People living in the Pacific Northwest will likely face a difficult choice in coming decades as global warming alters the region’s climate — they can have water for hydroelectric power or water for salmon runs, but not both.
That’s one conclusion of a new study, the most rigorous to date of potential impacts on water issues in the Western United States during the next 50 years as greenhouse warming begins to heat up the planet.
“The choices are rather stark,” said Dennis Lettenmaier, professor of civil and environmental engineering at the UW and one of the researchers contributing to the study. “We asked the question, ‘Could you mitigate the effects by operating the reservoir system differently?’ And the answer, at least in terms of the fish, is probably not.”
The study looks at the implications of climate impact as it relates to water resources in three major hydrologic basins in the Western United States: the Columbia River; California’s Central Valley, which includes the Sacramento and San Joaquin rivers; and the Colorado River. The work was conducted by the Accelerated Climate Prediction Initiative, a consortium of a number of agencies, including the UW, Pacific Northwest National Laboratory and Scripps Institution of Oceanography.
Along the Columbia River basin, the major issue isn’t changes in the amount of precipitation. It has more to do with a receding snowpack, which will reduce natural water storage and affect when water is available. The model predicts that, by mid-century, the yearly average snowpack in the Washington and Oregon Cascades could be reduced by as much as 50 percent.
That would result in big changes in water flows and temperatures in Cascade rivers and streams.
“If you warm things up, the snow melt peak occurs earlier and the summer flows are lower so the reservoir is working harder to move that water from when it wants to occur naturally to later in the year when you want it for fish,” Lettenmaier said. “That is a hurdle you basically just can’t get over.”
In fact, the window for successful salmon reproduction in the Pacific Northwest may become so shortened by climate change that some species could cease to exist, regardless of what water policies are adopted.
The water resources outlook for the next half-century appears troubled for not just the Pacific Northwest. Climate changes could have a devastating impact in California and along the Colorado, too.
The Colorado River reservoir system, according to the study, will not be able to meet all of the demands placed on it — including supplying sufficient water for Southern California and the inland Southwest. Reservoir levels are projected to be reduced by as much as one third and releases by as much as 17 percent.
The reason is that the Colorado system is currently in a sort of fragile equilibrium — currently the annual average flow is only slightly larger than the amount of water taken out.
“So you’re fairly close to a balance point,” Lettenmaier said. “But the problem is that you have slight projected decreases in precipitation. And if you reduce the flow by just a bit, that makes a big difference in the average storage in the system.”
The upshot is that all users of Colorado River hydroelectric power will be affected by lower reservoir levels and flows, which will result in reductions in hydropower by as much as 40 percent. In addition, the ability to deliver water to lower basin users will be impacted, as will the ability to provide water to Mexico in amounts required by international treaty.
In California, the study indicates that it will be impossible to meet current water system performance levels. As a result, water supplies will be less reliable, as will hydropower and instream flows. With less fresh water available, the Sacramento Delta’s ecosystem could be disrupted by a dramatic increase in salinity.
Bill Pennell, director of the Atmospheric Science and Global Change Division at Pacific Northwest National Laboratory, said the study underlines the need for planners and policy makers to pay attention to climate change.
“Population and economic growth already are placing severe strains on water resources in the West,” Pennell said. “Climate change is one more very important factor that has to be taken into account when thinking about the future.”
Of particular note is the physics of the model, developed by the National Center for Atmospheric Research in Boulder, Colo. In general, the model is somewhat conservative with respect to its projections of global warming, in part because of its representation of the thermal inertia of the world’s oceans. Also, use of Department of Energy computers allowed evaluation of a broader range of possible future outcomes, in recognition of the chaotic nature of the earth’s response to changes in global emissions of greenhouse gases
“One implication of this is that this model tends to give climate sensitivities that are toward the low end when compared with some of the other global models,” Lettenmaier said. “In other words, one could say that this is an optimistic scenario.”