Into thin air?

Global warming has spawned a call for new dams — but there may not be any water to fill them.

  • Mountain Snowpack and reservoir storage as of April 1

    Sources: USDA, Natural Resources Conservation Service, National Water and Climate Center

The dream of the infinitely expanding West is getting beaten over the head with an empty bucket. Flows in the Colorado River, which is now entering its eighth straight year of drought, will be less than half of average this year. In the Sierra Nevada, snowpack is just 46 percent of normal. And those numbers are likely to get even worse in the future, as droughts are amplified by global warming.

Now, water managers in the region are contemplating how to react, and new dams have been pitched as one possible response. In California, Gov. Arnold Schwarzenegger, R, announced on Jan. 9 that “with California’s booming population, and with the impact that global warming will cause to our snowpacks, we need more infrastructure.” Just two days later, California state Sen. Dave Cogdill, R, picked up the battle standard and introduced a bill to raise $4 billion in bond money for two new dams, saying, “We have to act now.”

Indeed, global warming is already changing the West’s waterscape. Average temperatures in the region have increased by about 2 degrees Fahrenheit since the 1970s. A greater portion of each year’s precipitation is falling as rain rather than snow, and snowpack — which serves as a natural, slow-release reservoir — is melting earlier than it used to; both changes are intensifying the pulse of winter and spring runoff.

Cogdill, who represents parts of the largely agricultural San Joaquin Valley, says that the continued loss of snowpack and the intensified bursts of seasonal runoff could exceed the storage capacity of California’s existing dams. “If you don’t do something about our ability to manage that water,” he says, “more and more of it will run into the ocean, and late in the year we’ll be in a real deficit.”

New dams can catch excess bursts of water when they come out of the mountains, then spread those flows throughout the year. But it is becoming clearer that, for the West as a whole, more dams may not stem future water crises. New climate projections are hinting that in the future, the region won’t just be warmer. It will also be drier.

Diminishing returns

Any effort to build more dams will be shadowed by the curse of diminishing returns. For one thing, there simply aren’t many places left to put a dam, either in California or in the West as a whole. One of the two proposed California dams would be built at Temperance Flat, in the Sierra foothills near Fresno, to catch floodwater in the San Joaquin River. Yet possible dam sites are so limited that the dam would have to be built in an existing reservoir called Millerton Lake. The second, Sites Reservoir, would be built in the hills on the western edge of the Sacramento Valley, and would hold water from the Sacramento River.

A very big question lurks behind the new-reservoir endeavor: While the dams are being pitched as part of a strategy to increase the reliability of California’s water supply, the reliability of the river flows needed to fill the new reservoirs is far from clear. In the case of Temperance Flat, for example, flow data from the past 57 years indicate that a dam could catch a considerable amount of water.

But past performance is no guarantee of future results. What will happen to river flows depends, in large part, on how much precipitation falls in a warming world. Michael Dettinger, a U.S. Geological Survey researcher at the Scripps Institute of Oceanography, has helped lead the way in assembling large “ensembles” that provide a composite view of various climate projections. Earlier projections, released about seven years ago, indicated that total precipitation in the West might actually increase with global warming. “Back then, the (climate-simulation model) that got most of the focus made the West just incredibly wet: It doubled the amount of precipitation, and more in some places,” Dettinger says. Now, “a lot of people think that was virtually a computer bug.”

In the years since, an increasing number of scientists have focused their efforts on climate change, largely in support of the most recent Intergovernmental Panel on Climate Change assessment, released this spring. “It’s still unsettled,” says Dettinger, “but as people have tinkered with their models and made the models better, there’s been a general drift to it getting drier” in California, and in the Colorado River Basin.

“Dueling models”

There is still considerable uncertainty in climate and water projections, and the Colorado River serves as an outsized example of the challenges facing researchers and water managers. Denver Water supplies 1.2 million people and gets about half of its water from the Colorado River. “The question is, does Denver have to recalculate what the firm annual yield of our water rights is because of global warming?” says Chips Barry, Denver Water’s manager. “And the answer is, maybe. How do you do that? The answer is, I haven’t got the foggiest idea, because you can assume things all over the map.”

Climate scientists are working to puzzle out the reasons behind the sometimes-contradictory projections generated by what they call “dueling models.” Temperatures are incontrovertibly rising. Less clear is how temperature will affect precipitation, how evaporation will affect runoff, and how both factors will affect the total volume of river water that can, in turn, be stored behind dams. And on the Colorado River, where future projections are especially bleak, “the question that’s out there,” says Dettinger, “is, ‘Is that going to be a large reduction in runoff, or a less-large reduction in runoff?’ ”

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