On the capacity of ICESat-2 laser altimetry for river level retrieval: An investigation in the Ohio River basin

The Advanced Topographic Laser Altimeter System (ATLAS) onboard the Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2), with a footprint of 17 m and an along-track spacing of 0.7 m, is expected to improve our ability to observe small inland water bodies. The objective of this study is to evaluate the capacity of ICESat-2 for water level retrieval of medium (<1000 m in width) to small (<50 m) rivers. A water level retrieval algorithm for small rivers is proposed based on geolocated photon data to fill gaps in inland water surface height product (ATL13). Validation results indicate that the retrieved river levels are consistent with in situ measurements (RMSE = 0.24 m, R = 0.99, n = 69), and in excellent agreement with those from ATL13 (RMSE = 0.12 m, R = 1.00, n = 187). The capacity of ICESat-2 is evaluated at 186 virtual stations (VSs) over rivers from 6 to 950 m wide in the Ohio River basin, USA. Water levels are retrieved for small and medium rivers using the proposed method and from ATL13, respectively. Terrain correction is carried out to relocate the VS to the gauge station so that pairs of river levels agree in position, enabling the comparison. The results show that the accuracy of river level estimates is significantly improved after terrain correction, with RMSE decreasing from 0.44 to 0.17 m, based on 186 VSs. The results reveal that ICESat-2 can resolve and measure rivers as narrow as 6 m. RMSE of retrieved river levels ranges between 0.19 and 0.25 m for rivers narrower than 50 m, and from 0.08 to 0.15 m for those wider than 50 m, with averages of 0.24 and 0.12 m, respectively. Moreover, our results suggest that river width and land cover type do not impact the performance of altimetric estimates in the study area. Instead, the validation method is likely to deteriorate accuracy metrics, and the accuracy of retrievals is also dependent on fluvial morphology. This is the first study to explore the potential of ICESat-2 to retrieve river levels, which demonstrates its superior capacity to accurately measure rivers just down to a few meters wide, providing a new and exciting asset to hydrological studies. The findings of this work will also contribute to enhancing our understanding of inland water behavior in global hydrological processes and water resource management.