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Publication Abstract

Operation of Rain Gauge and Groundwater Monitoring Networks for the Imperial Valley Water Authority Year Twelve: September 2003-August 2004 Wilson, Steven D., H. Allen Wehrmann, Kevin L. Rennels, Nancy E. Westcott, 2008  Illinois State Water Survey, Champaign, IL,  ISWS CR 2008-06    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 that 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 2004. 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 12 years of precipitation data and 10 years of groundwater observations.

For the period September 2003-August 2004, the network received an average of 29.64 inches of precipitation, 5.73 inches lower than the network 11-year 1992-2003 average precipitation of 35.37 inches, as well as the 30-year averages at Havana and Mason City (37.82 and 35.70 inches, respectively). Below average precipitation fell in winter and summer months, and average precipitation fell during the fall of 2003 and the spring of 2004.

In 2003-2004, groundwater levels in many wells tended to follow the usual pattern of rising water levels in early spring with peaks in mid-summer before evapotranspiration and irrigation demands caused water levels to decline. However, as in Year Eleven, groundwater levels in some wells experienced essentially no water-level recovery during the spring and summer, and exhibited a general decline throughout the year.

Total irrigation for the June-September period was estimated to be 42 billion gallons (bg), which was the fifth highest total since monitoring began in 1995 and ranked just below the fourth highest total of 46 bg in 2003. The years 2001 and 2002 had the second and third highest total withdrawals, respectively, with only 1996 having more pumpage than each of the last four years. Some of this increase in pumpage can be attributed to the growth of irrigation systems in the Imperial Valley, which now has 1,889 systems.

To improve our understanding of the relationship between groundwater, stream discharge, and irrigation, an irrigation test site was established in April 2003 (Year Eleven) near Easton, Illinois. Nine observation wells were installed in close proximity to an irrigated field that abuts Crane Creek. Transducers with data loggers were installed in three wells in 2003 to monitor groundwater levels, and an additional data logger was installed in Crane Creek to monitor stream stage. Two additional wells were outfitted with data loggers in June 2004. Discharge measurements indicate groundwater discharge into Crane Creek at the test site.

The groundwater data show a rapid (within a day) response of groundwater levels to precipitation, probably due mostly to the increase in stage in Crane Creek in this area of prevalent sandy soils. Shallow water levels also contribute to this rapid response.

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