Monitoring Reservoir Storage Using SWOT Satellite Observations and a Reservoir Operation Model

Reservoirs play a critical role in water management, yet comprehensive and real-time observations of reservoir storage remain limited, especially outside the U.S. Observations of two reservoir-related attributes, Water Surface Elevation (WSE) and Surface Area (SA), which can be used to calculate reservoir storage change, are often desynchronized, hindering precise estimation. The recently launched (December 2022) Surface Water and Ocean Topography (SWOT) satellite mission (science observations began August 2023) uses a cutting-edge interferometer to provide global, simultaneous WSE and SA maps of Earth's water bodies, which can be leveraged to estimate reservoir storage. We evaluate the accuracy of SWOT-based estimates of reservoir storage in comparison with in situ-based observations for 12 reservoirs in the Western U.S., of which four are in California, four are in the Upper Colorado River Basin (UCRB), and four are in the Columbia River Basin (CRB). Our results show that SWOT produces WSE measurements with less than 20 cm median absolute error (MAE) taken across all 12 reservoirs and 19 months of observations and storage estimates with MAE less than 10%. Model-based reservoir storage estimates (constrained by SWOT observations) can fill temporal gaps accurately and efficiently, even if fewer than one-quarter of SWOT observations are valid. Our results motivate further study of the potential for estimation of reservoir storage where few or no in situ observations are available via assimilation of SWOT observations into a reservoir simulation model.