Quantifying the Net Impact of SWOT on Reservoir Monitoring

Reservoirs store and regulate water for hydropower, irrigation, and flood control, yet global reservoir storage remains poorly quantified because water-surface elevation and surface area are rarely observed together across many reservoirs and long periods. The Surface Water and Ocean Topography (SWOT) mission, launched in December 2022, measures both across reservoirs worldwide. We quantify the net impact of SWOT on three applications (present-day storage estimation, historical storage reconstruction, and global uncertainty accounting), comparing SWOT-enabled approaches with pre-SWOT alternatives and validating against in-situ storage records. For present-day estimation, SWOTNOW combines SWOT water-surface elevation with surface area and a learned correction for storage change between SWOT observations. On 113 gauged reservoirs, it outperforms the pre-SWOT alternative in correlation, amplitude, and anomaly R 2 , and it provides daily estimates of storage change. For historical reconstruction, SWOTRACE uses SWOT data to constrain area-elevation relationships and applies them to satellite-derived area records, producing monthly storage from 1984 to 2021 for 9,401 reservoirs that previously lacked the documentation catalogue-based methods require; these relationships outperform digital elevation model-based relationships on anomaly R 2 ; Finally, for global accounting, extending an error relationship fitted on gauged reservoirs to 30,887 reservoirs with usable SWOT records yields an estimated 18.5% reduction in the uncertainty of globally summed storage. SWOT's net impact on the current state of the art of reservoir monitoring thus arrives through distinct pathways supported by evidence of different strengths.