Embedded sensor network design for spatial snowcover Robert Rice1, Noah Molotch2, Roger C. Bales1 1Sierra Nevada Research Institute, University of California, Merced (rrice@ucmerced.edu) 2UCLA, Department of Civil and Environmental Engineering Introduction Scaling point observations of snow water equivalent (SWE) to model grid-element scales is particularly challenging given the considerable sub-grid variability in snow accumulation over complex terrain. In an effort to capture this sub-grid variability and provide spatially explicit ground-truth snow data an embedded snow sensor network was designed and installed in Yosemite and in the Valles Caldera National Preserve. Extensive snow surveys were used to guide the installation of the network and to relate the observations to more detailed spatial SWE fields. Four years of continuous spatial and temporal data from Yosemite National Park and the and three-years in the Valles Caldera indicate that accumulation and ablation rates can vary as much as 50% based on variability in topography and vegetation. These snow distribution patterns are especially apparent in the open forests of Yosemite and the Valles Caldera where vegetation structure largely controls variability in snow distribution. Snow Survey Gin Flat-2006: Comparisons of snow depth estimates with historical snow course data shows that a single point measurement is a poor estimator of snow depth over a homogenous area, but 4 or more measurement points can reduce the uncertainty by 50%. Range of snow depth estimates from choosing 1-10 points with identical physiographic features (flat, open) for 3 different years, as % on mean snow depth: historical peak (1983), low (1988), & average (1982). This analysis from the historical snowcouse data indicated that an optimal snow depth network should consists of 7 to 10 snow depth sensors. Range of snow depth estimates from choosing 1-10 snow depth sensors within the distributed measurement network with terrain characterized by varying physiographic features (mixed conifer, slope, aspect). Again, as with the snow course 10 only slightly replicate better than 3. However, given the variability in the terrain the uncertainty is far greater than when compared to homogenous terrain. Using 4 or more snow depth sensors can reduce the uncertainty by 40%. Having 10-15 sensors per cluster provides for replication. Upper Merced River Basin Gin Flat: Elevation:2100-m Forested Complex terrain Ease of access Extensive snow surveys in February and April 2006 verified that the existing location of the distributed snow depth network provides details on the spatial variability of snow depth. The box plots represent the modeled snow depth over the 1-, 4-, and 16- km2 study areas for the 1st and 3rd quartiles with the spatial average. The plot demonstrates that at Gin Flat snow pillow/sensors and snow courses overestimate snow depth by 25% and indicate that these point measurements are not good indicators of the spatial average, but merely a point within variability. Results from the 2006 snow surveys determined that the current location of the distributed snow depth network are in optimal locations for both accumulation and ablation. Conclusions The specific objective of measurement network is capture the accumulation and ablation across topographic variables with the aim of providing guidance for future larger scale observation network designs. These spatial and temporal measurement arrays will improve remotely sensed and modeled SWE estimates across complex terrain by providing robust, spatially explicit ground-truth values of snowpack states. A distributed network is currently being installed in Yosemite National Park along an elevational transect using Tioga Pass Road (HWY 120). This will extend the current measurement array at Gin Flat from 1500-m to 2700-m. In addition, this will complement the basin transects that are installed in Sequoia National Park and the Kings River Experimental Watershed. Results and discussion-Gin Flat Accumulation & ablation rates over a 0.4 ha m2 of as much as 50%. Distributed snow measurements: The distributed snow measurement network is located at Gin Flat in Yosemite National Park at an elevation of 2100-m and deployed across a mixed conifer 0.4 ha site (Gin Flat) in the Upper Merced River basin. The distributed network consists of 10 ultra sonic snow depth sensors continuously logging snow depth every 1-hr. since December 2003. Gin Flat is located near the existing snow course (1930-present) and snow sensor (1980-present) sites. This site was chosen because of its close proximity to existing long term data sets, ease of access, and variable terrain. An additional network was deployed within the 36,000-hectare Valles Caldera National Preserve (VCNP) in the Jemez Mountains, New Mexico (35° 53’N, 106° 32’W). The primary forest type of the study site is a mixed conifer forest, consisting of Douglas fir, white fir, blue spruce, and southwestern white pine. The region’s principal sources of moisture are the Pacific Ocean and the Gulf of California, approximately 800 km to the west and southwest, respectively; approximately 50% of the 720 mm of average annual precipitation (1980-2004) falls between November and April in the form of snow. Snow depth was recorded hourly using a network of nine ultrasonic snow depth sensors placed under the canopy, at the edge of the canopy, and in open areas. Results and discussion-Valles Caldera 10 20 30 40 50 60 70 80 90 kilometers 1500 1800 2100 2400 2700 Instrument sites Strategy: rather than spreading instruments across a whole basin, this transect statistically samples the variability in the Tuolumne & Merced basins, taking advantage of the Tioga Pass Road as infrastructure Instrument sites leverage operational & research investments Observations from ultra-sonic snow depth sensors showed a 56% increase in snow depth in open versus under-canopy locations at maximum accumulation. Snow settling rates between 15 March and 1 April were greater in open areas versus those at the canopy edge; likely a result of more rapid snow metamorphism in areas exposed to greater solar irradiance. Total ablation rates after maximum accumulation on 15 March, were 54 and 46% lower in under-canopy locations versus canopy edge and open areas, respectively. Continuous (hourly) measurements of the ultra sonic depth sensors and temperature from 2003- 2007. The Gin Flat snow depth operated by CA DWR and USGS, as well as, the monthly snow course measurements are plotted and represents the open homogenous terrain. The snow as measures by the distributed network can vary as much as 50%, where tree canopy density of >60% can influence distribution patterns. In addition, the distributed snow measurement network is depleted of snow as much as 4 weeks earlier than the CA DWR site. Acknowledgements Support was provided by NASA Grant NNG04GC52AREASoN CAN “Multi-resolution snow products for the hydrologic sciences”. In addition, UC Merced, with the cooperation of Yosemite National Park is acknowledged.