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
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

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 CA
CRB GB
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
( 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 ( ) 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. Daily Precipitation, Tmax, Tmin
Long-Term Meteorological Driving Data for the West
Result:
Daily Precipitation, Tmax, Tmin

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

12. TMIN 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

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

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

17. Trends in April 1 Snowpack

18. 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

19. 1950-1997 relative trends in April 1 SWE vs DJF temperature
Obs
VIC
Obs
VIC
Obs
VIC
Obs
VIC
VIC grid cells (red) and snow courses (blue)
Cascades: west of 120 longitude, south of BC
Rockies: north of central Colorado, includes Wasatch and Uintas in Utah
Dry interior: Nevada, AZ, NM, southern and western Utah, southern Idaho, eastern Wash & OR

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. Trends in SWE 1916- 1997 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) FP
DJF Temp (C)
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. winter flows rise and summer flows drop
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):
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.

26. 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)

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

32. Cumulative Trends in the Date of Hydrologic Events
50% WY Runoff
80% Max SM
50% WY ET
Cumulative Trends in the Date of Hydrologic Events
( ) 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
1915
2003

38. Simulated Changes in the 20-year Flood Associated with 20th Century Warming
DJF Avg Temp (C)
X / X
Fig year flood A spatial scale
DJF Avg Temp (C)
X / X
X / X

39. 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

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.