Capturing the Onset of Mountain Snowmelt Runoff Using Satellite Synthetic Aperture Radar.

The timing of snowmelt runoff is critical for water resource applications, but its spatiotemporal evolution remains poorly understood. We present a scalable approach to map snowmelt runoff onset using Sentinel‐1 synthetic aperture radar data for the past 8 years with 10 m spatial resolution and a median temporal resolution of 3.9 days. A systematic analysis of stratovolcanoes in the Western United States showed that snowmelt runoff onset is strongly dependent on elevation (r = 0.81), with a median runoff onset lapse rate of 4.9 days per 100 m of elevation gain. During the 2015 snow drought, we observed snowmelt runoff onset 25 days early relative to the 2015–2022 median. We document a median shift in snowmelt runoff onset of +2.0 days later in the year per year between 2016 and 2022. Our open‐source tools can be used to create snowmelt runoff onset maps anywhere on Earth. Plain Language Summary: Snowmelt timing is important–knowing when water leaves mountain snowpack is critical for downstream water resource applications like irrigation and hydropower. Snowmelt timing is also affected by regional climate change. However, it is hard to make detailed measurements of when and where snow melts across mountainous regions. We developed an improved method to map snowmelt using satellite based radar data, and we applied this method to study snow on mountains in the Western United States. We documented the detailed relationship between elevation and snowmelt, and how this relationship changed over the past 8 years. In general, at higher elevations and latitudes, snow melts later in the year. We also observed snow melting much earlier than it usually does during the 2015 snow drought, which helps us prepare for future years with low snow accumulation. Finally, from 2016 to 2022, we documented a shift toward snowmelt happening earlier in the year, which means earlier spring flow in rivers. We publicly released interactive, user‐friendly software, so anyone can use our method to study snowmelt timing anywhere on Earth. Collectively, our work will help scientists better understand regional climate change and allow water managers to better manage water resources today and in the future. Key Points: We present a scalable method and open toolbox to map snowmelt runoff onset timing with high spatial and temporal resolutionWe quantify the topographic and geographic controls on snowmelt runoff onset for stratovolcanoes in the Western United StatesWe document early snowmelt runoff onset during the 2015 snow drought and intra/interannual variability from 2016 to 2022 [ABSTRACT FROM AUTHOR]