1. Hydrologic Changes in the Western U.S. from 1916-2003
Center for Science in the Earth System
Climate Impacts Group
and Department of Civil and Environmental Engineering
University of Washington
November, 2005
Alan F. Hamlet,
Philip W. Mote, Martyn Clark,
Dennis P. Lettenmaier

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

3. Trends in Temperature
and Precipitation

4. Trends in Winter (Oct-Mar) Precipitation and Temperature
Tmax
Tmin
1916-
2003
DJF Avg Temperature
Rel. Trend %/yr
Trend (°C/yr)
Trend (°C/yr)
1947-
2003
Red = PNW
Blue = CA
Green = CRB
Black = GB
DJF Avg Temperature
Rel. Trend %/yr
Trend (°C/yr)
Trend (°C/yr)

5. 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, as well as other hydrologic effects which are not typically measured (evaporation and soil moisture).

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

7. Trends in Snowpack

8. 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

9. Overall Trends in April 1 SWE from 1947-2003
Red = PNW
Blue = CA
Green = CRB
Black = GB
DJF avg T (C)
Trend %/yr
Trend %/yr
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):

10. Temperature Related Trends in April 1 SWE from 1947-2003
Red = PNW
Blue = CA
Green = CRB
Black = GB
DJF avg T (C)
Trend %/yr
Trend %/yr
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):

11. Precipitation Related Trends in April 1 SWE from 1947-2003
Red = PNW
Blue = CA
Green = CRB
Black = GB
DJF avg T (C)
Trend %/yr
Trend %/yr
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):

12. 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

13. Trends in Runoff Timing

14. 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.

15. 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)

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

17. Trends in the “Runoff Ratio” (runoff/precipitation) and Annual Flow

18. Trend Runoff Ratio
DJF Temp (°C)
Trend Oct-Mar PCP
Trend Runoff Ratio
Trend %/yr
Trend Apr-Sep PCP

19. Trend in Simulated Annual Average Flows at The Dalles for a Constant Precipitation Input Each Year
Columbia River at The Dalles, OR
Cumulative Trend is -1.7%
or 0.2% per decade

20. Despite small increases in evaporation, trends in annual flows have been dominated by precipitation trends in the historic record.
Columbia River at The Dalles, OR
Black = constant precipitation set at long term mean
Pink = observed precipitation

21. Conclusions
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, and systematic changes in runoff timing (more runoff in March, less runoff in June).
Because these effects are shown 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.
Trends in the runoff ratio and annual runoff are primarily linked to winter precipitation trends, which are not necessarily related to global warming.