STC-MODSCAG Snow Disappearance Date (SDD) for Western US 2001-2021

Snow Disappearance Date (SDD) was derived from Spatially and Temporally Complete MODIS Snow Covered Area and Grain-size (STC-MODSCAG), a daily fractional snow cover product derived from Moderate Resolution Imagining Spectrometer (MODIS) surface reflectance bands that produces a spatially and temporally complete dataset. Using multiple-end member spectral mixture analysis, MODSCAG solves for subpixel snow-covered area (SCA) and grain size of the fractional snow cover. MODSCAG allows the snow spectral reflectance to vary by pixel to account for the spatial heterogeneity of snow in mountainous terrain. STC-MODSCAG is known to underestimate snow under the canopy. We calculated SDD for each water year by working backwards to identify the first date at which each pixel switches from “snow” (greater than 15% snow cover) to “no snow” (less than 15% snow cover) following Crumley et al. (2020). We required at least 10 days of snow cover preceding the calculated SDD to ensure ephemeral late-season snow events were not captured. We also set a threshold of 30 days of snow per water year to count as a snowy pixel of interest to avoid ephemeral snowpacks. We masked out pixels with greater than 70% canopy cover, as determined by the National Land Cover Dataset (NLCD). SDD was calculated for each pixel in the seasonal snow zone across the western U.S. for the 20 years (WY 2002-2022) of STC-MODSCAG.