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Publication Abstract

Operation of Rain Gauge and Groundwater Monitoring Networks for the Imperial Valley Water Authority: Year 14: Sept. 2005-Aug. 2006 Wilson, Steven D., Nancy E. Westcott, and Kevin L. Rennels, 2008  Illinois State Water Survey, Champaign, IL,  ISWS CR 2008-12    Full Text Available

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 2006. Precipitation is recorded continuously at 20 rain gauges. Groundwater levels are measured the first day of each month at 13 observation wells. The database from these networks consists of 14 years of precipitation data and 12 years of groundwater observations.

The Year Fourteen network precipitation of 27.74 inches was below average, 6.1 inches lower than the network 14-year average of 33.84, and 6.57 inches below the previous 13-year average of 34.31 inches. It was the fifth driest year in the 14 years of network operation and the third year in a row with less than 30 inches of precipitation. Every season in Year Fourteen received below-average seasonal total precipitation.

In 2005-2006, groundwater levels continued to decline because of below-average precipitation. Only one storm with significant rainfall provided a recharge response that was evident in the water-level data. The dry growing season also had an effect on irrigation water demands, with the amount of irrigation pumpage being the third highest total, following the 72 billion gallons pumped in 2005 and the 52 billion gallons pumped in 1996. Total irrigation for the June-September period was estimated to be 50 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 logger was installed in Crane Creek to monitor stream stage. 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 mostly to the increase in stage in Crane Creek in this area of prevalent sandy soils, though shallow water levels are also a contributing factor.

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