The focus of this Technical Working Group (TWG) is on improving estimates of water supplies, including a more accurate accounting of the many components of the water balance and whether or how they are changing, including precipitation, run-off, groundwater, diversions, consumptive use and evaporation. Since water supplies are a function of climate, this group is examining the contribution of climatic factors to the variability of upper Great Lakes water levels in order to provide a plausible array of supply conditions to test the robustness and performance of alternative regulation plans. As part of this work, the TWG is extending climate change general circulation models (GCMs) and higher resolution regional circulation models (RCMs) to estimate future supplies to the Great Lakes. In addition, they will attempt to gain greater understanding of the long term variability of the past water supplies and extending those modes into the future. Ultimately, the results of these climate studies will be used as inputs to hydrological, channel routing and lake regulation models to assess impacts on various resource groups.