Estimating Water Surface Slope of Rivers Using ICESat-2 Observations

The water surface slope of rivers is an essential variable for estimating river 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. Still, only rough and mean estimates of water surface slope are obtainable using classical satellite altimetry because of its coarse time and space resolution.

Using the unique measurement geometry of ICESat-2 with six parallel laser beams, we derive instantaneous reach-scale water surface slope along and across the satellite's ground track. The method can be applied globally and provides extending insights into the time- and space-variability of the water surface slope of any river with increasing mission duration.

We compare the ICESat-2 water surface slope estimates with time-variable slopes derived from in-situ data from multiple gauging stations and with static datasets (e.g., from SWORD). We also show the possible performance gain at multiple virtual stations in the "Database for Hydrological Time Series of Inland Waters" (DAHITI, https://dahiti.dgfi.tum.de) applying the water surface slope estimates as a correction of the orbit-drift which can be a few kilometers for repeat missions such as Jason-2/3. However, the largest impacts are expected for non-repeat orbit missions such as CryoSat-2 or Saral (after July 2016).