Dennis P. Lettenmaier Alan F. Hamlet JISAO Climate Impacts Group and the Department of Civil and Environmental Engineering University of Washington July, 2001 Effects of Climate Change on the Hydrology and Water Resources of the PNW and Columbia River Basin

Elevation (m) Areas with December temperatures near freezing Topography of the Pacific Northwest

Annual PNW Precipitation (mm)

Winter Precipitation Summer Precipitation (mm)

Hydrologic Characteristics of PNW Rivers

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

Historic Analogues for the Effects of Climate Change

Ollalie Meadows (3700 ft elevation) WY 1995 Near Normal Precipitation Near Normal Temperatures normal precipitation normal snowpack Normal Conditions

Ollalie Meadows (3700 ft elevation) WY 1992 Near Normal Precipitation Warm Temperatures ( F) normal precipitation normal snowpack Unusually Warm Year

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

Quantifying and Evaluating the Hydrologic Impacts of Climate Change

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

Climate Change Scenarios 2020s

Climate Change Scenarios 2040s

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

Columbia Basin Average Snow Water Equivalent HadCM2 (Warm/Wet) and ECHAM4 (Warm/Dry) Scenarios Conclusion: Both Warm/Wet and Warm/Dry scenarios result in reduced SWE

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

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

Effects to the Cedar River (Seattle Water Supply) for “Middle-of-the-Road” Scenarios

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

Quantifying Uncertainties

Changes to PNW Annual Temperature (High, Medium, Low)

Changes to PNW Winter Precipitation (High, Medium, Low)

Range of Uncertainties in Summer Streamflow Simulations at The Dalles for 2040s Scenarios

Range of Uncertainty in Water Resources Sensitivity for 2040s Scenarios

Frequency of Drought in the Columbia River Comparable to Water Year 1992 (data from ) x 2 x 4.7 x 1.3

Limited reservoir storage is available, and there is little opportunity to build more (storage/streamflow ratios are 10% to 30% in most basins--vulnerable to timing shifts) Water systems are operated closer to their supply limits now than in the past (effective management is more important) Use of historic streamflow record for long-range planning Use of statistical streamflow forecasting tools based on 30-year streamflow record Inflexibility and fragmentation of water management institutions and entities Very limited use of available streamflow forecasts incorporating climate change information Changes in intra-regional water availability (e.g. different changes in Canada and US) may disrupt existing management framework and agreements Areas of Concern for Water Management

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. 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).

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.)