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~ AUGUST 2006 DRESSLER ET AL. 705 A Comparison of Snow Telemetry and Snow Course Measurements in the Colorado River Basin K. A. DRESSLER Penn State Institutes of the Environment, The Pennsylvania State University, University Park, Pennsylvania S. R. FASSNACHT Watershed Science, Colorado State University, Fort Collins, Colorado R. C. BALES School of Engineering, University of California, Merced, Merced, California (Manuscript received 16 February 2005, in final form 4 October 2005) ABSTRACT Temporal and spatial differences in snow-water equivalent (SWE) at 240 snow telemetry (SNOTEL) and at 500 snow course sites and a subset of 93 collocated sites were evaluated by examining the correlation of site values over the snow season, interpolating point measurements to basin volumes using hypsometry and a maximum snow extent mask, and variogram analysis. The lowest correlation at a point (r = 0.79) and largest interpolated volume differences (as much as 150 mm of SWE over the Gunnison basin) occurred during wet years (e.g., 1993). Interpolation SWE values based on SNOTEL versus snow course sites were not consistently higher or lower relative to each other. Interpolation rIDse was comparable for both datasets, increasing later in the snow season. Snow courses correlate over larger distances and have less short-scale variability than SNOTEL sites, making them more regionally representative. Using both datasets in hy- drologic models will provide a range of predicted streamflow, which is potentially useful for water resources management. 1. Introduction Much of the stream flow in the Colorado River and other rivers in the southwestern United States is de- rived from seasonal melt of mountain snowpack (Ser- reze et al. 1999). Estimates of water stored as snow, that is, snow-water equivalent (SWE), are essential for fore- casting runoff, understanding climate change (McGin- nis 1997), managing forest resources, and many other applications-yet making accurate basin-scale mea- surements has remained an unmet challenge for de- cades (Johnson and Schaefer 2002). As a result, spa- tially distributed models have not yet seen wide use or acceptance in operational forecasts of snowmelt runoff (Daly et al. 2000). Corresponding author address: Kevin A. Dressler, The Penn- sylvania State University, 129 Land and Water Research Building, University Park, PA 16802. E-mail: kxdI3@psu.edu @ 2006 American Meteorological Society In the western United States, SWE has been mea- sured at key index sites, or snow courses, since the 1930s. Historically, empirical forecasts of seasonal streamflow volume were based on at least 20 yr of data, regressing snow course SWE with observed streamflow (Pagano et al. 2004). Forecasts by the National Re- sources Conservation Service (NRCS) have incorpo- rated seasonal precipitation and climate indices (Garen 1992), while forecasts by the National Weather Service have focused on forcing a conceptual hydrologic model with an ensemble of historical meteorological data to estimate basin conditions in two model layers (snow- pack and soil water) (Day 1985). In 1963 the U.S. De- partment of Agriculture (USDA) Soil Conservation Service began the installation of an automated network of snow telemetry (SNOTEL) sites that are now oper- ated by the USDA NRCS. The system replaced some manual snow courses and provides continuous real- time SWE data for seasonal streamflow volume fore- casting (Johnson and Schaefer 2002). SNOTEL sites