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Alan F. Hamlet Dennis P. Lettenmaier

Estimating Changes in Flood Risk due to 20th Century Warming and Climate Variability in the Western U.S. Alan F. Hamlet Dennis P. Lettenmaier. Cool Season Climate of the Western U.S. PNW. GB. CA. CRB. DJF Temp ( °C). NDJFM Precip ( mm). Pacific Decadal Oscillation.

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Alan F. Hamlet Dennis P. Lettenmaier

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  1. Estimating Changes in Flood Risk due to 20th Century Warming and Climate Variability in the Western U.S. Alan F. Hamlet Dennis P. Lettenmaier

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

  3. Pacific Decadal Oscillation El Niño Southern Oscillation A history of the PDO A history of ENSO warm warm cool 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

  4. Effects of the PDO and ENSO on Columbia River Summer Streamflows PDO Cool Cool Warm Warm Red=warm ENSO Green=ENSO neutral Blue=cool ENSO

  5. April-September Naturalized Flow for the Columbia River at The Dalles, OR from Historic Water Years vs January Nino 3.4 Anomalies. 1916-2002

  6. Global T as a Predictive Variable for TMAX Trends Over the West TMAX R2 = 0.62

  7. Global T as a Predictive Variable for TMIN Trends Over the West TMIN R2 = 0.81

  8. 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

  9. 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

  10. Regionally Averaged Cool Season Precipitation Anomalies PRECIP

  11. Predominant Mechanisms Associated with Climate Related Changes in Flood Risk Snowmelt Dominant Rivers: Changes in spring peak snowpacks Transient Snow Rivers: Changes in antecedent snowpack at the time of storm Change in effective basin area due to rain/snow area distribution during storm Changes in storm intensity Rain Dominant Rivers: Changes in antecedent soil moisture Changes in storm intensity

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

  13. Evaluating the Hydrologic Model Simulations in the Context of Reproducing Flood Characteristics Ln (X100 / Xmean) OBS Avg WY Date of Flooding OBS Avg WY Date of Flooding VIC Ln (X100 / Xmean) VIC Red = PNW, Blue = CA, Green = Colo, Black = GB

  14. 100-yr Red = VIC Blue = OBS 50-yr X100 GEV flood/mean flood 20-yr 10-yr 5-yr Zp

  15. Detrended Temperature Driving Data for Flood Risk Experiments “Pivot 2003” Data Set Temperature Historic temperature trend in each calendar month “Pivot 1915” Data Set 2003 1915

  16. Trends in January TMIN for a VIC cell in the Cascades + 2.8° C

  17. Use of a Hydrologic Model with Long Precipitation and Temperature Records • Meteorological Records from 1915-2003 • De-trended Temperatures • Observed Precipitation Variability VIC Hydrology Model Variability of Runoff In Different River Basin Types for A Consistent “Early” and “Late” 20th Century Temperature Regime

  18. Simulated Changes in the 20-year Flood Associated with 20th Century Warming DJF Avg Temp (C) X20 2003 / X20 1915 DJF Avg Temp (C) X20 2003 / X20 1915 X20 2003 / X20 1915

  19. X100 2003 / X100 1915 X100 2003 / X100 1915 X100 2003 / X100 1915 DJF Avg Temp (C) DJF Avg Temp (C) DJF Avg Temp (C) X100 2003 / X100 1915 X100 2003 / X100 1915 X100 2003 / X100 1915

  20. X100 wPDO / X100 2003 X100 nPDO / X100 2003 X100 cPDO / X100 2003 DJF Avg Temp (C) DJF Avg Temp (C) DJF Avg Temp (C) X100 wPDO / X100 2003 X100 nPDO / X100 2003 X100 cPDO / X100 2003

  21. X100 wENSO / X100 2003 X100 nENSO / X100 2003 X100 cENSO / X100 2003 DJF Avg Temp (C) DJF Avg Temp (C) DJF Avg Temp (C) X100 wENSO / X100 2003 X100 nENSO / X100 2003 X100 cENSO / X100 2003

  22. Effects of Cool ENSO on Flood Risks in Larger Basins X100 cENSO / X100 2003

  23. Regionally Averaged Cool Season Precipitation Anomalies PRECIP

  24. 20-year Flood for “1973-2003” Compared to “1916-2003” for a Constant Late 20th Century Temperature Regime DJF Avg Temp (C) X20 ’73-’03 / X20 ’16-’03 X20 ’73-’03 / X20 ’16-’03

  25. Summary and Conclusions • Flood risks appear to be declining overall in the West due to systematic warming, but the simulations suggests that flood risks are increasing in many moderate elevation areas where tradeoffs between loss of antecedent snow and increasing basin size favor increasing basin size (typically basins in near-coastal mountains). • Flood risks are affected by interannual and decadal climate variability. During warm ENSO and warm PDO years, flood risks are generally lower in the PNW and the highest flood risks typically occur in either cool ENSO or ENSO neutral years. In phase PDO signals increase the strength of these relationships overall. • Changes in cool season precipitation variability since the mid 1970s have resulted in substantial increases in flood risk in many areas of the West. Are these related to warming?

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