Climate Change Tempers Good News About Colorado River Basin Water Supply

By Tom Yulsman

As much of the nation sweltered this past July 4th, a good turnout of skiers and boarders — some in bikinis and festive costumes — were busy carving tracks down a snowy slope at Colorado’s Arapahoe Basin ski area. (A herd of possibly patriotic mountain goats served as spectators.)

Skiiers take advantage of an extra-long season at Colorado’s Arapahoe Basin ski area on July 4, 2011. Credit: tab2space/Flickr.

Thanks to snowpack of nearly epic proportions in much of the Colorado Rockies this spring and early summer, Arapahoe Basin got to stay open on July 4th for the first time since 1997.

And it wasn’t just skiers and boarders who had reason to celebrate.

Melting snowpack in the mountains of the nearly quarter-million-square-mile Colorado River Basin provides much of the water used by almost 35 million people in Arizona, California, Colorado, Nevada, New Mexico, Utah and Wyoming. But for 11 long years, the Basin has suffered through the worst drought since record-keeping began more than 100 years ago.

By last November, Lake Mead, the giant reservoir on the lower Colorado that stores water for use in the Colorado’s lower basin — Arizona, California and Nevada — was flirting with a record low level.

But now, for the first time in a very long while, people across the region can enjoy a modicum of good news about water. So much has been flowing into Lake Mead, for example, that while it is still at a low level, the trend during the past several months is now positive.

The lake’s level has mostly been dropping over the past 11 years. But since it bottomed out last November, the level has come up by more than 20 feet. And the U.S. Bureau of Reclamation projects that the reservoir could rise by another 30 feet by the end of the year, causing people in Las Vegas, Nevada, which gets almost all of its water from the reservoir, to breathe a huge sigh of relief.

And the good news is not limited to the snowpack and river flows. A report released in June by the Pacific Institute, a non-profit research group, reveals that municipal users of Colorado River Basin water have become remarkably more efficient.

According to the report, funded by the Bureau of Reclamation, even as the number of people using water from the Colorado River Basin grew by 10 million from 1990 to 2008, per capita use dropped by about 1 percent each year, on average.

The study looked only at municipal use, which includes residential, commercial, industrial, and institutional uses of water, as well as some landscape irrigation. But it excluded use of water for agriculture, energy production, and mining. Municipal use comprises just 15 percent of total water consumption in the Colorado Basin, whereas agriculture accounts for 70 percent. But in some sense, municipal use is where the action is. That’s because farmers’ use of water has been relatively stable. The growth in demand for water has come mostly from the cities.

“This study shows that people aren’t just sitting around with a false sense of security about how much water the Colorado Basin can provide,” says Carly Jerla, a Bureau of Reclamation researcher overseeing a major assessment of the water situation in the region. A good thing, too, since what nature gives in one season she can easily take away the next.

Lake Mead's level has mostly been dropping over the past 11 years, but since it bottomed out last November, the level has come up by more than 20 feet. Credit: Marie Berne/Flickr.

In other words, a one-year respite does not mean that a drought is over. Far from it.

“It’s common to get a big flow year within a longer drought,” Jerla says. Researchers have documented many examples of this in the historical record, and also in the “paleo” record, meaning the time prior to written records, whose climate can be reconstructed through, for example, tree ring data.

Will this year’s bounty wind up serving as another example of that phenomenon? Only time will tell.

Climate change clouds the long-term outlook

There’s another reason for users of Colorado River Basin water not to develop a false sense of security: climate change.

An interim report of the water assessment Jerla is overseeing was released in June. It offers a sobering outlook for how climate change due to increasing concentrations of greenhouse gases in the air might affect Colorado Basin water supplies.

Climate change can alter water supplies in a variety of ways, including reducing the overall amount of precipitation, but also by affecting drought and the timing of spring snowmelt.

Based on computer modeling, the report projects that the natural flow of the Colorado River past Lees Ferry in Arizona, the dividing line between the upper and lower Basins, will decrease by almost 10 percent over the next 50 years.

If that doesn’t sound like much, consider that both the frequency and duration of droughts are likely to increase in coming decades as well. “Droughts lasting five or more years are projected to occur 40 percent of the time over the next 50 years,” the interim report states.

Climate change is also likely to increase climate variability in the region, according to Jerla. In other words, along with more frequent and longer-lasting droughts, we’ll also see short periods with even higher flows than have been normal in the past.

Is this phenomenon already at work with this year’s abundant snowpack and high river flows? “I’m posing the question,” Jerla says, answering: “It could be.”

With such an unsettling picture, the Pacific Institute’s recent report was all the more welcome, because it suggests that for users of water in most cities and towns, the trend is at least reassuring about what society can do to improve its resilience to changes in water availability through conservation and efficiency.

In fact, some urban areas have achieved particularly impressive gains in efficiency.  In Albuquerque, New Mexico, for example, per capita rates of water delivery declined 38 percent between 1990 and 2008. In Phoenix, the number was 30 percent, the study found.

Irrigation crop circles near the border of New Mexico and Arizona. Credit: atmtx/Flickr.

Denver, which gets water from the Colorado Basin through tunnels bored beneath the Continental Divide, performed even better. Despite a 30 percent rise in population served by the city’s water supplier, the city actually used 7 percent less water in 2008 than in 1990.

Similarly, during a time period when the population of Southern California grew by almost 3.6 million people, water agencies in the giant urban area managed to deliver 4 percent less water in 2008 than in 1990.

A variety of factors led to these achievements, including financial incentives to conserve water, restrictions on when lawns may be watered, policies encouraging the use of low-water-use plants in landscaping, and public education programs.

Perhaps just as important, the 11-year drought has raised public awareness. “It has sensitized people to the value of water,” says Michael Cohen, author of the Pacific Institute’s report.

“My hope is that urban efficiency continues to increase, and that agencies learn from Denver and Southern California,” he says.

Unsustainable long-term increase in water demand

There is much that they can learn — and much room for improvement. For the Basin as a whole, he points out, per capita use may have gone down, “but total water use was actually up.”

Between 1990 and 2008, overall deliveries of Colorado Basin water to municipal users increased by 600,000 acre-feet. (An acre-foot of water is enough to cover an acre of land to a depth of one foot; it is roughly enough to supply two suburban households in the West with the water they need for a year — including their thirsty lawns.)

“This is not a trend that can be sustained, even without climate change,” Cohen says.  That’s because of one stark fact: Since 2002, total use of water in the Colorado Basin (by everyone — city dwellers, industrial users, and farmers) has exceeded what actually flows through the Basin.

Disaster has been averted thanks to the hydrologic equivalent of savings banks: reservoirs. The two largest in the system are Lake Mead and Lake Powell, the later being the gargantuan impoundment on the upper Colorado.

“Storage has gotten us through,” Jerla says. “We’re not in dire straights because of the huge amount of storage on the system.”

But with 11 years of drought, and demand exceeding supply, there has been a massive run on the banks, leaving both Mead and Powell at low levels.

Factor in continuing population growth, along with climate change, and it becomes clear that the situation is not sustainable. Something will have to give.

One thing that’s likely to give is agriculture — literally. Farmers, who hold relatively senior water rights, meaning that they get to obtain their water with relative precedence over other users with less senior rights, will probably continue to lease or sell them to cities, transferring water from agricultural to municipal use. This can help delay an ultimate day of reckoning. But as irrigation water – the lifeblood of agriculture in the sere West – dries up, agricultural communities could do so also.

And this is one more reason why the improvements in efficiency by urban areas are so important. With greater efficiency “you get a bigger bang for your buck,” Jerla says — meaning less water will need to be transferred from farms to cities.

Farmers will almost certainly be thankful for that.