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2015 Puget Sound Factbook Book | v3.0 Granshaw et al., 2006) to a −14% decline in average area on Mt. Rainier (1970-2007, Sisson et al., 2011). There are no published trends for glaciers in the Olympic Mountains. 23. Projected decline in spring snowpack for Puget Sound. On average spring (April 1 st ) snowpack in Puget Sound is projected to decline by −56 to −74% by the 2080s (2070- 2099, relative to 1970-1999, Mote et al., 2015). 16 17 Streamflow 24. Observed shift to earlier peak streamflow in the Pacific Northwest. The spring peak in streamflow is occurring earlier in the year for many snowmelt-influenced rivers in the Pacific Northwest as a result of decreased snow accumulation and earlier spring melt – the shift ranges from no change to about 20 days earlier (observed over the period 1948- 2002; Stewart et al., 2005). 25. Projected shift to earlier peak streamflow timing in Puget Sound. Peak streamflow is projected to occur 4 to 9 weeks earlier by the 2080s (2070-2099, relative to 1970-1999 Elsner et al., 2010) in four Puget Sound watersheds (Sultan, Cedar, Green, Tolt). 17 26. Projected increases in winter streamflow for Puget Sound. Winter streamflow is projected to increase by +25 to +34% on average for Puget Sound by the 2080s (2070- 2099, relative to 1970-1999; Hamlet et al., 2013). 17 27. Projected decreases in summer streamflow for Puget Sound. Summer streamflow is projected to decrease by −22 to −31% on average for Puget Sound by the 2080s (2070- 2099, relative to 1970-1999, Hamlet et al., 2013). 17 28. Flooding a. Projected increases in peak river flows for Puget Sound. Projected increases for a selection of 17 streamflow sites across Puget Sound range from an increase of +15 to +90% for the magnitude of the 100-year peak flow event (Tohver et al., 2013). Changes depend on the location and specific characteristics of each watershed, such as the amount of winter snow accumulation within the basin.. Projections for specific Washington locations can be found here: http://warm.atmos.washington.edu/2860/products/sites/ b. Increases in heavy rainfall events could further increase flood risk. Heavy rainfall events are projected to become more severe by mid-century. In Puget Sound, the yearly maximum 24-hour rainfall is projected to increase by +4 to +30% for the 2050s (relative to 1970-1999), based on results from 10 global models and a low (RCP 4.5) and high (RCP 8.5) greenhouse gas scenario (Mote et al., 2015). Preliminary results suggest an increase in the number of heavy rain 17 Average projected change for ten global climate models, averaged over Washington State. Range spans from a low (B1) to a medium (A1B) greenhouse gas scenario. 50
Climate change events occurring in early fall (Salathé et al., 2014). These changes may result in more severe flooding in rain dominant and mixed rain and snow basins. c. Changes in flood management may not be sufficient to mitigate increases in flood risk. In the upper Skagit basin, for instance, with current flood management practices, the 100-year flood is projected to increase by 24% by the 2080s (2070-2099, relative to 1916-2006) 18 ; simulations indicate that changes in water management can only mitigate 7% of this projected increase (Lee and Hamlet 2011). d. Sea level rise will exacerbate coastal river flooding. Higher sea level can increase the extent and depth of flooding by making it harder for flood waters in rivers and streams to drain to the ocean or Puget Sound. Initial research on this issue suggests that the amount of area flooded in the Skagit would increase by up to 74% by the 2080s when accounting for the combined effects of sea level rise and larger floods (Hamman 2012). 29. Projected decreases in minimum flows for Washington State. Low summer streamflow conditions are projected to become more severe in about 80% of watersheds across Washington State. Projected decreases for a selection of 17 streamflow sites across Puget Sound range from a decrease of −9 to −51% for the magnitude of the 10-year in average 7-day flows (Tohver et al., 2013). Changes depend on the location and specific characteristics of each watershed, such as the amount of winter snow accumulation within the basin. Projections for specific locations can be found here: http://warm.atmos.washington.edu/2860/products/sites/. 19 Stream temperature 30. Projected increases in stream temperatures for Washington State. Stream temperatures are projected to increase in response to warming and decreases in summer streamflow. Projections for 124 stream temperature locations across the state find that more sites will experience temperatures that elevate stress for adult salmon (EPA, 2007). Many will exceed thermal tolerances for the entire summer season by 2080 (2070-2099), despite rarely being in excess of these temperatures in the recent past (Mantua et al., 2010). References Elsner, M.M. et al., 2010. Implications of 21st century climate change for the hydrology of Washington State. Climatic Change 102(1-2): 225-260. Environmental Protection Agency, 2007. Biological evaluation of the revised Washington water quality standards. US EPA, Seattle. 18 Projected change based on the ECHAM5 global climate model and the A1B greenhouse gas scenario. 19 Results for a low (B1) and medium (A1B) greenhouse gas scenario for 112 medium-sized watersheds in Washington. 51
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