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

2. Motivation

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

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

5. If global warming has affected the PNW significantly over the 20 th 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 Cool Warm PDO Red = Warm ENSO Green = ENSO neut. Blue = Cool ENSO

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

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

10. Met Data 1915-1997 VICSWE Linear Trend Analysis Overview of Simulation and Analysis 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: Experiments: Base—combined effects of temp and precip trends Static Precip—effects of temperature trends only Static Temp—effects of precipitation trends only

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

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

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

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

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

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

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

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

19. Both Temp and Precip Precip Effects Only Temp 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. b) Max Accumulationc) 90 % Melta) 10 % Accumulation Change in Date DJF Temp (C) Change in Date DJF Temp (C) Change in Date DJF Temp (C) Change in Date DJF Temp (C) Change in Date DJF Temp (C) Change in Date DJF Temp (C) Change in Date DJF Temp (C) FT FP

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

24. 20 th Century Climate Change Impacts in the Cascades

25. Elevation (m) Cascades Sub Domain

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

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

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

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

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

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

32. 20 th 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 Fraction of Annual Flow from June-Sept -10% In 82 yrs

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

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.