understanding irrigation water use (evapotranspiration, ET) by crops with respect to native vegetation (including grass and forest) on WSP, and other sand plains in humid temperate regions with shallow depth to groundwater. Arguably irrigated crops should be viewed as simply another vegetation type on the landscape, with characteristic temporal patterns of evapotranspiration loss and groundwater recharge, albeit strongly driven by human manipulation of soil wetness through irrigation. Foster and Chilton (2003) note the heavy exploitation of groundwater in recent years. They suggest that most consumptive use of pumped groundwater is by irrigated agriculture. We initiated a research project in summer of 2007 to attempt to obtain quantitative data on the causes of changes in groundwater elevation relative to groundwater use for irrigated crop in comparison to natural vegetation on WSP over recent decades. Water use by the differences vegetations will be accomplished via computer model simulation and indirect measurements of groundwater recharge rates.