Climatologists and ecologists have long known that the high aerosol loads reducing visibility in 1970’s and 1980’s also reduced solar radiant energy to the ground surface. In principle, we also knew that this would reduce evaporation from vegetation and land and water surfaces, but we assumed the region-wide effect would be small. Until recently, few people made any calculations as to the effect of the high aerosol burden on the water balance and hydrology of large watersheds such as the Great Lakes Basin. This changed, however, when the Global Circulation Models (GCM’s) had to be amended to include aerosols if the recent patterns of climate change were to be captured accurately and future trends forecast.

Energy and water balance calculations now show that the high sulfur and nitrogen emissions of the 1970’s and 1980’s in the greater Great Lakes region produced an aerosol load that reduced evaporation and contributed to unusually high water levels in the Lakes for over 30 years (1965-1998). The large–scale programs to reduce SO2 emissions are leading now to reduced acid aerosol loads, especially in Wisconsin, Michigan and parts of Ontario, with improved radiant energy and associated increases in evaporation from Great Lakes basin watersheds. At the same time, human consumption of water has increased, due both to the expanded population and increased per capita use, an impact that was readily overlooked during the years when evaporation from the basin was being limited by aerosols. The rapid reduction in “net basin supply” of water at the end of the 1990’s, however, is not readily explained by the usual climate variables. This presentation will seek to answer whether the combination of increased consumptive use and restored evaporation (due to reduced aerosols) accounts for the current pattern of low water supply to the Great Lakes and the tributary streams.