Drought Vulnerability of Illinois' Community Surface Water Systems - Notes and Terminology, Illinois State Water Survey

Illinois Drought

Notes and Terminology

Drought Vulnerability Classification

The drought vulnerability classification is based solely on the quantity of water available to a system from its various supply sources during a severe drought condition. The adequacy of a system to meet water quality standards was not evaluated in this study.

Inadequate System - There is less than a 50% probability that the current system would be able to fulfill the community's current water demand through a severe drought similar to the drought of record.

At-Risk System - There is less than a 90% probability that the current system would be able to fulfill the community's current water demand through a severe drought similar to the drought of record.

Marginal System - Although there is greater than a 90% probability that the current system would have sufficient water during a drought similar to the drought of record, the pending threat of potential shortages during the drought might still force the community to take extraordinary measures (enacting severe water use restrictions or development of alternative supply sources) to avoid shortages.

Adequate System - There is greater than a 90% probability that the community will not experience any water shortages or threat thereof during a severe drought similar to the drought of record.

Source Types

GW Groundwater

IR Impounding Reservoir

OCR Off-Channel Reservoir

QBP Quarry/Borrow Pit

RW River Withdrawal

SWP Surface Water – Purchased

Other Notes

  • This study updates the 1989 study by McConkey and Singh that analyzed the adequacy of community surface water systems in Illinois.
  • Gage stations listed can be used to compute estimates of the streamflow available at ungaged locations, including reservoir inflow and intakes from which water is pumped to off-channel reservoirs.
  • The yield estimates presented are based upon estimates of reservoir capacity in 2010.  Ongoing sedimentation may reduce the yields of systems in the future.
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