Site characteristics: Soil moisture summary for 376 SNOTEL station locations

Satellite-based synthetic aperture radar (SAR) has been used to assess and
quantify snowmelt across large spatial and temporal scales. While there
have been recent advancements in SAR-based snow water equivalent (SWE)
retrieval methods, obtaining accurate estimates of SWE requires knowledge
of the amount of liquid water content in the snowpack given its strong
impact on radar velocity. Recent studies have utilized Sentinel-1 SAR to
identify snowmelt runoff onset in complex, high-elevation terrain based on
a seasonal minimum backscatter time-series, however, detailed
investigations into the snowpack state before and after snowmelt runoff
onset are lacking. In this study, we integrated repeat field measurements,
SNOw TELemetry (SNOTEL) station data (n = 260) from across the western
U.S., and paired Sentinel-1 SAR estimates of snowmelt runoff onset to 1)
assess the snowpack state prior to and after Sentinel-1 SAR-derived runoff
onset estimates, and 2) evaluate Sentinel-1 SAR estimates of runoff onset
with SNOTEL-derived estimates of melt output via soil moisture “pulses”.
We found that SNOTEL soil moisture pulses preceded Sentinel-1 SAR
estimates of snowmelt runoff onset by a median of 3 days (standard
deviation = ± 25.3 days) and post-dated peak SWE by a median of 3 days
(standard deviation = ± 18.2 days). Soil moisture pulse dates occurred
earliest in montane forests/prairie snowpacks and latest in maritime
snowpacks. Snow density and number of positive degree days on soil
moisture pulse date increased with latitude and longitude and decreased
with elevation. While satellite-based estimates of snowmelt runoff onset
provide a promising methodology for improving spaceborne retrievals of
SWE, we emphasize the importance and influence of local climatological
conditions on runoff onset signal clarity for both in-situ and
satellite-based estimates.