Observing High Altitude Hydrologic Processes Using Space-Based Lidar: Lessons Learned from ICESat-2

Since 2018, the ICESat-2 Advanced Topographic Laser Altimeter System (ATLAS) has routinely observed global high altitude inland lakes and rivers using a six-beam 532 nm Lidar with photon counting detectors providing decimeter vertical accuracy. These observations have led to improved monitoring of alpine hydrologic processes for water body shapes over 0.1 sq km in area or 100m width. Principal published inland water products derived from analysis of georeferenced returned photons for each beam include surface water height, slope, significant wave height, wind speed, subsurface beam attenuation, crossing length, and coarse water depth. Two suites of Inland Surface Water Data products are generated: the continuous along-track high resolution product or ATL13 and the mean crossing product or ATL22 derived from ATL13 products. ATL13 products are produced for each beam at an along-track rate of 25-100 m depending on signal photon return density. Both suites of products are updated approximately every 8 months based on improvements to photon georeferencing and retrieval algorithms. The most recent versions (ATL13 Ver 7, ATL22 Ver3) together with updated documentation are made publicly available at the National Snow and Ice Data Center with each subsequent release reprocessed from October 2018 to present.

A five year evaluation of representative examples of ICESat-2 Inland Water products over alpine lakes is presented for a range of water body types, sizes and locations based on direct product analysis and published literature. Results demonstrate the high accuracy of ATL13 and ATL22 products in alpine areas for quantifying processes typical for hydrologic science and water resources applications including stream discharge, water body height, crossing slope, storage change, water clarity and bathymetry. Lessons learned from ICESat-2 inland water product development, Lidar's uniqueness for observing high altitude hydrologic processes, and its potential use future Lidar altimeters are also addressed.