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

Snake River at Ice Harbor Three changes

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

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 10-40 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.

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

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.

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

Part 0. Climate

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

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

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

1. snow

figure courtesy Noah Knowles, UCSD Knowles et al. 2006 Trend in SFE/P, 1949-2004 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

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

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

April 1 SWE trends, 1950-2002 aT<T> aP<P>

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

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

2. Streamflow and runoff Timing

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 10-40 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.

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

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

3. Flood Risk

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 3 20 year flood A spatial scale DJF Avg Temp (C) 2003 Flood/1915 Flood 2003 Flood/1915 Flood

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