Climatic controls on the snowmelt hydrology of the northern Rocky Mountains, USA.

Much of the western U.S. and Canada is characterized by an arid to semi-arid climate, and the majority (50 to 80% or more) of surface water in this region originates from mountain snowpack (Bales et al. 2006). A growing number of studies have demonstrated that since 1950, western North America has experienced a substantial decline in peak snow water equivalent (SWE; e.g. Mote et al. 2005; Pierce et al. 2008), a reduced and earlier snowmelt runoff (e.g. Hidalgo et al. 2009; Stewart et al. 2005), with a substantial proportion of the observed declines linked to antropogenically driven increases in winter and spring minimum temperatures (e.g. Barnett et al. 2008). Documented changes in both the quantity and temporal characteristics of snowpack and streamflow across the western U.S. have important implications for water resources and biophysical processes in high-mountain environments. In a forthcoming study (Pederson et al. in press), we build upon previous work by investigating the oceanic and atmospheric controls underlying changes in timing, variability, and trends documented across the entire hydroclimatic monitoring system within critical Northern Rocky Mountains (NRM) watersheds. This study was motivated, in part, by increasing pressure on state and federal resource management agencies to incorporate this type of climate information into sustainability- and adaptation-planning efforts. Additionally, the region serves as a critical headwaters area, is arguably one of the most intact and functional temperate ecosystems in the world, and contains a number of the longest-running, high-resolution stream gage, meteorological, and snow observation stations in North America.


Analyses were conducted using records from 25 snow telemetry (SNOTEL) stations, 148 April 1 snow course records, stream gage records from 14 relatively unimpaired rivers, and 37 valley meteorological stations. Daily SNOTEL SWE data were used in analyses of snowpack dynamics, and results are most representative of changes occurring at mid-elevations (avg. station elevation = 1743 m ± σ259 m). Over the past four decades, SNOTEL records show a tendency towards decreased snowpack with peak snow water equivalent (SWE) arriving and melting out ~ 8 days earlier, with an average of 14 more snow-free days per year. Corresponding with the changes in snowpack, temperature records show significant seasonal and annual decreases in number of frost days (days ≤ 0°C), and changes in spring minimum temperatures that correspond with atmospheric circulation changes and surface-albedo feedbacks in March and April. The amount and melt-out timing of snowpack and streamflow in the NRMs is strongly controlled by warming spring temperatures, increasing spring precipitation, and changes in winter stormtracks associated with variations in Pacific Ocean sea-surface-temperatures (SSTs; Fig. 1). Records of snow course April 1 SWE show the composite time-series are good metrics of SNOTEL peak SWE, and that since 1936 a major feature of the records is the strong variability on interannual- to decadal-scales that coincide with changes in streamflow discharge, and North Pacific SSTs. Spring temperatures coupled with increases in the mean and variance of spring precipitation control the timing of snow melt-out, an increased number of snow-free days, and observed changes in streamflow timing and discharge. Importantly, increased spring precipitation after 1977 appears to substantially buffer streamflow timing from what should otherwise be a considerably earlier decline in flow due to the recent decades of low snowpack with above-average temperatures. The atmospheric controls shown to underlie observed changes in temperature, snow melt-out, and runoff, also control multiple phenological and ecological aspects of spring onset across western North America. This implies that the associated snowmelt related ecosystem impacts are much broader than the Northern Rockies study area, and if regional projections of 21st century hydroclimatic change prove correct, major changes are in store for aquatic and terrestrial ecosystems, and consequently land and water resource managers.

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Project co-authors:
Greg Pederson, U.S. Geological Survey | NOROCK and University of Arizona
Stephen Gray, University of Wyoming
Toby Ault, Geosciences Department, University of Arizona
Wendy Marsh, Big Sky Institute, Montana State University
Daniel Fagre, U.S. Geological Survey, Glacier Field Station
Andrew Bunn, Huxley College, Western Washington University
Connie Woodhouse, Geography Department and Laboratory of Tree Ring Research, University of Arizona
Lisa Graumlich, School of Natural Resources, University of Arizona

Citation: Pederson, G. T., S. T. Gray, T. Ault, W. Marsh, D. B. Fagre, A. G. Bunn, C. A. Woodhouse, and L. J. Graumlich, December 8, 2010: Climatic controls on the snowmelt hydrology of the Northern Rocky Mountains, USA. Journal of Climate, Early on-line edition.

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