| | 1: | | Title: | | | | | Volume/Number: | 2002 | | | | Issuing Agency: | | | | | 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 and municipal, industrial, 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. 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 rain gauge network and the groundwater observation well network collect long-term data to determine the rate of groundwater level decline in dry periods and during the growing season, and the rate of groundwater level recovery during recharge periods. This report presents data accumulated from the rain gauge and observation well networks since their inception through August 2001. 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 nine years of precipitation data and seven years of groundwater observations. At the beginning of groundwater observations in late 1994, the water levels were at their highest in the seven 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 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 dry years, a recovery in wet 1997-1998, and a leveling off in near-normal 1998-1999, followed by declines in dry 1999-2000. Despite a dry July, near-normal precipitation in 2001 brought a return to more typical seasonal hydrographs. This report includes new regression analyses of data collected through August 2001, similar to regression analyses first conducted on data collected through August 1998. The analyses indicate that groundwater levels are affected by 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; e.g., a high correlation between June groundwater levels and the Illinois River stage or precipitation that occurs in April or May. However, additional data collected since 1998 did not improve the results of the regression analyses. In fact, coefficients of determination for many regressions worsened. This suggests that regressions of observed groundwater levels versus river stage and precipitation are not adequately describing all the variables affecting groundwater levels. Using the data collected to verify, test, and improve the existing Imperial Valley groundwater flow model is highly recommended. Continued data collection also is recommended to create long-term data sets of precipitation and groundwater levels for use in modeling analyses. Collection of additional groundwater level and irrigation pumpage data also is highly recommended. | | | | Date Created: | 9 24 2004 | | | | Agency ID: | CR-2002-07 | | | | ISL ID: | 000000000869 Original UID: 999999994345 FIRST WORD: Operation | |
|
