1. Hydrologic implications of 20th century warming in the western U.S.
Alan Hamlet, Philip Mote, and Dennis Lettenmaier
University of Washington

2. Snake River at Ice Harbor
Three changes

3. Figure by Iris Stewart, Scripps Inst. of Oceanog. (UC San Diego)
Western snowfed streamflow has been arriving earlier in the year in recent decades
Spring-pulse dates
Figure by Iris Stewart, Scripps Inst. of Oceanog. (UC San Diego)
Center
time
Spring
pulse
Centers of Mass
Stewart et al., 2005

4. winter flows rise and summer flows drop
As the West warms,
winter flows rise and summer flows drop
Figure by Iris Stewart, Scripps Inst. of Oceanog. (UC San Diego)
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.

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

6. To evaluate the effects of these temperature and precipitation changes on the Northwest’s water resources, we have used the Variable Infiltration Capacity (VIC) model, developed at the University of Washington. VIC includes multiple soil levels, energy and water balance at the surface, and a sub-grid distribution of land surface types and infiltration values. We have implemented VIC over the Columbia River Basin at 1/8 degree by 1/8 degree horizontal resolution.
Source for this and the next 6 slides: Hamlet and Lettenmaier, 1999, J. Amer. Water Resources Assn.

8. Three Integrations 1916-2003 Observed T and P
Observed T, P with interannual variability removed (“fixed P”)
Observed P, fixed T

9. Part 0. Climate

10. Regionally averaged cool season temperature: standardized anomalies (1961-90 mean)
Tmin
1920
1940
1960
1980
2000

11. Regionally averaged Oct-Mar precipitation: standardized anomalies
1920
1940
1960
1980
2000

12. Regionally averaged Apr-Sep precipitation: standardized anomalies
1920
1940
1960
1980
2000

13. 1. snow

14. figure courtesy Noah Knowles, UCSD Knowles et al. 2006
Trend in SFE/P,
Winter SFE/P trends: symbol area is proportional to trends, measured in standard deviations as indicated. Circles indicate high trend confidence, squares lower confidence. stations had to have reasonably complete records and mean annual snowfall of at least 25mm.
figure courtesy Noah Knowles, UCSD
Knowles et al. 2006

15. Declining April 1 snowpack, 1950-1997
Trends not calculated for lowest-elevation points (mean SWE > 5mm)

16. 1950-1997 relative trends in April 1 SWE vs DJF temperature
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

17. April 1 SWE trends,
aT<T>
aP<P>

18. Trends in April 1 SWE for the WA and OR Cascades 1950-2003
-35%
Effects of Temperature
And Precipitation
1960
1980
2000
-23%
Effects of Temperature
Alone
1960
1980
2000

19. Trends in April 1 SWE for the WA and OR Cascades 1916-2003
-19%
Effects of Temperature
And Precipitation
1920
1940
1960
1980
2000
-25%
Effects of Temperature
Alone
1920
1940
1960
1980
2000

22. 2. Streamflow and runoff Timing

23. winter flows rise and summer flows drop
As the West warms,
winter flows rise and summer flows drop
Figure by Iris Stewart, Scripps Inst. of Oceanog. (UC San Diego)
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.

24. 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)

25. Trends in March Runoff
DJF Temp (°C)
T and p
fixed p
Trend %/yr

26. 3. Flood Risk

27. Color: month of usual flood red=Jan, purple=Feb, lt green=Mar,
Simulated Changes in the 20-year Flood Associated with 20th Century Warming
Color: month of usual flood
red=Jan, purple=Feb, lt green=Mar,
dark green=Apr, blue=May, black=Jun
Fig year flood A spatial scale
DJF Avg Temp (C)
2003 Flood/1915 Flood
2003 Flood/1915 Flood

28. Conclusions
Observations, empirical analysis, and VIC simulations show consistent hydrologic responses to western warming in sensitive basins:
reductions in snowpack
shifts in flow from summer to winter
changed flood risk