Outlook for future water supplies to the Great Lakes

The net water supply to the Great Lakes basin is calculated from net inflows and outflows, precipitation, evaporation, and changes in storage. To predict future water supplies to the basin, it is necessary to predict the components of runoff, precipitation, and evaporation; these components include air temperature, humidity, and wind speed. Current predictability of water supplies is limited at best to ca 3 months into the future, which is inadequate for planning and analysis of water pipelines and diversions. Insight into potential future water supply conditions over a longer period can be provided by such approaches as climate models, statistical projections, climatic matching, and paleoclimate analysis. Discussion of these approaches focuses on climate scenarios based on the past recorded water supplies and on global warming models. A survey of past recorded precipitation and air temperatures in the Great Lakes basin shows a low precipitation trend from about 1890 to the early 1960s, with the 1970s to the present being a relatively high precipitation period. Examination of air temperatures shows a relatively cool period from about 1900 to 1929, a warm regime during 1929-59, and cool regime during 1960-80, and a relatively warm regime from 1980 on. Both the temperature and precipitation regimes appear to change abruptly as step functions, which cannot be predicted. If a global air temperature rise of 1-4 degree C occurs as predicted in many global warming models, Great Lakes water supplies will be reduced due to increased evapotranspiration and lake evaporation. Models cited indicate net basin supply to the lakes would decrease by 23% to 50%. 5 refs., 3 figs., 1 tab