The launch of the Surface Water and Ocean Topography (SWOT) satellite marks a new era in global hydrological monitoring, providing unprecedented spatial coverage of river water surface elevation (WSE) measurements. However, similar to previous nadir altimetry missions, the sampling frequency of SWOT is still insufficient to capture the rapid dynamics of flood events. In this study, we present the first approach for generating daily WSE time series from multi-mission satellite altimetry (SWOT, Sentinel-3A/B, Sentinel-6A). The approach, named Reach-Reg, benefits from the unique SWOT measurement geometry, which observes multiple river reaches simultaneously. This enables the construction of a linear regression for each neighbouring reach pair, which are subsequently used in a chained approach to transfer WSE between river locations. To account for the time lag, river velocity is estimated using the Manning formula and the at-a-station hydraulic geometry simplification. Finally, the application of outlier rejection, smoothing, aggregation, and interpolation yields the daily WSE time series. Reach-Reg has been validated on 95 stations located on 8 rivers (Elbe, Ganges, Mississippi, Missouri, Oder, Po, Rhine, Solimões), obtaining a mean RMSE, NRMSE, and NSE of 0.30 m, 4.2%, and 0.94, respectively. Reach-Reg significantly outperforms previous densification methods in terms of both accuracy and temporal resolution. The method is computationally efficient, automated, and relies solely on altimetry observations, offering high scalability for monitoring medium to large rivers globally. This marks a fundamental step towards operational hydrological monitoring using satellite altimetry. • Daily river WSE estimation using SWOT, S-3A/B, & S-6A multi-mission altimetry. • New densification method (Reach-Reg) uses chained regressions between reaches. • Validated on 95 stations (8 rivers): mean RMSE 0.30 m, NRMSE 4.2%, NSE 0.94.
Halicki et al. (Sun,) studied this question.