Historic climate and hydrological records are used as a solid basis for quantifying relationships between climate changes and the amount of water in rivers and aquifers. Projecting future climate and hydrological conditions, especially those modified by human influences such as an enhanced greenhouse effect, however, is highly uncertain.
Long-range water supply planning requires selecting a future time frame for conducting analyses. Two regional planning groups in northeastern and east-central Illinois selected 2050 as the time horizon for planning studies. Recognizing inherent uncertainties, the Illinois State Water Survey (ISWS) has chosen to construct future climate, water availability, and water demand scenarios rather than make firm predictions or forecasts.
Scenario analyses of the sensitivity of water availability and demand to climate change will provide estimates of how much more or less water will be available on the surface and in aquifers, and how much more or less water will be needed by consumers if climate changes by specified amounts.
Climate, water availability, and water demand scenarios are based on a set of specified reasonable assumptions about the future, including population and economic growth. Planners must understand these assumptions and plan for the future, recognizing great uncertainties. Strategic planning must be a process of risk assessment leading to risk management.
ISWS scenarios to 2050 are based on the assumption that climatic conditions experienced from 1971 to 2000 (the period for which ‘normal’ climate conditions are calculated) will continue into the future. These three scenarios include (1) a current trend scenario; (2) a higher-demand scenario; and (3) a lower-demand or conservation scenario.
In addition, Wittman Hydro Planning Associates, Inc. and Professor Ben Dziegielewski at Southern Illinois University will provide sensitivity analyses on the current-trend water demand scenario to 2050. The following climate scenarios will be used in the water availability and water demand studies:
• An increase in temperature from 1971-2000 normal conditions ranging from 0°F to +6°F
• A change in annual precipitation from 1971-2000 normal ranging from -5 inches to +5 inches
These analyses will include a variation of the affects of these temperature and precipitation scenarios on the Lake Michigan water level, as well as surface water and groundwater quantities. Water demand studies for northeastern and east-central Illinois will be completed in the spring of 2008 and draft water availability studies will be available in October 2008.