Evaluation and Comparison of the New Generation of Spaceborne Laser Altimeters for Surface Water Level Measurements in Wetlands

Wetlands ecosystems face external threats that have caused the loss of approximately 35% of their area globally in the last decades. Consistent water level monitoring of wetlands is necessary for their management, conservation, and restoration. However, in-situ monitoring worldwide is unfeasible due to costs and access limitations. Space-based water level measurement has been demonstrated to be an essential complement to gauge networks, and is sometimes the only source of information in non-instrumented areas. The increasing availability of data from new missions poses an opportunity to advance space-based hydrological applications, but it requires a consistent evaluation of the accuracy for such a purpose. This study evaluates the new generation of satellite laser altimeters, ICESat-2 and GEDI, for water level retrieval in wetlands. The evaluation was conducted in the south Florida Everglades using data from ICESat-2 ATL08 and GEDI L2A products acquired between 2020-2021. We evaluated the effect of land cover and sensors beam modes on water level estimation accuracy. We used the water surface model version 3 from the Everglades Depth Estimation Network (EDEN) as a reference for the analysis. Preliminary results show that both products provide reasonable water level retrievals, with Root Mean Square Error (RMSE) of 0.32 m and R2 of 0.98 for ICESat-2, and RMSE of 1.79 m and R2 of 0.87 for GEDI. ICESat-2 outperformed GEDI water level estimations in all land covers with lower errors and higher R2. Both products performed better on herbaceous wetlands than on closed forests. Analysis of beam modes suggests that the weak beams of ICESat-2 perform better than strong beams for the wetlands in the Everglades. Regarding GEDI, the full power beams presented a better performance than coverage beams. Overall, the analysis demonstrated the potential of the new generation of laser altimeters to monitor water level changes in wetlands. Both missions can contribute to strengthening hydrological monitoring in gauge and ungauged areas.