Alan F. Hamlet, Phil Mote, Martyn Clark, Dennis P. Lettenmaier Center for Science in the Earth System Climate Impacts Group and Department of Civil and Environmental Engineering University of Washington October, 2005 Effects of Climate Change on Snowpack, Runoff, Soil Moisture and Evaporation in the Western U.S. from

Background

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

Trends in Temperature and Precipitation

Natural Climate InfluenceHuman Climate Influence All Climate Influences Natural AND human influences explain the observations of global warming best.

Rel. Trend %/yr Trend (°C/yr) Precipitation TmaxTmin DJF Avg Temperature Rel. Trend %/yr Trend (°C/yr) Trends in Winter (Oct-Mar) Precipitation and Temperature

Rel. Trend %/yr Trend (°C/yr) Precipitation TmaxTmin DJF Avg Temperature Rel. Trend %/yr Trend (°C/yr) Trends in Summer (Apr-Sept) Precipitation and Temperature

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).

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

Trends in April 1 Snowpack

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 April 1 SWE

Trend %/yr DJF avg T (C) Trend %/yr Overall Trends in April 1 SWE from

Trend %/yr DJF avg T (C) Trend %/yr Temperature Related Trends in April 1 SWE from

Trend %/yr DJF avg T (C) Trend %/yr Precipitation Related Trends in April 1 SWE from

Effects of Temperature And Precipitation Effects of Temperature Alone -19% -25% Trends in April 1 SWE for the WA and OR Cascades -2.84% per decade -2.15% per decade

Effects of Temperature And Precipitation Effects of Temperature Alone Trends in April 1 SWE for the WA and OR Cascades -35% -23% -4.25% per decade -6.48% per decade

Obs. Summer Water Availability is Declining 55 years Figures courtesy of Matt Wiley and Richard Palmer at CEE, UW

b) Max Accumulationc) 90 % Melta) 10 % Accumulation Change in Date DJF Temp (C) Change in Date DJF Temp (C) Change in Date DJF Temp (C) Change in Date DJF Temp (C) Change in Date DJF Temp (C) Change in Date DJF Temp (C) Change in Date DJF Temp (C) FT FP Trends in SWE

Trends in Runoff Timing

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)

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

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

Trends in Soil Moisture

April 1 July 1 Trends in Simulated Soil Moisture from Trend %/yr DJF Temp (°C) Trend %/yr

DJF Temp (°C) Trend %/yr DJF Temp (°C) Trends in April 1 SMTrends in July 1 SM

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

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

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 21 st century. Trends in the runoff ratio are primarily linked to winter precipitation trends, which are not necessarily related to global warming

Trends in Potential Evaporation

Average July PotET over the Southern Plain Region in the Snake River Basin Current Climate vs. MPI2040 scenario (Effects of temperature increases alone) Current Climate MPI C PotET (mm/day)

Trends in July Avg PotET over the Southern Plain Region from

Will Global Warming be “Warm and Wet” or “Warm and Dry”? Answer: Probably BOTH!

Implications for Water Planning

Water resources managers and planners should expect systematically decreasing natural flows in summer in river basins with substantial winter snow accumulation. Hydrologic changes due to global warming are likely to exacerbate future stresses on water systems due to increasing population, and may intensify existing conflicts between water supply and other objectives such as instream flow for fish. There is little evidence in the historic record for systematic changes in winter precipitation associated with global warming in the West. Water managers should be prepared to cope with extended periods of both “warm and wet” and “warm and dry” conditions at different times in the future. Effects to potential evaporation (irrigation demand) remain unclear and appear to be strongly affected by uncertain changes in cloudiness and humidity in addition to temperature. Straight-forward methods to include these kinds of hydrologic changes in water planning studies are readily available.

Selected References: Hidalgo HG, Cayan DR, Dettinger MD, 2005: Sources of variability of evapotranspiration in California, J. of 6 (1): 3-19 Hamlet A.F., Lettenmaier D.P., 2005, Production of temporally consistent gridded precipitation and temperature fields for the continental U.S., J. of Hydrometeorology, 6(3), 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 (in press) Hamlet A.F., Mote P.W., Clark M.P., Lettenmaier D.P., 2005: 20th Century Trends in Runoff, Evapotranspiration, and Soil Moisture in the Western U.S., J. of Climate (in review) Mote P.W., Hamlet A.F., Clark M.P., Lettenmaier D.P., 2005, Declining mountain snowpack in western North America, BAMS, 86 (1), Stewart IT, Cayan DR, Dettinger MD, 2005: Changes toward earlier streamflow timing across western North America, J. Climate, 18 (8):