Long-term variations in seasonal, weather conditions important to water resources in Illinois

An analysis of historical relationships between seasonal weather conditions and water resource conditions in Illinois provides insights to the challen ges of projecting such relationships under conditions of climate change. In Illinois for 1901-1997 there were major temporal shifts in types of season al conditions that have positive and negative effects on surface water and ground water supplies and their quality. Major seasonal effects came in the spring and summer seasons and when either wet-and-warm or dry-and-warm wea ther conditions prevailed in either season. Sixty percent of the summer sea sons creating negative impacts occurred during only 40 years: 1911-1940 and 1951-1960. Seasons creating impacts relate well to the frequency of cyclon e passages and to the incidence of El Nino or La Nina conditions. This reve als that future climate fluctuations that shift the frequency of cyclones a nd/or ENSO events will have profound effects on Midwestern seasonal conditi ons that affect water resources. Projecting future effects of climate chang e on water resources will need to consider how shifts in water use and wate r management technologies act to re-define the seasonal weather conditions that are critical.