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Hydrologic Implications of 20th Century Warming in the Western U.S. Center for Science in the Earth System Climate Impacts Group and Department of Civil and Environmental Engineering University of Washington March, 2006. Alan F. Hamlet, Philip W. Mote, Dennis P. Lettenmaier.
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Hydrologic Implications of 20th Century Warming in the Western U.S. Center for Science in the Earth System Climate Impacts Group and Department of Civil and Environmental Engineering University of Washington March, 2006 Alan F. Hamlet, Philip W. Mote, Dennis P. Lettenmaier
Winter Climate of the Western U.S. PNW GB CA CRB DJF Temp (°C) NDJFM Precip (mm)
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).
PNW GB CA CRB Schematic of VIC Hydrologic Model and Energy Balance Snow Model Snow Model
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
As the West warms, winter 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
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)
Trends in March Runoff Trends in June Runoff DJF Temp (°C) DJF Temp (°C) Trend %/yr Trend %/yr
Trends in Simulated Soil Moisture from 1947-2003 DJF Temp (°C) April 1 Trend %/yr July 1 DJF Temp (°C) Trend %/yr
Trends in April 1 SM Trends in July 1 SM DJF Temp (°C) DJF Temp (°C) Trend %/yr Trend %/yr
Trends in the “Runoff Ratio” (runoff/precipitation) And Annual Flow
Simulated Trends in Naturalized Annual Flow for the Columbia River at the Dalles, OR For a Constant Precipitation Input Each Year
Temperature Related Downward Trends in Annual Streamflow at The Dalles Compared with the Effects of Precipitation Variability
Effects of Cool Season Precipitation Trends on Trends in the Runoff Ratio Trend Runoff Ratio Trend Oct-Mar PCP
Changes in Flood Risk Associated with 20th Century Warming
Simulated Changes in the 20-year Flood Associated with 20th Century Warming DJF Avg Temp (C) 2003 Flood/1915 Flood DJF Avg Temp (C) 2003 Flood/1915 Flood 2003 Flood/1915 Flood
20-year Flood for “1973-2003” Compared to “1916-2003” for a Constant Late 20th Century Temperature Regime DJF Avg Temp (C) ’73-’03 Flood/’16-’03 Flood ’73-’03 Flood/’16-’03 Flood
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 summer soil moisture. • Trends in the runoff ratio are primarily linked to winter precipitation trends, which are not necessarily related to global warming • Flood risks appear to be declining overall due to warming, but changes in precipitation variability since 1975 suggest increasing flood risks. • 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 Temperature and Precipitation
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
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