ICESat-2 Based River Surface Slope and Its Impact on Water Level Time Series From Satellite Altimetry

The water surface slope (WSS) of rivers is essential for estimating flow velocity and discharge. It is also helpful as a correction applied to range measurements of satellite altimetry missions to derive water level time series at a virtual station. Using radar altimetry, WSS can only be roughly estimated and is limited to wide rivers because of its coarse spatiotemporal resolution. In contrast, the lidar sensor onboard Ice, Cloud, and Land Elevation Satellite 2 (ICESat-2) can also observe small rivers. Using ICESat-2's unique measurement geometry with six parallel laser beams, we derive instantaneous WSS along and across the satellite's ground track, time-variable WSS (with an average of 5 days of records in the studied epoch between October 2018 and October 2021), and average WSS on reach-scale. Although the method can be applied globally, this study is limited to 815 reaches in Europe and North America where sufficient validation data is available. We compare the ICESat-2 WSS with time-variable WSS derived from multiple gauges and constant data from the "SWOT River Database." For 89% of the studied reaches, ICESat-2 can be used to estimate the average WSS with a median absolute error of 23 mm/km. We also show the possible performance gain at multiple virtual stations (VS) in the "Database for Hydrological Time Series of Inland Waters" (), applying the WSS as a correction for altimetry satellites' ground track variability. We correct 137 VS for the derived ICESat-2 WSS and yield improvements in the root mean square error by up to 30 cm or 66%.