Trends in Runoff and Soil Moisture in the Western U.S. 1916-2003 Center for Science in the Earth System Climate Impacts Group and Department of Civil and Environmental Engineering University of Washington December, 2004 Alan F. Hamlet, Phil Mote, Martyn Clark, Dennis P. Lettenmaier

Changes in Simulated April 1 Snowpack for the Cascade Range in Washington and Oregon for Middle-of-the-Road Climate Change Scenarios Current Climate “2020s” (+1.7 C) “2040s” (+ 2.5 C) -44% -58% April 1 SWE (mm)

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

In temperature sensitive areas of the West, we should be able to see the effects of 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. What can we say about less sensitive areas?

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

Overview of Simulation and Analysis Met Data 1915-2003 Linear Trend Analysis VIC SWE 1916-2003 1924-1976 (warm to cool PDO) 1947-2003 (cool to warm PDO) 1924-1946 with 1977-1997 (warm to warm PDO) Linear Trends: Base—combined effects of temp and precip trends Static Precip—effects of temperature trends only Static Temp—effects of precipitation trends only Experiments:

Trends in April 1 SWE 1950-1997 Mote P.W.,Hamlet A.F., Clark M.P., Lettenmaier D.P., 2005, Declining mountain snowpack in western North America, BAMS (in press)

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

winter flows rise and summer flows drop As the West warms, winter flows rise and summer flows drop Stewart IT, Cayan DR, Dettinger MD, 2004, Changes toward earlier streamflow timing across western North America, J. Climate (in review) 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 fraction of annual runoff 1947-2003 (all cells) Relative Trend (% per year)

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

Runoff June June Effects of temp and precip Effects of temp only 1916- 2003 Relative Trend (% per year)

Trends in Total Column Soil Moisture 1947-2003 (% per year) Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

Soil Moisture Apr 1 Apr 1 Effects of temp and precip Effects of temp only Apr 1 Apr 1 1925- 2003 Relative Trend (% per year)

Soil Moisture Sep 1 Sep 1 Effects of temp and precip Effects of temp only Sep 1 Sep 1 1925- 2003 Relative Trend (% per year)

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, more runoff in March, less runoff in June, and corresponding increases in simulated spring soil moisture and decreases in late summer and fall soil moisture. 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.