Historical Analysis of Remotely Sensed Snow Properties' Relation to Streamflow

In snowmelt dominated river basins, snow properties near peak accumulation are used to assess spring and summer runoff. Forecast models rely on estimates of the water stored in the snowpack to determine the contribution of snowmelt to runoff. We can measure snow-covered area and albedo from available satellite sources such as MODIS, which provides daily imagery at 500 m spatial resolution, and Landsat, which provides imagery every 16 days at 30 m. In California's Sierra Nevada, daily snow pillow measurements of snow water equivalent are the main source of in situ information. We investigate the relationship of snow-covered area to snow water equivalent in Sierra Nevada watersheds with varying latitudes, orientations, and elevations. Based on monthly unimpaired runoff volumes, we selected a set of years during the Landsat historical record (1985-2007) that encompass 80% of the range of variability in runoff during the last century. We use multiple endmember spectral unmixing to estimate the fraction of snow in each 30 m pixel and the albedo of that fractional snow cover for the American, San Joaquin and Kern watersheds for five years that represent the minimum, quartiles and maximum April, May, and June unimpaired runoff. Recent years have produced similar variability in runoff, and daily fractional snow cover is estimated from MODIS for these years at 500 m resolution. In addition to snow- covered area and albedo, we estimate the spatial distribution of snow water equivalent by blending a hypsometric method with snow-covered area. The correlations of these estimates with streamflow are evaluated for each of the watersheds.