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

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

Trends in Temperature and Precipitation 1916-2003

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)

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).

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

Trends in Snowpack

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, 86 (1), 39-49

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): 4545-4561

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): 4545-4561

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): 4545-4561

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

Trends in Runoff Timing

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): 1136-1155 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 Trends in June Runoff DJF Temp (°C) DJF Temp (°C) Trend %/yr Trend %/yr

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

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

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

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

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.