2014/07/16 Council Agenda Packet

21: NORTHWEST jected to shift three to four weeks earlier than the 20th century average, and summer flows are projected to be substantially lower, even for an emissions scenario that assumes sub- stantial emissions reductions (B1).24 In some North Cascade rivers, a significant fraction (10% to 30%) of late summer flow originates as glacier melt;25 the consequences of eventual glacial disappearance are not well quantified. Basins with a significant groundwater compo- nent may be less responsive to climate change than indicated here.26 Changes in river -related flood risk depends on many factors, but warming is projected to increase flood risk the most in mixed basins (those with both winter rainfall and late spring snowmelt-related runoff peaks) and remain largely unchanged in snow -dominant basins. Regional climate models project increases of 0% to 20% in extreme daily precipitation, depending on location and definition of "extreme" (for example, annual wettest day). Observed Shifts in Streamflow Timing June Strearnitow Trends (tractlor l Howl 942-2000 • •15%to-8 • .8% to -4% • 1%to.2% o -29t. to-1% o .t%t 0% 0 r#to*1% o *1%to•2% • *2+6 to *3% tLLlevmfon ea( oo It 1 33oon-15094 sbo It - 3000 to f ,300o a = OK* It 5oo4 h Figure 21.1. Reduced June flows in many Northwest snow -fed rivers is a signature of warming in basins that have a significant snowmelt contribution. The fraction of annual flow occurring in June increased slightly in rain -dominated coastal basins and decreased in mixed rain -snow basins and snowmelt- dominated basins over the period 1948 to 2008.21 The high flow period is in June for most Northwest river basins; decreases in summer flows can make it more difficult to meet a variety of competing human and natural demands for water. (Figure source: adapted from Fritze et al. 201121). Future Shift in Timing of Stream Flows - H' A1B 29435 A1B 2060s Reduced Summer Flows Figure 21.2. (Left) Projected increased winter flows and decreased summer flows in many Northwest rivers will cause widespread impacts. Mixed rain -snow watersheds, such as the Yakima River basin, an important agricultural area in eastern Washington, will see increased winter flows, earlier spring peak flows, and decreased summer flows in a warming climate. Changes in average monthly streamflow by the 2020s, 2040s, and 2080s (as compared to the period 1916 to 2006) indicate that the Yakima River basin could change from a snow -dominant to a rain -dominant basin by the 2080s under the A1B emissions scenario (with eventual reductions from current rising emissions trends). (Figure source: adapted from Elsner et al. 2010)24. (Right) Natural surface water availability during the already dry late summer period is projected to decrease across most of the Northwest. The map shows projected changes in local runoff (shading) and streamflow (colored circles) for the 2040s (compared to the period 1915 to 2006) under the same scenario as the left figure (A1B).29 Streamflow reductions such as these would stress freshwater fish species (for instance, endangered salmon and bull trout) and necessitate increasing tradeoffs among conflicting uses of summer water. Watershedswithsignificant groundwater contributions to summer streamflow may be less responsive to climate change than indicated here. U.S. GLOBAL CHANGE RESEARCH PROGRAM 490 CLIMATE CHANGE IMPACTS IN THE UNITED STATES