Seasonal and Interannual Variability in Colorado River and Rio Grande Snowcover Patterns: 1995-2002

Snow covered area (SCA) maps gridded at 1-km² were analyzed to assess the persistence of snowcover in the Southwestern U.S. The maps used were derived from AVHRR scenes using a three-part cloud masking procedure and spectral unmixing algorithm, and give fractional snow cover per 1-km² pixel. Areas with persistent snowcover exceeding 25% SCA were relatively reproducible from year to year, corresponding to higher elevations. However, there were different January through June patterns of snowcover persistence during recent drought years as compared to previous years. Differences reflect both seasonal timing of snowfall and different temperature patterns. The annual maximum snow extent, or area with any snowcover during the year, exhibited significant interannual variability, and was not well correlated with maximum snow water equivalent. Cloud persistence over the January through June snow season also showed significant interannual variability, and was in part responsible for differences in the apparent SCA. There were as few as 3 and as many as 18 cloud-free days when snow on all or part of the area of interest could be mapped.