1. Alan F. Hamlet Andy Wood Dennis P. Lettenmaier JISAO Climate Impacts Group and the Department of Civil Engineering University of Washington September, 2001 Climate Change in the Pacific Northwest and Implications for Water Management in the Columbia River Basin

2. Winter Precipitation Summer Precipitation (mm)

3. Hydrologic Characteristics of PNW Rivers

4. Temperature warms, precipitation unaltered: Streamflow timing is altered Annual volume stays about the same Precipitation increases, temperature unaltered: Streamflow timing stays about the same Annual volume is altered Sensitivity of Snowmelt and Transient Rivers to Changes in Temperature and Precipitation

5. Cedar River Western Cascades (caused predominantly by warm temperatures) Columbia River at The Dalles (caused both by warm temperatures and decreased precipitation) Effect of 1992 Winter Climate on Two PNW Rivers

6. ColSim Reservoir Model VIC Hydrology Model Changes in Mean Temperature and Precipitation from GCMs

7. Climate Change Scenarios 2020s

8. Climate Change Scenarios 2040s

9. The main impact: less snow April 1 Columbia Basin Snow Extent

10. Columbia River at The Dalles 2020s “Middle-of-the-Road” Scenario

11. Columbia River at The Dalles 2040s “Middle-of-the-Road” Scenario

12. Water Resources in the Columbia River Basin System objectives affected by winter flows Winter hydropower production (PNW demand) System objectives affected by summer flows Flood control Summer hydropower production (California demand) Irrigation Instream flow for fish Recreation

13. Simulated Reliability of Water Resources Objectives for “Middle-of-the-Road” Scenarios

14. 1 2 3 4 1 Palisades 2 Milner 3 Oxbow 4 Ice Harbor 5 Kiona 5

15. Snake River at Milner

16. Snake River at Ice Harbor

17. Yakima River at Kiona

18. Conclusions PNW hydrology is predominantly controlled by winter conditions in the mountains. Warmer temperatures produce streamflow timing changes in most PNW basins. Changes in precipitation produce changes in streamflow volumes. Basins encompassing the mid- winter snow line are most sensitive to warming. Basins at high elevations with cold winter temperatures are less sensitive. The primary impact of warming in the PNW is loss of mountain snowpack. For the scenarios investigated, both warm/wet and warm/dry scenarios result in decreased snow water equivalent in the Columbia basin. Warmer temperatures generally results in higher winter flows, lower summer flows, and earlier peak flows Effects to the Columbia water resources system are largely associated with reduced reliability of system objectives affected by summer streamflows (water supply, irrigation, summer hydropower, instream flow).

19. There are significant uncertainties regarding changes in precipitation and the resulting intensity of reductions in summer streamflows and increases in the frequency of droughts. However, a consistent and robust result is that some reduction in summer streamflow and increase in drought frequency is present in all scenarios by the 2040s for the Columbia basin. The greatest impacts to the Columbia system are for the warm/dry scenarios, which produce the strongest reductions in summer streamflows and the greatest increases in drought frequency. The reductions in summer streamflows in these scenarios are likely to exacerbate existing conflicts over water, the impacts of regional growth, and weaknesses in infrastructure, water management practice, and management institutions. Conclusions (cont.)