ANALYSIS OF THE SPATIAL VARIABILITY OF SNOW COVER DEPLETION IN AN ALPINE WATERSHED, TOKOPAH BASIN, SIERRA NEVADA, CALIFORNIA, U.S.A.

In this study we quantified the spatial and temporal depletion of snow covered area (SCA) in order to obtain estimates of peak snow water equivalence (SWE) in the 19.1 km2 Tokopah Basin. We sought to answer the following questions: How do we improve the use of snow cover depletion curves by incorporating additional data and spatial analysis? How can we use snow cover depletion curves to estimate peak SWE earlier in the snowmelt season and continue to have leverage to update our estimates throughout the ablation season? What meteorological fields or combination of meteorological fields yield the most leverage in updating SWE estimates? In order to quantify snow cover depletion at different elevations and under different topographic environments, we analyzed snow cover depletion patterns in different elevation zones and in sub-basins of varying topographic heterogeneighty. We used 30m remotely sensed SCA data, hourly meteorological data, ground-based surveys of SWE and a 30m digital elevation model (DEM). We generated depletion curves of SCA using two methods of varying complexity: SCA versus modeled accumulated degree-days and SCA versus modeled net radiation. We found that SWE over the basin during the April, May and June snow surveys was 995mm, 701mm and 283mm respectively. Degree-day accumulation produced decreases in SCA at lower elevations (<2823m) before SCA decreased at higher elevations (>3222m). Degree-day depletion curves did not represent observed temporal variability in SWE during the ablation season. Depletion curves using net radiation as the response variable proved to better represent temporal SWE variability. We found that incremental decreases in SCA at higher elevations (>3222m) resulted in a greater decrease of SWE than the corresponding SWE decrease at lower elevations (<2 82 3m)