Midwinter snow ablation patterns, drivers, and hydrologic consequences in the Western U.S.

Climate warming is expected to reduce radiation-driven snowmelt rates from deep mountain snowpacks, and increase the duration of low-intensity melt periods, especially from shallow snowpacks. It should also increase the humidity-dependence of melt rates, especially in winter. Taken together, these effects will raise the importance of snow ablation in midwinter, the magnitude of which has implications for shifting surface runoff regimes, groundwater recharge, and flood risk. We address two related questions: first, what is the relative effect of radiation, turbulent energy exchange, and precipitation as they relate to midwinter snow ablation, and how important are they to seasonal and annual streamflow generation; and second, how do midwinter ablation episodes and their drivers vary regionally and topographically, and how have those characteristics changed over time (if at all)? We answer these questions using a network of over 500 SnoTel snow pillows across the Western United States that have recorded daily snow water equivalent (SWE) since 1985 or earlier. With ancillary climate data, we identify distributions of midwinter snow ablation episodes along with their trends and sensitivity to meteorological drivers. We then focus on 32 small catchments with nearby daily streamflow and SWE measurements to assess regional differences in the importance of midwinter ablation to winter and annual flows.