As demand in the Southwest increases for dwindling water resources, thirsty communities are casting their eye on the Colorado River as the lifeblood for their future.
But there already are too many straws in the river, a system that studies indicate is over-allocated and highly variable.
Spanning parts of Wyoming, Utah, Colorado, Arizona, Nevada, California and New Mexico, the Colorado River and its tributaries provide water to nearly 40 million people for municipal use, irrigate nearly 5.5 million acres of land and is the lifeblood for at least 22 Native American tribes, seven national wildlife refuges, four national recreation areas and 11 national parks, according to a U.S. Bureau of Reclamation study released in late 2012.
Los Angeles, Las Vegas, Denver, Phoenix, Tucson and Albuquerque are among the many large cities dependent on Colorado River water. It also is vital to Mexico to meet agricultural and municipal needs in that country. In addition, hydropower facilities along the river provide more than 4,200 megawatts of generating capacity to help meet the power needs of the West.
The Colorado River system is operated in accordance with the Law of River, a complex series of treaties, compacts, decrees, statutes, regulations, contracts and other legal documents and agreements.
Under the 1922 Colorado River Compact, both the upper basin that includes Wyoming, Utah and Colorado, and lower basin of California, Arizona and Nevada were allocated 7.5 million acre-feet each. In 1944, Mexico was allocated 1.5 million acre-feet.
It has since become clear that the early decades of the 20th century, the period upon which the 1922 compact was based, has been the wettest period in the Colorado River gage record.
“The Colorado River is the backbone of our dependable water supply,” said Herb Guenther, former Yuma County lawmaker who served as director of the Arizona Department of Water Resources for eight years and is now a consultant for water issues.
“And it has the most competition,” he said.
A tree-ring study by a University of Arizona research team in 2007 documented the year-by-year natural variability of stream flows in the upper Colorado River Basin back to A.D. 762 with various dry periods. The biggest drought found in the entire record was an epic drought during the mid-1100s. That 60-year drought was remarkable for the absence of any wet years interspersed with the dry ones. At the core of the drought was a period of 25 years in which the river averaged 15 percent below normal.
Drought has again come to the Colorado River basin with a number of years experiencing below average stream flows. While the consistent year-after-year below average flows of the drought in the 1100s hasn't been experienced in the last decade, projections for the near future are not encouraging.
Guenther said estimates for over-allocation of the river range from 2 million to 4 million-acre feet.
And new research by UA scientists indicates an unprecedented combination of heat plus decades of drought could be in store for the Southwest sometime this century.
For every 1.8-degree warming in the future, Colorado River flow is projected to decrease between 2 and 8 percent, Connie Woodhouse, a UA professor of geography and development, wrote in her 2009 paper, “A 1,200-year perspective of 21st century drought in Southwestern North America.”
She noted that in recent decades, temperatures have been higher than during the previous 1,200 years and future temperatures are predicted to be even warmer.
In addition, she said, other research predicts that changes in atmospheric circulation will reduce the amount of winter precipitation the Southwest receives in the future. And new tree-ring research indicates that long-term droughts in the Southwest often mean failure of both summer and winter precipitation.
“Droughts similar to those that occurred in the past could occur in the future, exacerbated by climate warming,” she said. “Even without warming, if you had one of those medieval droughts now, the impact could be devastating. Our water systems are not built to sustain us through that length of drought.”
Relief apparently isn't in sight anytime soon.
Current Colorado River basin snowpack is 77 percent of average, according to the USBR website. Total river storage system was 52 percent of its 60 million acre-foot capacity as of May 5, compared with 62 percent last year. Lake Mead is at 49 percent of capacity and Lake Powell 47 percent. That puts Mead at 38 feet above the trigger to declare a shortage.
And another hot, dry summer is forecast for the West.
“While much of the eastern half of the nation has cooler and wetter conditions relative to last summer, the West will bear the brunt of this summer's drought and heat,” reported Paul Pastelok, head of the AccuWeather.com Long Range Forecast Department.
“It's a bit scary,” Steve Hvinden, chief Boulder Canyon operations officer for the USBR, said of the situation during a water summit in Yuma. He noted that the latest models show a “significant chance of a shortage by 2016.”
And that's without any new straws in the river.