Closing the water balance of the Great Lakes: developing a new historical record reconciling bias and uncertainty

The purpose of this project is to develop a new historical record of monthly runoff, over-lake evaporation, over-lake precipitation, and connecting channel flows for each of the Great Lakes using a novel statistical model that (through an explicit acknowledgment of bias and uncertainty) reconciles discrepancies between model- and measurement-based estimates of each component while closing the Great Lakes water balance. This project leverages a similar project completed in early 2015 between NOAA and ECCC that successfully developped new estimates of the monthly water budget for Lakes Superior and Michigan-Huron for the historical period between 2004-2015. This project is expected to result in the first comprehensive water budget estimates for the Great Lakes system that systematically close the entire water balance while addressing both measurement bias and uncertainty. It will lead to a modeling framework that is designed to incorporate qualitative perceptions of measurement uncertainties that can be incorporated, through a process of Bayesian updating, into the final estimates. Furthermore, the newly-developed water budget estimates are expected to provide a much more robust basis (relative to previous estimates) for decision making not only because they include an expression of uncertainty (a gap in decision making that has been evident for decades) but also because they provide a full explanation for changes in water levels from month-to-month. Finally, this approach can help in resolving the regional water budget across monthly and inter-annual time scales and represents an important stepping stone towards addressing a long-standing need in the Great Lakes for clear and defensible differentiation between hydrological, climatological, geological, and anthropogenic drivers behind seasonal and long-term changes in Great Lakes water.