Satellite-based water surface slope over a small mountain river in northern China

Satellite altimetry has become a crucial alternative for measuring inland water surface elevations apart from ground observations. Water surface slope (WSS), a fundamental parameter in river discharge simulation, also allowing for assimilating satellite and ground-observed water level data with different observation positions. There are various satellite data products offering precise measurements of river water levels and estimates of WSS on a global scale. However, satellites observation of water surface over small rivers, particularly those in mountainous areas with narrow river channels, continues to be difficult. This study focuses on evaluating the precision of the ICESat-2 ATL03 photon height data in estimating WSS throughout the mountainous river section of Yongding River, which flows across the Hebei Province and Beijing City in northern China. The ICESat-2 ATL03 data with minimum along-track sampling interval of 0.7 m makes it possible to accurately estimate the WSS for small river sections that ranged from 50 to 100 m in width. A histogram-based statistical approach was used for identifying the satellite lidar photon height located right over the water surface. The eleven pairs of satellite virtual stations selected for extracting river water surface elevation and estimating river WSS demonstrated an overall validation correlation coefficient of 0.98. The range of relative error in WSS estimation is between 0.13 % and 9.02 %. The results of this work have important implications for using satellite-based measurements to estimate critical hydrological parameters in small and mountainous river basins in the absence of ground observations.