Running Toward Empty?: Part Two

By Brendon Bosworth and Tom Yulsman

A section of the Green River in Utah, at the mouth of Horseshoe Canyon. The Green River is an integral part of the Colorado River Basin. Copyright Tom Yulsman.

In part one of this series, we examined what’s happening now in the Colorado River Basin, including the impact of a protracted, multi-year drought on the region. Today, we take a look at what the future might bring.

Climate change research offers little reason to believe that the gloriously snowy start to the year in the mountains of the Colorado Basin are a harbinger of wetter times to come.

To begin with, some research suggests that changes to the climate, thanks in part to rising concentrations of greenhouse gases in the atmosphere, are already responsible for at least a portion of the drought that has been plaguing the region since 2000.

During the past few decades of rapid growth in water use, “the hydrological cycle in the region began to change,” write Tim Barnett and David Pierce of the Scripps Institution of Oceanography, in a 2009 study published in the Proceedings of the National Academy of Sciences.

“Snowpack declined in the western mountains, temperatures increased, and many streams gradually shifted their peak flow to earlier in the year,” they continued. “It has been shown, with very high statistical confidence, that a substantial portion of these changes are attributable to human-induced effects on the climate.”

But Brad Udall, director of the Western Water Assessment of the University of Colorado, is a bit more cautious: “The way science and statistics work is that there’s a really high bar set to say, ‘Okay, this particular event is actually climate change and not just natural variability.’ In fact that bar is so high, frequently all you can say is, ‘hey, this is consistent with what we think climate change will bring.’ I think this is in many ways is where we are in the Colorado River.”

Regardless of whether the recent drought has a man-made component or not, computer modeling of the climate system is not reassuring about the future. It indicates that the Colorado River basin will become warmer and more arid in coming decades. In fact, this is one of the more robust findings shared among most of the climate models.

Barnett and Pierce started their 2009 paper by noting this sobering fact: “Global climate models almost unanimously project that human-induced climate change will reduce runoff in this region by 10 –30 percent.” Based on those projected reductions, they estimated the impact on delivery of Colorado River water to its many users.

To fully appreciate what they found, it’s helpful to understand how water managers for the U.S. Bureau of Reclamation, which regulates flow from lakes Powell and Mead (and others), respond to shortages. Their most important job, of course, is to do whatever they can to avoid completely draining those hydrological bank accounts.

They will also do all in their power to make sure that Lake Mead does not drop below the water intakes for Las Vegas, Nevada, which gets most of its water from the reservoir. To do this when drought conspires with excessive use of river water, the bureau’s managers will simply have to cut deliveries to many water users. (Meanwhile, the city of Las Vegas has been working frantically to install a new 20-foot diameter, three-mile long intake tunnel that will stretch farther and deeper into the reservoir than the existing ones.)

This is where Barnett and Pierce’s study comes in: Given the range of reductions in runoff from climate change, how much might the bureau have to reduce scheduled deliveries of water to users (which are actually supposed to increase in the future)? 

The researchers found that if climate change reduces runoff by just 10 percent, scheduled deliveries of water to users will be missed almost 60 percent of the time by the year 2050. If runoff should drop by 20 percent, those deliveries will be missed 88 percent of the time.

This sounds worrisome enough. But these estimates are based on the long-term average flows experienced through most of the 20th century. And the record of climate over a longer time span reveals that period to have been unusually wet.

Tree ring studies indicate that the 20th century was the wettest in the last 500 to 1,200 years. Credit: istock

Tree ring studies indicate that the 20th century was the wettest in the last 500 to 1,200 years. In fact, the tree ring evidence shows that the current drought in the Colorado basin was dwarfed by a series of what researchers have called “megadroughts” that afflicted the entire West between 900 AD and 1400 AD. During this 500-year period, more than half of the entire American West routinely was in severe drought, according to Richard Seager at Columbia University’s Lamont-Doherty Earth Observatory.

“The climate system clearly has the capacity to get ‘stuck’ in drought-inducing modes over North America that can last several decades to a century or more,” Seager and colleagues wrote in a paper published in 2009.

The researchers also point out that the megadroughts occurred without any intervention from human beings. So they could well happen again. It’s also very possible that human-caused warming could bring a return to megadroughts by inducing the same climatic conditions that appear to have been associated with them in the past.

Barnett of Scripps comments that “almost certainly, mother nature, without global warming, is going to reduce the amount of water in the Colorado. When you combine that with human-induced changes, then I think you really get into some big numbers that can’t be handled.”

He points to the impact of exceptionally dry years when storage is low. Flows in the river could drop to six million acre-feet (MAF), versus the 15 MAF experienced through much of the 20th century. If that should happen, “the margins are pushed down so low that when you have a couple of those [years] in a row you really have trouble.”

Barnett and Pierce are not the only researchers whose work is causing concern about the long-term sustainability of water use in the Colorado Basin. In 2009, Udall and colleagues published a similar study in the journal Water Resources Research.

It found that given projected increases in demand for water on the river, and a 20 percent reduction in its annual flow by 2057 due to climate change, there would be a nearly 10-fold increase in the chances that lakes Mead and Powell would become depleted.

But there was also a ray of hope: In the next 20 years, the risk that lakes Mead and Powell would become depleted is actually relatively low — providing time for policy makers to devise new management strategies.

Lake Mead in 1985 (above), as compared to Lake Mead in 2010. Credit: NASA

At the same time, Udall and his colleagues emphasize that the small risk of drying in the next two decades “should not lull policy makers into inaction,” because waiting too long to do something would significantly reduce the ability of water managers to mitigate the large risks after that point.

Over the short term, if flow in the river does indeed continue to decline, the first cut in deliveries would happen when Lake Mead drops below the trigger-point elevation of 1,075 feet, as many experts suspect it will. In that case, deliveries to the lower basin states overall would be curtailed by 333,000 acre-feet per year.

Ninety-six percent of that hit would be borne by the parts of Arizona receiving water from the Central Arizona Project, which serves users in Pima, Pinal and Maricopa counties, including the cities of Phoenix, Mesa and Scottsdale, agricultural irrigation districts, and 12 Indian tribes.

Meanwhile, Nevada would shoulder the other four percent reduction. But California would remain unaffected by this first cut.

If the surface elevation of Lake Mead should keep dropping, further reductions in deliveries would occur first at the 1,050-foot mark, and then at 1,025-feet.

The Bureau of Reclamation has launched a planning study covering the next 50 years to assess likely supply and demand scenarios and future strategies to keep the system viable. As part of its planning, the bureau is working with climate experts.

In the meantime, water managers already have a good idea how they’ll cope with what may be coming in the shorter term. According to Fulp, the first shortage is quite manageable. “If it goes past 1,025 feet and it looks like we’re heading for 1,000 we’ll sit down to renegotiate,” he says, speaking of how water is allocated among the states. 

Gregg Garfin, a climatologist at the School of Natural Resources and the Environment at the University of Arizona, says the first cut would at most nominally affect the city of Phoenix, which also pumps local groundwater and receives water from river basins within Arizona.

“There are options,” he says. “It’s not like all of a sudden people will turn on the tap one day and no water will be flowing.”

Garfin explains that water managers in Phoenix are also prepared to deal with possible shortages in the future through water trading, and pumping ground water on an emergency basis. He also emphasizes that Phoenix has eight or nine water supplies that have not been fully developed.

Barnett is not terribly optimistic about these strategies. He says 20 years worth of scientific work has been giving the same answer about water sustainability — and the problem lies with the demands of development.

“Who would have imagined in 1922 when they made the compact that Phoenix would be the fourth biggest city in the United States? It’s out in the desert,” he says. “They’re drilling like crazy to try and get ground water, but a lot of it is not very good. Do you think they can support millions and millions of acre-feet taken out from underground? How is that replenished, particularly as the desert moves northward?”

One way to cope with the situation is to start curbing development, he argues. “Practically, it’s the thing to do.” But politically he acknowledges that it would probably be untenable.

During a presentation to the mayors of the ten biggest cities in California, Barnett says he brought up that option. “They all should just shook their heads and said ‘no way.’ Then of course I realized, their campaigns are paid for by big developers, so that was a dumb thing for me to say.”

So will there come a day when residents of Phoenix, Las Vegas and other cities in the region will be told, “Sorry, no more water”?

In their recent study, Udall and his colleagues found that a reduction of just six percent in demand for water would result in a 33 percent drop in the risk that the reservoirs would dry out. That’s a pretty big bang for the conservation buck. 

And already, water consumption in the upper basin states has peaked and started to decline during the recent drought, thanks to the systems already in place.

Udall says this demonstrates how much room there is to substantially reduce long term risks, even with climate change — but only if water managers and water users in the region continue working hard to find ways of reducing consumption. 

He also emphasizes that the Colorado Basin is not unique in the challenges it faces. “When you look around the planet with regard to water, places like India and China are particularly at risk given growing population and use of water for food and all these competing uses,” he says.

During the next century, one of the most pressing environmental issues we’ll face will be “the intersection of carbon and climate change with all these other problems. And water is one of the first ones that is arising. The Colorado River Basin is the classic case in the United States.”

Brendon Bosworth is a master’s student at the University of Colorado, Boulder. His advisor, Tom Yulsman, is co-director of the Center for Environmental Journalism and a frequent contributor to Climate Central.