Evaluating the performance of the surface water and ocean topography (SWOT) mission for monitoring reservoirs across the conterminous United States

Remote sensing provides critical insights into reservoir operations at sites with minimal in-situ data. The Surface Water and Ocean Topography (SWOT) mission improves upon previous satellite altimetry missions with two-dimensional water surface representations, enhanced discharge estimates, and simultaneous water surface elevation (WSE) and surface area/width data. We found median agreement (Spearman rho) greater than 0.9 between SWOT and 773 United States Geological Survey (USGS; ρ = 0.96) and US Army Corps of Engineers (USACE; ρ = 0.94) WSE time series at Conterminous US reservoirs. At USGS sites, 68% of observations had relative errors compared to in-situ observations (after adjusting for bias using time series average WSE) with an absolute value under 10 cm, versus 22 cm at USACE sites. However, low agreement clusters appeared in the Pacific Northwest and Southeastern US. For SWOT-derived downstream discharge, agreement (ρ) with in-situ data was lower (USACE: 0.57, USGS: 0.39) with a low performance cluster in the Pacific Northwest and a high performance cluster in the central US. At USGS sites, 68% of observations had relative errors with an absolute value under 108 cubic meters per second (m 3 /s), compared to 133 m 3 /s at USACE sites. This research supports the ability of SWOT to accurately represent reservoir water levels at ungauged sites while recognizing potential for improvement on downstream discharge estimates.