An assessment of snowcover in major river basins of Sierra Nevada Network parks and potential approaches for long-term monitoring

Rice, Robert; Bales, Roger

The Sierra Nevada Network Inventory & Monitoring Program (SIEN) performs long-term natural resource monitoring within four units of the National Park Service (Devils Postpile National Monument-DEPO, Sequoia and Kings Canyon National Parks-SEKI, and Yosemite National Park-YOSE). Since 85% and 91% of the elevations are above 1800 m in YOSE and SEKI respectively, snow is the fundamental driver of hydrologic processes in these parks. Snowpack provides seasonal storage of water in the Sierra Nevada for soil moisture, lakes, and streams. This water storage is particularly important given the seasonal cycle of precipitation and the long summer-fall dry period in central and southern Sierra Nevada. Due to its importance to park ecosystems and the Sierra Nevada region overall, snow was identified as a specific climatic indicator by the SIEN program. The goals of this project were to use existing data and techniques to assess the trends and current status of snowpack for six watersheds within SIEN parks and to recommend approaches for monitoring and modeling snowpack changes. To do so, we first retrieved, assembled, documented, and performed quality assurance and quality control of temperature and snow data for the six main river basins of the SIEN parks. Daily snow-covered area data were tabulated and snowmelt calculated for 300-m elevation bands over a ten-year period, 2000-2009, for each of the six basins. Daily snow-covered area estimates from satellite data were used to determine where snowmelt could occur. The magnitude of snowmelt was estimated from average daily temperature for the elevation band and a melt factor. Data from snow-measurement sites in each basin were used to estimate daily melt factors. Evaluation of the satellite-derived snow-covered area (SCA) for the period 2000-2009 from MODIS showed that the average snowmelt progression, or the rate at which the snowline moves upslope during snowmelt, for the six basins is on average 17.0 m day-1, and that each successively higher 300-m elevation band melts out about 20 days later than the elevation band below it. When the MODIS SCA was coupled with a temperature index model, the majority of the estimated snowmelt in the Tuolumne and Merced River basins was derived from mid-elevations between 2400-3000 m, and the majority of the estimated snowmelt in the San Joaquin, Kings, Kaweah, and Kern basins was derived from elevations above 3000 m. Three climate change scenarios were considered: 2, 4, and 6 oC. Each 2oC rise in temperature corresponded to the current average temperature about 300 m lower in elevation. Analysis showed that temperature increases will shift the amount of snow covered area as less precipitation falls as snow. To a first approximation, this can be viewed as a shift of 300 m in current SCA patterns. Currently across all six basins there is an elevation dependence of winter SCA, with each successive lower 300-m elevation band having greater inter-seasonal variability. The day of snowmelt was also projected, and achieved by imposing a temperature change on the basin and its snowpack by shifting the corrected SCA fractions up by 300 m per 2oC change in temperature, which is consistent with the current observations that each 300 m corresponds to a 2oC shift in temperature. Overall, the timing of snowmelt shifts toward earlier in the spring at a rate of -6 to -7 days per oC. That is, with a temperature increase of 2oC, 50% of the seasonal snowmelt will be depleted about two weeks earlier than present A minimum protocol for continued assessment of snowpack should follow the approach used in this report.

An assessment of snowcover in major river basins of Sierra Nevada Network parks and potential approaches for long-term monitoring

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