Water Supply vs. Distribution: Forecasting Out-of-Basin Diversions in the Laurentian Great Lakes

The capacity for the Laurentian Great Lakes to support basin ecology along with regional economic and recreational activity is highly dependent on their water levels. Precipitation and evaporation patterns, which govern the hydrologic regime of these lakes, are sensitive to a rapidly changing climate. However, climate change may also influence lake levels through more subtle, human avenues. A warming global climate presents unique challenges that vary by geographic region. The American West faces drier summer months due to decreasing snowpack as well as increasing length and intensity of droughts. Whereas in the Midwest, the Great Lakes have recently seen record water levels after a 5-year period of relative water abundance. These differing trends suggest that the largest water challenge facing the United States may be one of distribution rather than supply. Although the Great Lakes Compact currently prohibits out-of-basin diversions, increasing pressure from high demand and dwindling water supply in the western states mean that an exemption may be considered. We will present findings on Great Lakes water levels under plausible out-of-basin diversion scenarios, each reflecting significant water transfers to the American West. Diversions from Lake Superior will be propagated into basin-wide water level predictions using a novel copula-based statistical model. In particular, the copula-produced net basin supply (NBS) outputs will be coupled with a lake-to-lake routing model to project monthly water levels for Lake Superior, Michigan-Huron, and Erie. This method faithfully replicates historical water levels and can be manipulated to reflect diversions from a given lake. Plausible scenarios will represent both chronic and seasonal water withdrawals from Lake Superior with water levels presented for each diversion scenario at 10, 25, and 50 year horizons. We hope our projected lake level impacts of varying diversion rates will facilitate a hydrologic data-driven policy analysis, ultimately setting a precedent for scientifically-informed policies to address emerging climate challenges.