Alan F. Hamlet, Philip W. Mote, Dennis P. Lettenmaier JISAO/CSES Climate Impacts Group Dept. of Civil and Environmental Engineering University of Washington Effects of Climate Change on Water Resources in the Pacific Northwest and Western U.S.

Example of a flawed water planning study: The Colorado River Compact of 1922 The Colorado River Compact of 1922 divided the use of waters of the Colorado River System between the Upper and Lower Colorado River Basin. It apportioned **in perpetuity** to the Upper and Lower Basin, respectively, the beneficial consumptive use of 7.5 million acre feet (maf) of water per annum. It also provided that the Upper Basin will not cause the flow of the river at Lee Ferry to be depleted below an aggregate of 7.5 maf for any period of ten consecutive years. The Mexican Treaty of 1944 allotted to Mexico a guaranteed annual quantity of 1.5 maf. **These amounts, when combined, exceed the river's long-term average annual flow**.

Despite a general awareness of these issues in the water planning community, there is growing evidence that future climate variability will not look like the past and that current planning activities, which frequently use a limited observed streamflow record to represent climate variability, are in danger of repeating the same kind of mistakes made more than 80 years ago in forging the Colorado River Compact. Long-term water supply planning and specific agreements influenced by this planning (e.g. water allocation agreements) should be informed by the best and most complete climate information available, but frequently they are not. What’s the Problem?

Image Credit: National Snow and Ice Data Center, W. O. Field, B. F. Molnia Aug, 13, 1941Aug, 31, 2004 Recession of the Muir Glacier

Pacific Northwest °C °C °C °C Observed 20th century variability +1.7°C +0.7°C +3.2°C Until mid-century, emissions scenarios play a minor role in the temperature impacts. Towards the end of the century they play a big role. Conclusions: 1) Adaptation will be an essential component of the response to warming over the next 50 years. 2) Mitigation of greenhouse gas emissions will play an important role in determining the scope of late 21 st century impacts.

Pacific Northwest % -1 to +3% -1 to +9% -2 to +21% Observed 20th century variability +1% +2% +6%

Hydrologic Changes Associated with Warming

Hydrologic Characteristics of PNW Rivers

The warmest locations that accumulate snowpack are most sensitive to warming +2.3C, +6.8% winter precip

April 1 SWE (mm) 20 th Century Climate“2040s” (+1.7 C)“2060s” ( C) -3.6%-11.5% Changes in Simulated April 1 Snowpack for the Canadian and U.S. portions of the Columbia River basin (% change relative to current climate) -21.4%-34.8%

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

Nooksack River Sensitivity of a Transient Snow Basin

Chehalis River Sensitivity of a Rain Dominant Basin

Mote P.W.,Hamlet A.F., Clark M.P., Lettenmaier D.P., 2005, Declining mountain snowpack in western North America, BAMS, 86 (1): Trends in April 1 SWE

As the West warms, spring 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):

Water Supply Impacts

Reductions in Supply Increases in Demand Conflicts with Other Water Resources Objectives Water Supply Impact Pathways Combined Impacts Climate Change Increasing Population

Transient SWE simulation from HadCM3 (A2) GCM run (with running 10 year average smoothing) Simulated from observed climate shows a declining trend of ~3KAF per decade ( ) HadCM3 simulated declines ~4KAF per decade Figure courtesy of Matt Wiley and Richard Palmer at CEE, UW

Master's Thesis: Wiley, M.W. (2004). "Analysis Techniques to Incorporate Climate Change Information into Seattle’s Long Range Water Supply Planning," University of Washington In sensitive areas, systematic reductions in summer water availability will affect the yield of water supply systems.

Changes in Hydropower Resources

Impacts on Columbia Basin hydropower supplies Winter and Spring: increased generation Summer: decreased generation Annual: total production will depend primarily on annual precipitation (+2C, +6%) (+2.3C, +5%) (+2.9C, -4%) NWPCC (2005)

Warming climate impacts on electricity demand NWPCC 2005 Reductions in winter heating demand Small increases in summer air conditioning demand in the warmest parts of the region

Source: Payne, J.T., A.W. Wood, A.F. Hamlet, R.N. Palmer and D.P. Lettenmaier, 2004, Mitigating the effects of climate change on the water resources of the Columbia River basin, Climatic Change Vol. 62, Issue 1-3, Climate change adaptation may involve complex tradeoffs between competing system objectives

Flood Control vs Reservoir Refill

Flood Control vs. Refill Full : Current Climate

Flood Control vs. Refill Streamflow timing shifts can reduce the reliability of reservoir refill Full : Current Climate o C : o C No adaption

Flood Control vs. Refill Streamflow timing shifts can reduce the reliability of reservoir refill Full : Current Climate : o C plus adaption o C : o C No adaption

Instream Flow Augmentation and Water Quality

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

Temperature thresholds for coldwater fish in freshwater +1.7 °C +2.3 °C Warming temperatures will increasingly stress coldwater fish in the warmest parts of our region –A monthly average air temperature of 68ºF (20ºC) has been used as an upper limit for resident cold water fish habitat, and is known to stress Pacific salmon during periods of freshwater migration, spawning, and rearing

Conclusions: Global climate change is expected to result in significant hydrologic changes in the PNW and western U.S. because of widespread impacts to snowpack and streamflow timing. Some important water resources impact pathways include: reductions in summer water supply, changes in the seasonality of hydropower resources, disruption in the balance between flood control and reservoir refill, increased need for streamflow augmentation, and impacts to water quality (particularly temperature). These climate-related impacts will force difficult tradeoffs between competing water resources objectives in an already conflicted policy arena.

Anticipate changes. Accept that the future climate will be substantially different than the past. Use scenario based planning to evaluate options rather than the historic record. Expect surprises and plan for flexibility and robustness in the face of uncertain changes rather than counting on one approach. Plan for the long haul. Where possible, make adaptive responses and agreements “self tending” to avoid repetitive costs of intervention as impacts increase over time. Approaches to Adaptation and Planning