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 532 nm six-beam Lidar with high resolution photon counting detection 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 products for each inland water crossing include surface water height, slope, significant wave height, wind speed, subsurface beam attenuation, crossing length and slope, and coarse water depth. Two suites of Inland Surface Water Data products derived from photon returns from each of six ICESat-2 532 nm beams are the continuous along-track high resolution product or ATL13 and the mean product or ATL22. The most recent ATL13 Release 7 products are produced for each beam at an along-track rate of 25-100 m depending on signal photon return density. The ATL22 Release 3 mean products are computed for each beam crossing. Both ATL13 and ATL22 are updated approximately every 8 months and 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 the above observed products and processes over alpine lakes is presented for a range of water body types, sizes and locations based on ICESat-2 product analysis and published literature. Results demonstrate the high accuracy and capability of ATL13 and ATL22 products and processes required for typical hydrologic science and water resources applications including stream discharge, water body height and slope, storage change, water clarity and bathymetry. Lessons learned applicable to potential future Lidar altimeters over inland and coastal waters are also addressed.