1. Alan F. Hamlet Marketa McGuire Elsner Ingrid Tohver Kristian Mickelson JISAO/CSES Climate Impacts Group Dept. of Civil and Environmental Engineering University of Washington Effects of Climate Change on Extreme Hydrologic Events in the Western U.S. and Washington State

2. Consensus Forecasts of Temperature and Precipitation Changes from IPCC AR4 GCMs

3. Simulated Changes in Natural Runoff Timing in the Naches River Basin Associated with 2 C Warming Impacts: Increased winter flow Earlier and reduced peak flows Reduced summer flow volume Reduced late summer low flow

4. Chehalis River

5. Nooksack River

6. Changes in Flood Risk in the Western U.S.

7. Tmin Tmax PNW CACRB GB Regionally Averaged Temperature Trends Over the Western U.S. 1916-2003

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

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

10. Summary of Observed Flooding Impacts Rain Dominant Basins: Possible increases in flooding due to increased precipitation variability, but no significant change from warming alone. Mixed Rain and Snow Basins Along the Coast: Strong increases due to warming and increased precipitation variability (both effects increase flood risk) Inland Snowmelt Dominant Basins: Relatively small overall changes because effects of warming (decreased risks) and increased precipitation variability (increased risks) are in the opposite directions.

11. Daily Time Step Streamflow Scenarios Associated with Changes in Climate Fit Probability Distributions To Estimate Flood and Low Flow Risks Compare Flood and Low Risks to Those in the 20 th Century

14. Some Conclusions Based on Preliminary Delta Method Runs As is the case for changes in flood risk associated with observed warming, estimated changes in flood and low flow risk associated with projected 21 st century warming show characteristic patterns related to how warm the basins are in mid winter. Rain dominant basins close to the coast tend to show small changes in the 100 year flood, and substantial large changes in low flow risk associated with projected dry summers. Transient snow and warmer snowmelt dominant streams in the Cascades show strongly increasing flood and low flow risks, especially on the east side of the Cascades. Colder snowmelt dominant basins in inland areas show reduced flood risks in central WA and increased flood risk on the West slope of the Rockies. Changes in low flow risk are generally less severe in the eastern most part of the state. These initial simulations will be extended to examine an ensemble of transient climate scenarios from 20 GCMs.