Topographic Distribution of Snow-Covered Area in the Sierra Nevada

Melting snow is the principal source of water supplies in the western US. Historical ground measurements in the Sierra Nevada seem to show a trend toward decreasing spring snow water equivalence at lower-elevation sites, particularly at more southern latitudes. Remotely sensed measurements of snow-covered area from Landsat augment these ground-based snow accumulation data. Overlaying the Landsat measurements with topographic data from the Shuttle Radar Topography Mission (SRTM) enables us to examine the relationship between snow-covered area and topographic variables--elevation, slope, and orientation--and to analyze systematic differences between a "binary" snow mapping method that categorizes every grid cell as either snow-covered or bare and a "subpixel" method that estimates the fraction of every grid cell that is snow-covered. Snow-covered area and snow-cover fraction increase with elevation, and analysis of selected basins reveals that slopes with a western orientation have more snow covered area than those of eastern orientation. Moreover, analyses of snow-covered area in basins with varying elevation distributions show that the binary method systematically underestimates snow at lower elevations and overestimates snow at higher elevations.