1. Hydrologic Implications of 20th Century Climate Variability and Global Climate Change in the Western U.S.
Alan F. Hamlet, Philip W. Mote,
Dennis P. Lettenmaier
JISAO/CSES Climate Impacts Group
Dept. of Civil and Environmental Engineering
University of Washington

2. Hydroclimatology of the Western U.S.

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

4. (mm)
Cool Season
Precipitation
Warm Season
Precipitation

5. Hydrologic Characteristics of PNW Rivers

6. A Seasonal Water Balance for the Naches River in Eastern Washington

7. Climatic Variations Associated with ENSO and PDO

8. Pacific Decadal Oscillation El Niño Southern Oscillation
A history of the PDO
A history of ENSO
warm
warm
cool

9. Effects of the PDO and ENSO on Columbia River
Summer Streamflows
PDO
Cool
Cool
Warm
Warm
high
high
low
low
Ocean Productivity

10. PDO Effects ENSO Effects PDO/ENSO Effects
Naturalized Streamflow for the Columbia River at The Dalles OR
PDO Effects
ENSO Effects
PDO/ENSO Effects

11. PDO Effects ENSO Effects PDO/ENSO Effects
Inflows to Chester Morse Lake in the Cedar River Basin
PDO Effects
ENSO Effects
PDO/ENSO Effects

12. Cool Season Precipitation Anomalies Compared to the PDO
-0.845
-0.264
-0.438
-0.053
(Regional to PDO
Correlation R )

13. X100 wENSO / X100 2003 X100 nENSO / X100 2003 X100 cENSO / X100 2003
Fig 8 Warm, neutral, cool ENSO 100-year flood
DJF Avg Temp (C)
DJF Avg Temp (C)
DJF Avg Temp (C)
X100 wENSO / X
X100 nENSO / X
X100 cENSO / X

14. Changing Climate
in the 20th Century

15. A Time Series of Temporally Smoothed, Regionally Averaged Met Data for the West

16. (% per century for precip, degrees C per century for temperature)
Linear Trends in Cool and Warm Season Climate for and
(% per century for precip, degrees C per century for temperature)

17. Tmax
Tmin
Figure 4

18. Differences in cool and warm season precipitation trends suggest different mechanisms (large-scale advective storms vs. smaller scale convective storms) and differing sensitivity to regional warming. Trends in warm season precipitation in the CRB are very different than the other regions and may function more like cool season precipitation (e.g. related to circulation rather than locally generated storms)

19. Regionally Averaged Cool Season Precipitation Anomalies

20. Trends in April 1 SWE
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

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

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

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

24. Wide-Spread Glacial Retreat has Accompanied 20th Century Warming.
Loss of glacial mass may increase summer flow in the short term and decrease summer flow in the long term.
1902
2002
The recession of the Illecillewaet Glacier at Rogers Pass between 1902 and 2002.
Photographs courtesy of the Whyte Museum of the Canadian Rockies & Dr. Henry Vaux.

25. Snowpack is melting earlier
Hamlet A.F.,Mote P.W, Clark M.P., Lettenmaier D.P., 2005: Effects of temperature and precipitation variability on snowpack trends in the western U.S., J. of Climate, 18 (21):
Simulated Change in Date of 90% Melt

26. spring flows rise and summer flows drop
As the West warms,
spring 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):
Spring snowmelt timing has advanced by days in most of the West, leading to increasing flow in March (blue circles) and decreasing flow in June (red circles), especially in the Pacific Northwest.

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

28. Flood risks have increased in many coastal areas with warm winter temperatures, whereas colder inland areas show decreases in flood risk.
Simulated Changes in the 20-year Flood Associated with 20th Century Warming
DJF Avg Temp (C)
Fig year flood A spatial scale
X / X
X / X

29. Regionally Averaged Cool Season Precipitation Anomalies

30. 20-year Flood for “ ” Compared to “ ” for a Constant Late 20th Century Temperature Regime
DJF Avg Temp (C)
X20 ’73-’03 / X20 ’16-’03
X20 ’73-’03 / X20 ’16-’03

31. Forecasting the Impacts of
Global Climate Change

32. Natural AND human influences explain the observations of global warming best.
Natural Climate Influence
Human Climate Influence
Red: observations of global average temperature
Grey: simulations with a climate model (huge computer program, like weather prediction model only run for hundreds of years)
Natural influence: volcanoes, solar variations – guesses before ~1970, better since then
Human influence: greenhouse gases, sulfate aerosols
it is possible to simulate the climate of the last 100 years, and the conclusion is that humans didn’t matter much before 1960 – the early warming and the cooling were largely natural, and the late-century warming was largely human-caused
All Climate Influences

33. Observed 20th century variability
Curves are fits to ln(CO2) for A2 (solid) and B1 (dashed)
Warming ranges are shown for 2020s, 2040s and 2090s relative to 1990s. Central estimates: 0.7C by 2020s, 1.7C by 2040s, 3.2C by 2090s. Pink box shows +/- 2 sigma for annual average temperature (sigma=0.6C). Red lines show previous generation of change scenarios. Until mid-century, emissions scenarios play a minor role in the temperature impacts. Towards the end of the century they play a big role. Conclusions: 1) Adaptation will be an essential component of the response to warming over the next 50 years. 2) Mitigation of greenhouse gas emissions will play an important role in determining the scope of late 21st century impacts. °C
Pacific Northwest

34. Observed 20th century variability%
-1 to +3%
+6%
+2%
+1%
Curves are fits to ln(CO2) for A2 (solid) and B1 (dashed)
Precip changes are shown for 2020s, 2040s and 2090s relative to 1990s. Central estimates: 1% by 2020s, 3% by 2040s, 6% by 2090s. Pink bar shows +/- 2 sigma for PNW annual precip.
Observed 20th century variability
-1 to +9%
-2 to +21%
Pacific Northwest

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

36. Shasta Reservoir inflows

37. Seasonal Water Balance Naches River Current Climate 2040s Scenario
More runoff in winter and early spring, less in summer
2040s Scenario
(+ 2.5 C)

38. For areas that accumulate snowpack in winter, the warmest locations are most sensitive to warming
+4.5% winter precip

39. Changes in Simulated April 1 Snowpack for the Canadian and U. S
Changes in Simulated April 1 Snowpack for the Canadian and U.S. portions of the Columbia River basin
(% change relative to current climate)
20th Century Climate
“2020s” (+1.7 C)
“2040s” ( C)
-3.6%
-11.5%
-21.4%
-34.8%
April 1 SWE (mm)

40. Simulated Changes in Natural Runoff in the Snowmelt-Dominant Naches River Basin Associated with 2 C Warming
Impacts:
Increased winter flow
Earlier and reduced peak flows
Reduced summer flow volume
Reduced late summer low flow

41. Simulated Changes in Natural Runoff in the Rain Dominant Chehalis River Basin Associated with 2 C Warming

42. Conclusions
Cool season precipitation and temperature play a crucial role in the hydrologic behavior of mountain watersheds in the western U.S.
Cool season climate is not stationary and is influenced by cyclical patterns like ENSO, decadal variability associated with the PDO, and by long term trends related to global warming and other unknown factors.
ENSO and PDO forecasts provide useful information for predicting streamflow and other hydrologic variables at seasonal to interannual time scales.
Regional expressions of global warming in the West are expected to result in systematically warmer temperatures, streamflow timing shifts in snowmelt and transient basins, increases in winter flows, and losses of summer water availability.
Systematic changes in cool season precipitation volumes appear to be modest both in observations and GCM simulations, but changes in cool season precipitation variability since 1975 suggest another possible impact pathway.