The Illinois State Water Survey (ISWS), under contract to the Imperial Valley Water Authority (IVWA), has operated a network of rain gauges in Mason and Tazewell Counties since August 1992. The ISWS also established a network of groundwater observation wells in the Mason-Tazewell area in 1994, which is monitored by the IVWA. The purpose of the rain gauge network and the groundwater observation well network is to collect long-term data to determine the impact of groundwater withdrawals in dry periods and during the growing season, and the rate at which the aquifer recharges. This report presents data accumulated from both networks since their inception through August 2007. Precipitation is recorded continuously at 20 rain gauges. Groundwater levels are measured at 13 observation wells. Ten of the observation wells are now outfitted with continuous digital recorders and three are hand measured periodically. The database from these networks consists of 15 years of precipitation data and 13 years of groundwater observations. The Year Fifteen network precipitation of 31.94 inches was below average, 1.77 inches lower than the network 15-year average of 33.71, and 1.90 inches below the previous 14-year average of 33.84 inches. Overall, precipitation was near average, although the spring and summer seasons in Year Fifteen were below average in seasonal total precipitation. In 2006-2007, groundwater levels continued to decline in some of the study area because of below average precipitation and increased irrigation demand. However, in much of the study area, water levels rebounded somewhat after the end of the 2006 irrigation season. The dry growing season of 2007 had an effect on irrigation water demands, with the amount of irrigation pumpage being the second highest total, second only to the 72 billion gallons pumped in 2005. Total irrigation for the June-September 2007 period was estimated to be 57 billion gallons. To improve our understanding of the relationship among groundwater, stream discharge, and irrigation pumpage, an irrigation test site was established in April 2003 (Year Eleven) near Easton, IL. Nine observation wells were installed in close proximity to an irrigated field that abuts Crane Creek. Transducers with data loggers have been installed in various wells since 2003 to monitor groundwater levels, and an additional logger was installed in Crane Creek to monitor stream stage. Data collection at the test site ended after the 2006 growing season. These data will be included in a groundwater flow model, currently in development. Data indicate there is groundwater discharge into Crane Creek at the test site even during irrigation withdrawals. The groundwater data indicate a rapid (within 24 hours) response of groundwater levels to precipitation, probably due to the increase in stage in Crane Creek in this area of prevalent sandy soils, though shallow water levels also are a contributing factor.
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