Alan F. Hamlet Philip W. Mote Martyn Clark Dennis P. Lettenmaier

1. Effects of Temperature and Precipitation Variability on Snowpack Trends in the Mountain West JISAO/SMA Climate Impacts Group and Department of Civil and Environmental Engineering University of Washington May 20, 2004 Alan F. Hamlet Philip W. Mote Martyn Clark Dennis P. Lettenmaier

2. Motivation

3. Changes in Simulated April 1 Snowpack for the Cascade Range in Washington and Oregon Current Climate “2020s” (+1.7 C) “2040s” (+ 2.5 C) -44% -58% April 1 SWE (mm)

4. Hydrologic effects to the Cedar River for “Middle-of-the-Road” Scenarios Man-made storage ~ 10% of annual flow + 1.7 C + 2.5 C

5. If global warming has affected the PNW significantly over the 20th century, we should see it in the Cascades in the historic snow and streamflow records. What can we say about less sensitive areas?

6. Why Do We Need Model Simulations of the Historic Record? • Longer Record (Avoids problems with PDO from 1950-1997) • Spatial Coverage (high and low elevations not in the observations) • Temporal Resolution (daily time step) • Consistency between temp, precip, SWE, and streamflow • Explicit sensitivity analysis for effects of temperature and precipitation

7. Effects of the PDO and ENSO on Columbia River Summer Streamflows PDO Cool Cool Warm Warm Red = Warm ENSO Green = ENSO neut. Blue = Cool ENSO

8. PNW GB CA CRB Schematic of VIC Hydrologic Model and Energy Balance Snow Model Snow Model

9. Result: Daily Precipitation, Tmax, Tmin 1915-1997

10. Overview of Simulation and Analysis Met Data 1915-1997 Linear Trend Analysis VIC SWE • 1916-1997 • 1924-1946 (cool to warm PDO) • 1947-1997 (warm to cool PDO) • 1924-1946 with 1977-1995 (warm to warm PDO) Linear Trends: • Base—combined effects of temp and precip trends • Static Precip—effects of temperature trends only • Static Temp—effects of precipitation trends only Experiments:

11. Trends in April 1 SWE 1950-1997 Source: Mote et al. (2004)

12. 1916-1997 Mar 1 Apr 1 May 1 Trend %/yr Trend %/yr Trend %/yr Red = PNW Blue = CA Green = CO Black = GBAS DJF T (C) DJF T (C) DJF T (C) Trend %/yr Trend %/yr Trend %/yr

13. 1916-1997 DJF AVG T (C) Relative Trend in April 1 SWE (% per year)

14. 1916-1997 Effects of Temp DJF AVG T (C) Relative Trend in April 1 SWE (% per year)

15. 1916-1997 Effects of Precip DJF AVG T (C) Relative Trend in April 1 SWE (% per year)

16. Both Temp and Precip Temp Effects Only Precip Effects Only

17. Both Temp and Precip Temp Effects Only Precip Effects Only

18. Both Temp and Precip Temp Effects Only Precip Effects Only

19. Both Temp and Precip Temp Effects Only Precip Effects Only

20. Physical Characteristics of the Mountain West Elevation (m) DJF Temp (C) NDJFM PCP (mm)

21. Trends from 1916-1997 Effects due to precip trends only

22. a) 10 % Accumulation b) Max Accumulation c) 90 % Melt Change in Date Change in Date Change in Date DJF Temp (C) DJF Temp (C) DJF Temp (C) Change in Date Change in Date Change in Date DJF Temp (C) DJF Temp (C) DJF Temp (C) FP Change in Date Change in Date Change in Date DJF Temp (C) DJF Temp (C) DJF Temp (C) FT Change in Date Change in Date Change in Date

23. a) 10 % Accum. b) Max Accum. c) 90 % Melt Effects of Temperature and Precipitation DJF Temp (C) DJF Temp (C) DJF Temp (C) Change in Date Change in Date Change in Date Effects of Temperature only DJF Temp (C) DJF Temp (C) DJF Temp (C) Change in Date Change in Date Change in Date Effects of Precipitation only DJF Temp (C) DJF Temp (C) DJF Temp (C) Change in Date Change in Date Change in Date FP FT

24. 20th Century Climate Change Impacts in the Cascades

25. Cascades Sub Domain Elevation (m)

26. Monthly Water Balance for the Washington Cascades ~1.8 trillion gallons

27. WA Cascades Water Balance from April-September 1916-1974 Outputs Inputs 58% 45% 42% 33% 22%

28. WA Cascades Water Balance from April-September 1975-1995 Outputs Inputs 56% 39% 44% 38% 23%

29. Trends in Simulated Average APR 1 SWE for the Cascades in WA and OR (1950-1995) Effects of TMP and PCP -54% SWE (mm) Effects of TMP -26% Effects of PCP -28% SWE (mm)

30. Trends in Simulated Average APR 1 SWE for the Cascades in WA and OR (1916-1995) Effects of TMP and PCP -22% SWE (mm) Effects of TMP -20% Effects of PCP -2.5% SWE (mm)

31. Summer Water Availability is Declining 55 years Figures courtesy of Matt Wiley and Richard Palmer at CEE, UW

32. 20th Century Climate Change Impacts in the Snake River Basin

33. Effects to SWE Upstream of Milner

34. Simulated and Observed Natural Streamflow for Snake River at Milner Fraction of Annual Flow from June-Sept

35. Simulated Natural Streamflow for N. Fork Clearwater at Dworshak Dam -10% In 82 yrs Fraction of Annual Flow from June-Sept

36. Simulated Natural Streamflow for Boise River at Boise -9.4% In 88 yrs Fraction of Annual Flow from June-Sept

37. Fine Scale Comparison Between VIC and April 1 Snow course Observations a) b) c) d) e) f) Pictures fo all 1144 sites available at: ftp://ftp.atmos.washington.edu/philip/VICsnowbands_obs.ps

38. Conclusions The Western US is experiencing large losses of SWE in sensitive areas (such as the Cascade mountain range) due to observed regional warming. Without precipitation trends, essentially the entire mountain west would be experiencing declines in April 1 SWE due to large-scale warming. Precipitation trends remain the major driver in areas with cold winter temperatures. Precipitation trends seem to be most strongly associated with regionally-specific decadal-scale climate variability. A consistent global warming signal for precipitation across the West is not apparent. Decadal variability is apparently not a good explanation for losses of snowpack associated with temperature trends. (Any period paired with 1977-1997 will show negative trends in SWE associated with temperature). These results are consistent with the broad features of many global warming scenarios—i.e. rapid warming since the mid 1970s, modest increases in winter precipitation, streamflow timing shifts.