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 2005. Precipitation is recorded continuously at 20 rain gauges. Groundwater levels are measured the first of each month at 13 observation wells. The database from these networks consists of 13 years of precipitation data and 11 years of groundwater observations. For the period September 2004–August 2005, the network received an average of 27.34 inches of precipitation, 7.55 inches lower than the network 12-year 1992–2004 average precipitation of 34.89 inches. Above average precipitation fell in the fall and winter months, and below average rain fell during the spring and summer of 2005. Year Thirteen had the third wettest fall and the second wettest winter of the 13 years of network operation. The spring and summer of 2005 were the driest warm seasons of the past 13 years. In 2004–2005, groundwater levels rose in response to the wet fall and winter, then fell dramatically in the spring and summer of 2005 due to a lack of precipitation. The dry growing season also had a dramatic effect on irrigation water demands; irrigation pumpage increased 40 percent from the previous high record of 52 billion gallons (in 1996). Total irrigation for the June–September period was estimated to be 72 billion gallons. To improve our understanding of the relationship among groundwater, stream discharge, and irrigation, 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 were installed in various wells since 2003 to monitor groundwater levels, and an additional data logger was installed in Crane Creek to monitor stream stage. Discharge measurements indicate that there is groundwater discharge into Crane Creek at the test site even during irrigation. Groundwater data show a rapid (within 24-hour) response of groundwater levels to precipitation, probably mostly due to the increase in stage in Crane Creek in this area of prevalent sandy soils, though shallow water levels also contribute to this rapid response.
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