10/11/2022
--- River slopes determined from spaceborne lidar onboard ICESat-2 ---
Knowledge of the water surface slope (WSS) is essential for estimating the discharge and flow velocity of rivers. These parameters are among the Essential Climate Variables (ECVs) as defined by the Global Climate Observing System (GCOS). They critically contribute to the characterization of Earth’s climate, and their determination on a global scale is thus of great scientific relevance.
Field surveys of WSS are, however, very costly, while remote sensing approaches are limited by several factors. While, for example, wide swath SAR interferometric measurements are relatively inaccurate, precise point measurements of radar altimetry lack simultaneous observations over short distances. This obstructs the observation of WSS that vary greatly in space and time. In contrast, the unique measurement geometry of ICESat-2 with six parallel laser beams enables instant, highly accurate WSS observations. A new approach uses two different methods, depending on the intersection angle between the satellite orbit and the river: If multiple beams cross a river reach nearly perpendicularly, the WSS between the crossings can be calculated with the across-track approach. Otherwise, if satellite orbit and river are nearly parallel, the along-track approach derives the WSS directly from the continuous water level observations along a single intersecting crossing beam. The method can be applied globally, and the long-repeat orbit pattern of ICESat-2 allows continuous WSS monitoring, revealing details of the highly varying WSS with little effort.
WSS is not only relevant for the derivation of river discharge, but also as a correction for the ground track variability of altimetry satellites. In this way, errors at regularly observed sites (so-called virtual stations) can be reduced by up to 30 cm or 66 %. Details of the study are presented in the article ICESat‐2 based River Surface Slope and its Impact on Water Level Time Series from Satellite Altimetry (Water Resources Research, 2022, DOI: https://doi.org/10.1029/2022WR032842, PDF: https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2022WR032842).
