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175 The Effects of Climate Change on Snow and Water Resources in the Columbia,Willamette, and McKenzie River Basins, U.S.A: A Nested Watershed Study Anne Nolin, Eric Sproles, and Aimee Brown Introduction Significance and Motivation Water management relies on predictions of water availability through time, and these predictions are necessary to the U.S. Climate Change Science Program Strategic Plan (US Global Change Research Program 2001; US Climate Change Science Program 2003). The snowmelt from the mountains of the Columbia River Basin provides critical water supply for agriculture, ecosystems, hydropower, and municipalities.Throughout the western United States, current and projected analyses show rising temperatures resulting in diminished snowpacks, leading to declines in summertime in-stream flow (Service 2004;Barnett et al.2005;Knowles et al. 2006). Annual average precipitation in the Cascade Range in the Pacific Northwest ranges from approximately 50 cm on the eastern side to about 3800 cm at high elevations on the western side of the range (Taylor and Hannan 1999). Even with high amounts of precipitation, watersheds on the wet, western side of the Cascade Range experience seasonal drought and summertime low stream flows on an annual basis.Winter precipitation (November–March) accounts for 70% of the annual total and roughly 40-60% of that falls as snow (Serreze, et al. 1999). Because snow in much of the Cascades accumulates close to the melting point, a 2o C warming would result in a projected 22% decrease in midwinter snow-covered area in the Oregon Cascades, as snowfall would convert to rainfall (Nolin and Daly 2006).The ensuing effects of such a change would be higher peak flows and greater flood risk in the winter and lower summer lows when the water is needed most. Such changes have already been documented in this region. Using measurements of April 1 snow water equivalent (SWE) dating back to 1950, Mote et al. (2005) noted that the Pacific Northwest has experienced the largest declines in snowpacks of any region in the western United States.This change can be primarily attributed to an increase in winter temperatures (Mote 2003, Mote et al. 2005). In addition, this increase in temperature appears to account for a nine- to eleven-day shift towards earlier snowmelt runoff in the Pacific Northwest (Stewart et al. 2005). While these studies suggest general trends across much of the mountainous western 176 Anne Nolin, Eric Sproles, & Aimee Brown U.S., a closer look reveals that not all areas will be affected to the same degree by these broad climatic shifts. For example, the lower elevations of the Cascade Range are predicted to exhibit the greatest differences in snow accumulation (Nolin and Daly 2006) and in the timing and magnitude of snowmelt (Payne et al. 2004).This western coastal mountain region is especially sensitive because its Mediterranean climate is characterized by cool, wet winters (when water demand is low) and warm, dry summers (when demand is high).The changing patterns of streamflow and the timing of peak flow are particularly relevant to the Columbia RiverTreaty because they present a challenge to water managers. In this chapter, we describe the projected effects of rising temperatures on snowpack at three nested watershed scales (Figure 1): the Columbia River Basin (the U.S. portion), the Willamette River Basin, and the McKenzie River Basin. Within the U.S.portion of the Columbia andWillamette River Basins,we map the extent of “at-risk” snowpack—areas of present day snow accumulation that, in a warmer climate, would experience rainfall instead of snow. In a more detailed case study, we then focus on the McKenzie River Basin, a sub-basin of the Willamette River Basin in western Oregon.For the McKenzie River Basin,we model presentday and future snow distribution, which we examine in the context of the presentday snow-observation network, and then offer some perspectives on snowpack monitoring now and for the future. We then briefly discuss the implications of changing snowpack on the Columbia River Treaty. Background Approximately 50–60% of the annual precipitation in the Cascades Range falls as snow during the winter months (Serreze et al. 1999). Much of the snow cover in this maritime mountain region accumulates at temperatures close to the melting point (Sturm et al. 1995).This is especially true of snow at elevations ranging from 1000–1800 m. Such lower-elevation snow in this relatively warm climate regime is at risk of falling instead as rain. The McKenzie River Basin (Figure 2) located in the central western...


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