Heterogeneity of Snow Water Equivalent Derived from MODIS Imagery and the Isnobal Snowmelt Model (Invited)

The spatial heterogeneity of snow water equivalent (SWE) affects the timing and magnitude of daily and annual melt and consequently a basin’s stream flow. Heterogeneous snow cover smoothes daily runoff, and areas of greater accumulation produce quasi- riparian zones of increased soil moisture that persist well into the summer. A grid cell with spatial heterogeneity in SWE will develop patchy snow coverage during the melt season; daily melt will be lower than for areas with homogeneous snow depth and snow will persist longer into the melt season. We combine an energy balance snowmelt model, Isnobal, with fractional snow-covered area (fSCA) derived from MODIS imagery, to estimate SWE heterogeneity retrospectively throughout the melt season for a 152km2 basin on the west side of the Sierra Nevada. If we assume that snow accumulation during the melt season is negligible, we can subsequently add up calculated daily melt to rebuild the snowpack. The temporal resolution of MODIS imagery allows us to observe the depletion of fSCA daily and makes it possible to compute the distribution of SWE within each 500m MODIS pixel. In addition to average SWE values, the degree of heterogeneity intrinsic to each pixel is correlated with landscape characteristics. Based on data from the Marble Fork of the Kaweah River in the Sequoia National Park, California from the years 2006-2008, we find that heterogeneity decreases with increasing slope and increases with more southerly aspect. These trends in heterogeneity are consistent between the three years of considerably different snow accumulation and maximum SWE. To verify our results, we compare average pixel SWE to spatially interpolated point measurements that were gathered during intensive snow surveys covering the upper parts of the basin.