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Alan F. Hamlet, Phil Mote, Martyn Clark, Dennis P. Lettenmaier

Effects of Climate Change on Snowpack, Runoff, Soil Moisture and Evaporation 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 October, 2005. Alan F. Hamlet,

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Alan F. Hamlet, Phil Mote, Martyn Clark, Dennis P. Lettenmaier

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  1. Effects of Climate Change on Snowpack, Runoff, Soil Moisture and Evaporation 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 October, 2005 Alan F. Hamlet, Phil Mote, Martyn Clark, Dennis P. Lettenmaier

  2. Background

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

  4. Trends in Temperature and Precipitation

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

  6. Trends in Winter (Oct-Mar) Precipitation and Temperature Tmax Tmin Precipitation 1916- 2003 DJF Avg Temperature Trend (°C/yr) Trend (°C/yr) Rel. Trend %/yr 1947- 2003 DJF Avg Temperature Trend (°C/yr) Trend (°C/yr) Rel. Trend %/yr

  7. Trends in Summer (Apr-Sept) Precipitation and Temperature Tmax Tmin Precipitation DJF Avg Temperature 1916- 2003 Trend (°C/yr) Trend (°C/yr) Rel. Trend %/yr DJF Avg Temperature 1947- 2003 Trend (°C/yr) Trend (°C/yr) Rel. Trend %/yr

  8. 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).

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

  10. Trends in April 1 Snowpack

  11. 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)

  12. Overall Trends in April 1 SWE from 1947-2003 DJF avg T (C) Trend %/yr Trend %/yr

  13. Temperature Related Trends in April 1 SWE from 1947-2003 DJF avg T (C) Trend %/yr Trend %/yr

  14. Precipitation Related Trends in April 1 SWE from 1947-2003 DJF avg T (C) Trend %/yr Trend %/yr

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

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

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

  18. 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) DJF Temp (C) FP 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

  19. Trends in Runoff Timing

  20. 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)

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

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

  23. Trends in Soil Moisture

  24. Trends in Simulated Soil Moisture from 1947-2003 DJF Temp (°C) April 1 Trend %/yr July 1 DJF Temp (°C) Trend %/yr

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

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

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

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

  29. Trends in Potential Evaporation

  30. 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 MPI2040 + 4 C PotET (mm/day)

  31. Trends in July Avg PotET over the Southern Plain Region from 1915-2002

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

  33. Implications for Water Planning

  34. 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.

  35. 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), 330-336 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), 39-49 Stewart IT, Cayan DR, Dettinger MD, 2005: Changes toward earlier streamflow timing across western North America, J. Climate, 18 (8): 1136-1155

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