A high use-to-yield ratio (e.g., >0.9) suggests an area where groundwater availability problems exist or could be impending. The area of influence of a well or well field often may extend beyond the township boundaries in which the pumpage is occurring. In such cases, township aquifer potential yields may appear to be approached or exceeded even though the withdrawal does not exceed total aquifer potential yield. Therefore, the delineation of high groundwater useto- yield areas by this method should be considered simply as a means for calling attention to areas to prioritize on a statewide basis for water resources planning and management.
Comparing groundwater withdrawals to potential aquifer yields in a GIS format is a useful technique for drawing attention to areas where stresses may occur (or are occurring). However, such analysis can not be substituted for local-scale investigations, particularly those that incorporate detailed information into groundwater flow models that can accurately assess local conditions. Areas may be unduly highlighted where large, relatively isolated, withdrawals occur within an extensive aquifer, such as occurs in the Mahomet aquifer near Champaign in eastcentral Illinois. The effects of such pumpage will be spread across a larger area than the townships where the wells are located, smoothing the use-to-yield ratio over a larger area. In at least one other area near East St. Louis, the withdrawal in one township is intentionally greater than the potential yield for purposes of dewatering to protect below-grade highway roadbeds.
However, areas where the aquifer may be confined to a narrow valley, where multiple pumping wells are located within a small area, or where withdrawals do indeed exceed the estimated recharge rate can be identified (e.g., Fox River valley, Peoria, Lewiston, Normal). Certainly, areas where multiple townships exhibiting high use-to-yield ratios are clustered together (e.g., the deep bedrock of northeastern Illinois) should signify locations where additional research, data collection, and water resource planning may be warranted.