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Publication Abstract

Kaskaskia Regional Water Supply Planning: Projected Water Needs from Lake Shelbyville and Carlyle Lake During Extreme Drought Conditions Knapp, Vernon, 2017  Illinois State Water Survey, Champaign, IL,  ISWS CR 2017-01    Full Text Available

Within the Kaskaskia River watershed are two large federally owned reservoirs, Lake Shelbyville and Carlyle Lake, which are two of the three largest reservoirs in Illinois. By way of contracts with the federal government, in 1983 the State of Illinois was allocated roughly 14 percent of the joint-use storages in Lake Shelbyville and Carlyle Lake for water supply. The combined designated water supply storage (57,406 acre-feet) is estimated to produce an annual average yield of 42 million gallons per day (mgd) during a calculated 50-year drought condition (excluding 5 mgd used to support minimum water quality releases from the reservoirs). The Illinois Department of Natural Resources (IDNR), which administers the rights to the water supply storage, has sub-allocated that yield to a total of nine entities, with five (three electricity-generating plants and two regional water supply systems) accounting for roughly 99 percent of the total allocation. Four of the five major entities do not directly withdraw water from the reservoirs but instead have intakes in the Kaskaskia River downstream of the reservoirs and when needed must request supplemental reservoir releases from upstream to augment the river’s flow at their intakes. The allocation contracts between IDNR and the five major water users were executed in 2000–2004. Each allocation has a 40-year contract period, after which it must be reviewed and renegotiated. As of 2017, the region’s droughts have not yet had sufficient severity to result in the use of reservoir releases.

This study simulates the use of the state’s designated water supply storage under observed historical drought condition sequences rather than the hypothetical 50-year drought which is statistically based. Water budget modeling of the two reservoirs (and the downstream flow conditions at each intake), using measured hydrologic and climatic records from historical droughts as input, provides a more thorough understanding of how often the major water supply allocation holders would need to request reservoir releases during such droughts. This in turn leads to a better analysis of the performance of the reservoirs’ water supply during droughts. Particular emphasis in this study is given to the water supply adequacy during conditions represented by the 1953–1955 drought, the worst drought on record and estimated to have a recurrence interval of approximately 100 years or greater.

The water budget simulations assume that each major allocation holder will operate at its full capacity, thus maximizing the release and withdrawal from the reservoirs’ water supply storage. However, there are two external factors that also can influence reservoir releases: 1) water needs at the Kaskaskia Lock, which controls commercial navigation between the Kaskaskia and Mississippi Rivers, and 2) the amount of the summer water quality release from each reservoir needed to maintain acceptable water quality in the river downstream. These factors particularly influence the water needs of two power plants, the Dynegy-Baldwin Power Station and the Prairie State Energy Campus, located downstream of Carlyle Lake that have direct withdrawals from the Kaskaskia Navigation Channel.

The results indicate that the State of Illinois’ combined water supply storage in Lake Shelbyville and Carlyle Lake is not only likely to be sufficient to meet the needs of all allocation holders during an extreme (1953–1955) drought condition, but there is also a greater likelihood that a sizeable portion of the water supply storage will remain unused during such an extreme event. Even if unused, however, it is not clear that additional allocations could be made given the current manner in which water is allocated by IDNR (and accounted by the U.S. Army Corps of Engineers) based on each user’s maximum annual use. But the finding generally bodes well regarding the future availability of water leading up to the 2040s when the existing allocation contracts come up for review.

The only future scenario in which the state’s water supply storage would not be sufficient during an extreme drought involves an unlikely combination of several factors, the greatest of which is a substantial increase in commercial navigation through the Kaskaskia Lock (to nearly 10 million tons per year); this in turn would likely require a ten-fold increase or greater in the exportation of locally mined coal to outside markets. In contrast, the exportation of coal has generally been declining since the late 1980s.

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