Monitoring river profile and discharge with FFSAR off-nadir and SWOT

Accurate monitoring of river profiles and discharge is critical for understanding hydrological dynamics and water resource management. However, current nadir radar altimetry is constrained by orbital spacing and the meandering nature of rivers. This study explores the potential of off-nadir processing methods using Fully-Focused SAR (FFSAR) data from Sentinel-3A/-3B and Sentinel-6A, complemented by observations from the Surface Water and Ocean Topography (SWOT) mission.

An automated off-nadir processing algorithm was developed to estimate time-evolving river profiles in the cross-track direction. By applying off-nadir slant range corrections to retracked ranges, we expanded the effective cross-track measurement range to 6.6 km for Sentinel-3A/-3B and 9.3 km for Sentinel-6A. Validation against in-situ data from the Rhine, Danube, and Oder rivers demonstrated water level accuracy, with a standard deviation of difference (STDD) between 0.04 m and 0.09 m. Slope measurements exhibited a precision of 0.7–1.3 cm/km. Comparative analyses of river profiles over 60-km channels revealed STDD values of 0.14 m for Sentinel-6A and 0.19 m for Sentinel-3B.

Additionally, discharge in Rhine, Danube, and Oder rivers are computed from FFSAR off-nadir (2016-2024) and SWOT (2023.04-2024) using Metropolis-Manning (MetroMan) algorithms. Both of the discharge are evaluated against gauges. The results were evaluated using the NRSME and NSE metrics on the reach, showing good agreement between discharge from FFSAR, SWOT and gauges. This study is to prove that the discharge from SWOT can be extended to earlier periods using nadir altimetry data collected prior to 2023.