GREAT-LAKES HYDROLOGY UNDER TRANSPOSED CLIMATES

Historical climates, based on 43 years of daily data from areas south and southwest of the Great Lakes, were used to examine the hydrologica l response of the Great Lakes to warmer climates. The Great Lakes Envi ronmental Research Laboratory used their conceptual models for simulat ing moisture storages in, and runoff from, the 121 watersheds draining into the Great Lakes, over-lake precipitation into each lake, and the heat storages in, and evaporation from, each lake. This transposition of actual climates incorporates natural changes in variability and ti ming within the existing climate; this is not true for General Circula tion Model-generated corrections applied to existing historical data i n many other impact studies. The transposed climates lead to higher an d more variable over-land evapotranspiration and lower soil moisture a nd runoff with earlier runoff peaks since the snow pack is reduced up to 100%. Water temperatures increase and peak earlier. Heat resident i n the deep lakes increases throughout the year. Buoyancy-driven water column turnover frequency drops and lake evaporation increases and spr eads more throughout the annual cycle. The response of runoff to tempe rature and precipitation changes is coherent among the lakes and varie s quasi-linearly over a wide range of temperature changes, some well b eyond the range of current GCM predictions for doubled CO2 conditions.