Snow Depth from Lidar: Challenges and New Technology for Measurements in Extreme Terrain

The Airborne Snow Observatory (ASO) uses an airborne LiDAR system to measure basin-wide snow depth with cm-scale accuracy at ~1m spatial resolution. This is accomplished by creating a Digital Elevation Model (DEM) over snow-free terrain in the summer, then repeating the flights again when the terrain is snow-covered and subtracting the elevations. Snow Water Equivalent (SWE) is then calculated by incorporating modeled snow density estimates, and when combined with coincident spectrometer albedo measurements, informs distributed hydrologic modeling and runoff prediction. This method provides SWE estimates of unprecedented accuracy and extent compared to traditional snow surveys and towers, and 24hr latency data products through the ASO processing pipeline using Apache Tika and OODT software. The timely ASO outputs support operational decision making by water/dam operators for optimal water management.

The water-resource snowpack in the western US lies in remote mountainous terrain, spanning large areas containing steep faces at all aspects, often amongst tree canopy. This extreme terrain presents unusual challenges for LiDAR, and requires high altitude flights to achieve wide area coverage, high point density to capture small terrain features, and the ability to capture all slope aspects without shadowing. These challenges were met by the new state-of-the-art Riegl LMS-Q1560 LiDAR system, which incorporates two independent laser channels and a single rotating mirror. Both lasers and mirror are designed to provide forward, backward, and nadir look capability, which minimizes shadowing and ensures data capture even on very steep slopes. The system is capable of logging more than 10 simultaneous pulses in the air, which allows data collection at extremely high resolution while maintaining very high altitude which reduces complete region acquisition time significantly, and allows data collection over terrain with extreme elevation variation. Our experience to-date includes acquisition of data over terrain relief of more than 3500m, and ranges of up to 6000m in a single swath. We present data acquired during spring of 2013 and 2014 in western Colorado and the central Sierra Nevada, which demonstrates the capability of the new LiDAR technology and shows basin-wide measured snow depth and SWE results.