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Publication Abstract

State Water Plan Task Force Special Report on Ground-water Supply and Demand in Illinois Bowman, Jean A., 1991  Illinois State Water Survey, Champaign, IL,  ISWS RI-116    Full Text Available

Ground-water supplies are available to meet most demands in Illinois. However, in some places and under certain conditions, the demand may exceed the supply. Because the demand for ground water for municipal, industrial, and agricultural irrigation uses continues to increase, there has been a growing interest in understanding the regional balance between ground-water supply and demand in Illinois.

This report compares present and projected ground-water uses with ground-water potential yields on a township scale. The potential yield information was gathered for deep sandstone, shallow bedrock, and unconsolidated sand and gravel formations as part of the 1967 Illinois Water Plan issued by the Technical Advisory Committee on Water Resources.

Information on present municipal and industrial ground-water use was obtained from the Illinois Water Inventory Program, which has documented and reported groundwater withdrawals since 1978. Present municipal and industrial ground-water withdrawals are based on averages for 1980-1987.

Present agricultural irrigation estimates are based on a soil- and weather-dependent water-balance model that determines irrigation demand. The demand is then extrapolated into a daily ground-water use value according to the present irrigated acreage in each township.

Projections are also made in this report for municipal, industrial, and agricultural irrigation ground-water withdrawals. Municipal (public water supply) projections are based on a per capita ground-water use for each public water supply system in the state using ground water, according to the population served by each system.

The population served by each system was adjusted by the 1995 projected population change for each county; those projections were made by the Illinois Bureau of the Budget. Adjusted population served by each public water supply system was multiplied by the per capita ground-water use for that system to arrive at projected 1995 municipal ground-water use.

Industrial ground-water use projections were made for industries that supply their own water from a well. Projections were based on adding and subtracting one standard deviation of the 1980-1987 mean ground-water use for each manufacturing category, and adding and subtracting 10 percent of the mean for non-manufacturing uses.

The correct percentage change for each manufacturing or non-manufacturing category was applied to each facility's pumpage in each township of the state for new township totals. The assumption in this method of projection is that given the large uncertainties in industrial ground-water use, the actual ground-water withdrawals will fluctuate above and below the average, as has been the case since 1980, when detailed industrial pumpage record-keeping began statewide.

Agricultural irrigation projections are based on the same water-balance model used for present irrigation estimates. "Irrigable" acreage in Illinois was determined on the basis of soil characteristics and ground-water availability.

The present balance between ground-water supply and demand shows a significant overpumpage problem in the Chicago metropolitan area and in the four surrounding "Collar Counties" (Cook, DuPage, Lake, and Will). This overpumpage is caused by large municipal and industrial demands. In addition, seasonal overpumpage may be experienced in several localized regions where agricultural irrigation is concentrated. This overpumpage is limited to the growing season and is almost entirely balanced by normal recharge over the course of a year. The amount of seasonal overpumpage is largely determined by weather conditions, since irrigation pumpage is greatly increased in dry years.

The projected balance between ground-water supply and demand shows a reduction in the overpumpage problems in the Chicago/Collar Counties region as a result of shifts from ground water to Lake Michigan water by numerous public water supply systems. Elsewhere in the state, anticipated changes in municipal and industrial pumpage are expected to be small enough or localized enough that they will have only minor effects on the ground-water supply-and-demand balance.

The possibility of large expansions in agricultural irrigation should be considered. This report concludes that expansions are most likely in areas with sandy soils and productive aquifers where irrigation is already being practiced with economic success. In those areas, large expansions in irrigation might exaggerate seasonal water-level declines, but average annual recharge should still provide for full resource recovery.

Extended droughts will continue to be a serious but temporary problem in two ways: 1) irrigation pumpage is greatly increased during droughts; and 2) annual ground-water recharge is reduced during droughts. A long-term climatic change could seriously alter the present balance in which annual recharge compensates for high seasonal irrigation pumpage.

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