Illinois State Library

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Title

• Operation of rain gauge and groundwater monitoring networks for the Imperial Valley Water Authority. Year Eight : September 1999-August 2000.

Description

• 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. These networks are located in the most heavily irrigated region of the state. The region's major source of water for irrigation, municipal, and domestic water supplies is groundwater pumped from thick sand-and-gravel deposits associated with the confluence of two major ancient river valleys, the Mississippi and the Mahomet-Teays. Relatively recent extreme weather events (e.g., the drought of 1988 and the great flood of 1993) resulted in large fluctuations in groundwater levels in the Imperial Valley area. The purpose of the rain gauge network and the groundwater observation well network is to collect long-term data to determine the rate of groundwater drawdown in dry periods and during the growing season, and the rate at which the aquifer recharges. This report presents data accumulated from the rain gauge and observation well networks since their inception through August 2000. Precipitation is recorded continuously at 20 rain gauges for each storm that traverses the Imperial Valley. Groundwater levels at the 13 observation wells are measured the first of each month. The database from these networks consists of eight years of precipitation data and six years of groundwater observations. At the beginning of groundwater observations in late 1994, the water levels were at their highest in the six years of observation. These high groundwater levels were the result of the very wet 1992-1995 period when annual precipitation was above the 30-year normals at both Havana and Mason City. From September 1995-August 1997, precipitation in the region was well below the 30-year normal followed by the 1997-1998 and 1998-1999 observation years with rainfall totals that were slightly above and slightly below normal, respectively. Groundwater levels in the observation wells reflected the multi-year rainfall patterns, showing a general downward trend during the dry years, a recovery in the wet 1997-1998 year, and a leveling off in 1998-1999. Precipitation in the region during observation year 1999-2000 was well below normal, mirroring the quite low totals observed during the dry years of 1995-1997. In response, groundwater levels fell to levels similar to those experienced in 1996-1997. Analysis indicates that groundwater levels are affected by both the precipitation in the Imperial Valley area and, for wells close to the Illinois River, by river stage. Generally, water levels in wells follow antecedent precipitation and Illinois River stage by one to two months, i.e., June groundwater levels are most highly correlated with the Illinois River stage or precipitation that occurs in April or May. The analyses conducted indicate the need for continued operation of both networks due to inconsistencies associated with groundwater levels, precipitation, and the Illinois River stage. For instance, although observation well number 2 (MTOW-2) is located near the center of Mason County, well away from the Illinois River, it has an equal correlation with the Illinois River stage and area precipitation. Additional data collection and analyses are needed to determine the reasons for this and other data disparities.

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1. Operation of rain gauge and groundwater monitoring networks for the Imperial Valley Water Authority. Year Eight : September 1999-August 2000. (20061002183852_ISWSCR2001-15.pdf).
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