Another year of very low precipitation and snowpack in the Rocky Mountains has spurred another year of low runoff on the Colorado River - the 10th such year out of the past 14.
The low runoff means the Central Arizona Project - which provides Tucson drinking water - has a 35 percent chance of experiencing its first water shortage in history in 2016, U.S. Bureau of Reclamation officials say.
For 2013, April-July runoff into Lake Powell at the Utah border is expected to be 38 percent of normal. That would be the fourth- lowest such total since the gates on Powell closed in 1964, says the Colorado River Basin Forecast Center of the National Oceanic and Atmospheric Administration.
Last year's runoff was more dismal. The lake received 29 percent of its normal flows - the third- lowest on record - from the Upper Colorado River Basin states of Utah, Colorado, New Mexico and Wyoming.
Officials who run the state-managed, Arizona CAP water project say, however, that they don't expect a shortage until at least 2017 unless next year is also dry. That would create a water three-peat - three years of below-average runoff in the river that is the West's lifeblood, serving at least 25 million people in Arizona and six other states.
The project, consisting of more than 300 miles of above-ground canals and underground pipelines, started delivering river water to Phoenix in 1985 and to Tucson in 1991. (The city of Tucson stopped taking CAP water two years later, and didn't permanently sign on for CAP water for nearly another decade.)
"It would have to be another awful year, on par with this year," for a 2016 shortage to occur, said Chuck Cullom, CAP's Colorado River programs manager. "Our expectation is that it's rare to have three really, really bad years in a row, but it's not uncommon to have two back-to-back dry years."
If a shortage does occur, Tucson and suburban water users such as Marana, Oro Valley and Metro Water wouldn't immediately see cutoffs or reductions in their CAP supplies, project officials say. The first shortages would cut off the recharge - for use in future years - of CAP water that is not allocated to individual cities, farms, mines or water companies. This water is commonly known as excess CAP water. About 300,000 to 320,000 acre-feet of CAP would be likely to be cut off in the first shortage, Cullom said.
This recharge is commonly done by state agencies such as the Arizona Water Banking Authority and the Central Arizona Groundwater Replenishment District, which would probably take about two-thirds of the shortage, Cullom said.
Then, non-Indian farmers with CAP water rights would get cut off. Only after those supplies were exhausted would cities, mines, other industries and Indian tribes with CAP water be stripped of their supplies. State and federal officials have long said they don't expect that to happen until the late 2020s at the earliest.
The state has faced and ducked the threat of CAP shortages once before. In October 2010, Lake Mead reached its lowest level since the 1930s - 1,082 feet, 7 feet above the level at which a shortage would have been declared. Had 2011 been as dry as 2010, the project's first shortage would have occurred in 2012.
Instead, the river was flush with water in 2011, receiving its fourth- largest total runoff on record. The river's reservoir storage system is still benefiting from that year, which is why shortage predictions have been put off for up to five years more.
"That's how the system works. The system has historically been driven by extreme events," Cullom said. Back in 1983 and 1984, the river was smacked by two extremely wet years that produced flooding or near-flooding conditions both years, he noted.
However, some climate scientists and environmentalists have raised concerns about the river's chronic low flows since 2000 - the most severe low-flow pattern on the river since records started being kept a century ago.
While there's no direct evidence that the current drought is being caused by climate change, some University of Arizona scientists have noted that the region's hotter weather in recent years has aggravated drought conditions by inducing more evaporation of river and reservoir water.
"Our expectation is that it's rare to have three really, really bad years in a row, but it's not uncommon to have two back-to-back dry years."
Chuck Cullom, CAP's Colorado River programs manager
By the numbers
• Lake Mead: It is 51 percent full, at 1,116 feet elevation above sea level. By the end of September, it's expected to drop to 1,104 feet - well above its historic low of 1,082 feet in October 2010.
• Lake Powell: It is 48 percent full at 3,597 feet above sea level. By the end of September, it's expected to drop to 3,584 feet.
Contact reporter Tony Davis at email@example.com or 806-7746.