Estimating spatial distribution of snow water equivalence in a montane watershed

TitleEstimating spatial distribution of snow water equivalence in a montane watershed
Publication TypeConference Proceedings
Year of Conference1997
AuthorsElder, K., Rosenthal W., and Davis R. E.
Conference Name65th Annual Western Snow Conference
Series TitleProceedings of the 65th Annual Western Snow Conference
Date PublishedMay 1997
PublisherWestern Snow Conference
Conference LocationBanff, Alberta
KeywordsKings River, LandSat, Modeling

We describe an approach to model distributed snow water equivalence (SWE) that merges field measurements of depth and density with remotely sensed snow-covered area (SCA). In 1993 two teams conducted an intensive snow survey in the 92.8 km2 Blackcap Basin of the Kings River. We measured snow depth at 709 points and density in five snow pits and along five transects using a Federal Sampler. Sample locations were chosen to be representative of the range of elevations, slopes and aspects of the basin. Regression tree models showed that net radiation, elevation, and slope angle account for 60- 70% of the variance in the depth measurements. Density was distributed over the basin on a 30 m grid with a multiple linear regression model that explained 70% of the observed variance as a function of the same three variables. The gridded depth estimates combined with modeled density produced spatially distributed estimates of SWE. An unsupervised spectral unmixing algorithm estimated snow cover fractions from Landsat-5 Thematic Mapper data acquired at the time of the snow survey. This method provides a snow cover fraction estimate for every pixel. We used this subpixel map as our best estimate for SCA and combining it with the SWE map allowed us to compute SWE volume. We compared the estimated volume using the subpixel SCA map with several SCA maps produced with simulations of binary SCA mapping techniques. Thresholds of 40%, 50% and 60% fractional cover were used to map binary cases of full snow cover or no snow cover. The difference in basin SWE volume was up to 13% depending on the threshold used to classify snow-covered versus snow-free areas. The percent differences in volumes roughly couesponded to the percent differences in SCA between the methods.