Hydrologic Implications of 20th Century Warming in the Western U.S.

1. Hydrologic Implications of 20th Century Warming in the Western U.S. • Alan F. Hamlet, Philip W. Mote, • Martyn Clark , Dennis P. Lettenmaier • JISAO/CSES Climate Impacts Group • Dept. of Civil and Environmental Engineering • University of Washington • Western Water Assessment, CIRES, • University of Colorado, Boulder

2. Cool Season Climate of the Western U.S. PNW GB CA CRB DJF Temp (°C) NDJFM Precip (mm)

3. Natural AND human influences explain the observations of global warming best. Natural Climate Influence Human Climate Influence All Climate Influences

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

5. Schematic of VIC Hydrologic Model and Energy Balance Snow Model PNW GB CA CRB 12 km 1/8th Deg. 12 km Snow Model

6. Seasonal Water Balance Naches River 20th Century Climate More runoff in winter and early spring, less in summer 2040s Scenario (+ 2.25 C + 4% Pcp)

7. In temperature sensitive areas of the West, we should be able to see the effects of observed global warming in the historic snow and streamflow records. Using models we should be able to more fully analyze these changes, and explore the effects on other hydrologic variables which are not typically measured (evaporation and soil moisture).

8. Overview of Research Questions: • How have variations in temperature and precipitation from the early 20th Century on (1916-2003) affected trends in hydrologic variables such as snowpack, volume and timing of runoff and baseflow, seasonal evaporation and soil moisture, and flood risk in the western U.S.? • Is a consistent global warming signal apparent over the western U.S. in this period, and is it possible to make a clear distinction between “natural” variations such as decadal precipitation variability and more systematic effects associated with global warming signals? Are temperature and precipitation different in this regard? • What role do regional climatic regimes and topographic variations play in defining the role of temperature and precipitation variability on hydrologic variations? What areas of the western U.S. are most sensitive to changes in temperature or precipitation changes and why?

9. Trends in Temperature and Precipitation

10. Long-Term Meteorological Driving Data for the West Result: Daily Precipitation, Tmax, Tmin 1915-2003

11. Regionally Averaged Cool Season Temperature Anomalies 0.74 0.63 0.76 0.62 (Regional to Global Correlation R2 ) TMAX

12. Regionally Averaged Cool Season Temperature Anomalies 0.84 0.87 0.94 0.73 (Regional to Global Correlation R2 ) TMIN

13. Cool Season Precipitation Anomalies Compared to the PDO -0.845 -0.264 -0.438 -0.053 (Regional to PDO Correlation R2 ) PNW Trend

14. Regionally Averaged Cool Season Precipitation Anomalies PRECIP

15. Trends in Cool Season (Oct-Mar) Precipitation and Temperature Tmax Tmin Precipitation 1916- 2003 DJF Avg Temperature Trend (°C/yr) Trend (°C/yr) Rel. Trend %/yr 1947- 2003 DJF Avg Temperature Trend (°C/yr) Trend (°C/yr) Rel. Trend %/yr

16. Trends in warm season (Apr-Sept) Precipitation and Temperature Tmax Tmin Precipitation DJF Avg Temperature 1916- 2003 Trend (°C/yr) Trend (°C/yr) Rel. Trend %/yr DJF Avg Temperature 1947- 2003 Trend (°C/yr) Trend (°C/yr) Rel. Trend %/yr

17. Trends in April 1 Snowpack

18. Trends in April 1 SWE 1950-1997 Mote P.W.,Hamlet A.F., Clark M.P., Lettenmaier D.P., 2005, Declining mountain snowpack in western North America, BAMS, 86 (1): 39-49

19. 1950-1997 relative trends in April 1 SWE vs DJF temperature Obs VIC Obs VIC Obs VIC Obs VIC

20. Overall Trends in April 1 SWE from 1947-2003 DJF avg T (C) Trend %/yr Trend %/yr

21. Temperature Related Trends in April 1 SWE from 1947-2003 DJF avg T (C) Trend %/yr Trend %/yr

22. Precipitation Related Trends in April 1 SWE from 1947-2003 DJF avg T (C) Trend %/yr Trend %/yr

23. a) 10 % Accumulation b) Max Accumulation c) 90 % Melt Trends in SWE 1916- 1997 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

24. Trends in Runoff Timing

25. As the West warms, winter flows rise and summer flows drop Stewart IT, Cayan DR, Dettinger MD, 2005, Changes toward earlier streamflow timing across western North America, J. Climate, 18 (8): 1136-1155

26. Trends in simulated fraction of annual runoff in each month from 1947-2003 (cells > 50 mm of SWE on April 1) June March Relative Trend (% per year)

27. Trends in March Runoff Trends in June Runoff DJF Temp (°C) DJF Temp (°C) Trend %/yr Trend %/yr

28. Trends in Soil Moisture

29. Trends in Simulated Soil Moisture from 1947-2003 DJF Temp (°C) April 1 Trend %/yr July 1 DJF Temp (°C) Trend %/yr

30. Trends in April 1 SM Trends in July 1 SM DJF Temp (°C) DJF Temp (°C) Trend %/yr Trend %/yr

31. Trends in the Dates of 50% WY runoff, 80% max soil moisture recharge, and 50% WY ET

32. 50% WY Runoff 80% Max SM 50% WY ET Cumulative Trends in the Date of Hydrologic Events (1947-2003) BR BR BR DJF Temp (°C) FPR FPR FPR Effects of Temp alone DJF Temp (°C) FTR FTR FTR Effects of Precip alone DJF Temp (°C) Trend days/50 yr

33. Trends in the “Runoff Ratio” (runoff/precipitation)

34. Effects of Cool Season Precipitation Trends on Trends in the Runoff Ratio Trend Runoff Ratio Trend Oct-Mar PCP

35. Temperature Related Downward Trends in Annual Streamflow in the Columbia River at The Dalles Compared with the Effects of Precipitation Variability Black trace = constant precip Magenta trace = with precip variability

36. Changes in Flood Risk Associated with 20th Century Warming and Increased Precipiatation Variability

37. Detrended Temperature Driving Data for Flood Risk Experiments “Pivot 2003” Data Set Temperature Historic temperature trend in each calendar month “Pivot 1915” Data Set 2003 1915

38. Simulated Changes in the 20-year Flood Associated with 20th Century Warming DJF Avg Temp (C) X20 2003 / X20 1915 DJF Avg Temp (C) X20 2003 / X20 1915 X20 2003 / X20 1915

39. 20-year Flood for “1973-2003” Compared to “1916-2003” for a Constant Late 20th Century Temperature Regime DJF Avg Temp (C) X20 ’73-’03 / X20 ’16-’03 X20 ’73-’03 / X20 ’16-’03

40. Summary • Large-scale changes in the seasonal dynamics of snow accumulation and melt have occurred in the West as a result of increasing temperatures. • Hydrologic changes include earlier and reduced peak snowpack, more runoff in March, less runoff in June, and corresponding increases in simulated spring soil moisture and decreases in summer soil moisture. • Trends in the runoff ratio are predominantly linked to winter precipitation trends, which are not necessarily related to global warming • Flood risks appear to be declining overall due to warming, but changes in precipitation variability since 1975 suggest increasing flood risks due to changes in precipitation variability. • Because these effects are shown in many cases to be predominantly due to temperature changes, we expect that they will both continue and increase in intensity as global warming progresses in the 21st century.

41. Implications for Water Management The hydrologic changes shown in these studies will create problems for water management in the western U.S. by disrupting the existing balance between water resources objectives such as flood control, hydropower production, water supply, instream flow augmentation, and water quality. Implications for Ecosystems Synchronized temperature changes affecting very large spatial scales have important implications for the structure and integrity of ecosystems that are affected by water or air temperatures. Some examples of this kind of large scale ecological change are the recent bark beetle outbreaks which have devastated forests in Canada and Alaska, changes in fire ecology, and impacts to cold water fish species such as salmon.