1. Hydrologic trends in the West Philip Mote Climate Impacts Group University of Washington Alan Hamlet, Martyn Clark, Dennis Lettenmaier With thanks to Dave Hart, Jon Lea, Greg Johnson, Randy Julander, Scott Pattee ftp://ftp.atmos.washington.edu/philip/SNOWPAPER/

2. 20th c.2020s2040s April 1 snow water equivalent (mm) The Columbia Basin’s snow in a warming world

5. Snake River at Ice Harbor

6. This talk Significant hydrologic changes will occur for 2-4°F warming The Northwest has already seen 1.5°F warming in the 20th century Have hydrologic changes been observed? Are they consistent with the warming and the model projections?

9. decrease increase Data from NRCS, CA DWR, BC SRM 824 snow courses/SNOTEL Trends in April 1 SWE, 1950-2000 Relative to 1950 value 73% – trends Large – trends PNW Some + trends SW

10. VIC simulation 1/8° long x 1/8° lat, west of Continental Divide Daily weather data, 1/1/1915 - 9/30/97 interpolated to VIC grid points from Coop stations Long-term trends interpolated from USHCN stations

11. Relative trends 1950-1997

12. Trends in timing of peak snowpack Change in Date Figure by Alan Hamlet, UW

13. Western Cordillera trends in Apr SWE/Mar SWE

15. 1950-1997 relative trends vs DJF temperature Obs VIC

16. Regional average April 1 SWE Obs o VIC x

17. Relating SWE to climate data US Historical Climate Network (USHCN) Use nearest 5 stations to form reference time series T(t) and P(t) Correlation between Apr 1 SWE and T or P Regression: SWE(t)=a T T(t) + a P P(t)+  (t) Components of trend a T and a P

18. Correlations between Nov-Mar climate and Apr 1 SWE X-direction: precip Y-direction: temp Coldest locations insensitive to temperature Cascades very sensitive

19. April 1 SWE trends, 1950-2002 a P a T

20. a P + a T

21. Changes in SWE vs changes in precip 1930s to 1990s1945-55 to 1990s Obs SWEVIC SWEPrecipObs SWEVIC SWEPrecip Cascades-14%+1%+4%-29%-16%-5% Rockies+11%+2%+9%-16%-9%+1% California+3%-14%+10%-2%-25%-1% Interior+9%-6%+10%-22%-18%+2%

22. Trends in timing of spring snowmelt (1948-2000) Courtesy of Mike Dettinger, Iris Stewart, Dan Cayan +20d later –20d earlier

23. As the West warms, winter flows rise and summer flows drop Figure by Iris Stewart, Scripps Inst. of Oceanog. (UC San Diego)

24. Conclusions Observations show substantial declines in western snowpack, corresponding changes in flow Largely temperature-driven Cascades, N. Calif most temp-sensitive Large increases in precipitation have offset warming in some places Warming trends very likely to continue ftp://ftp.atmos.washington.edu/philip/SNOWPAPER/

25. April 1 SWE trends, 1930-2002 a P a T

26. SNOTEL data: cold-season melt events vs Apr 1 SWE

27. At almost every USHCN station, winters warmed + signs: warming but not statistically significant

28. Winters wetter in much of the West Drier in some of Northwest (PDO)

29. Declining snowpack in the Cascades From the VIC simulation