The federal government ratcheted up its risk estimates for Central Arizona Project shortages on Wednesday.

The odds of a shortage in water deliveries to Arizona and other Lower Colorado River Basin states in 2016 are now 33 percent, up from 21 percent as predicted in January, the U.S. Bureau of Reclamation said.

By 2017, the odds rise to 75 percent, compared to a January prediction of 54 percent.

The bureau raised the odds of shortages three days after Lake Mead on the Colorado River hit another record low level — the third time that’s happened since 2010.

Snowpack levels in the Colorado River’s Upper Basin are significantly lower now than in January, which reduces runoff into the river and into Lake Powell, which releases water to Lake Mead at the Nevada border. Water for the CAP canal system is stored in Lake Mead.

If there is a shortage next year, it would reduce total CAP deliveries 320,000 acre-feet, or 20 percent. Seventeen irrigation districts would get less water. There would be no water delivered to the state to recharge into the ground as a set-aside for future shortages. Cities and Indian tribes probably wouldn’t lose CAP water for at least five years.

The bureau will make a decision as to whether to declare a shortage for next year as early as August or as late as December.

The prospect of shortages highlights the state’s continuing vulnerability to droughts and overuse of Colorado River water compared to its supply.

“This isn’t a crisis for Arizona, but it’s really a signal that all users in the basin need to address the long-term imbalance between supply and demand,” said Chuck Cullom, CAP’s Colorado River program manager.

Less water in the river means less for electric power generation at Hoover Dam, less for the river environment and less for recreation, he noted.

Nevada and Mexico would also absorb cuts in water deliveries from the river if a shortage is declared, but the reductions would be far less than Arizona would suffer. California is exempt from any cuts in Colorado River deliveries unless the CAP loses its entire 1.5 million acre-foot supply, under the 1968 federal law that authorized CAP’s construction.

Cullom also expressed concern that the latest forecasts show increasing odds of more serious shortages in 2018, 2019 and 2020.

In 2019, for instance, the odds are 24 percent that the project would lose 400,000 acre-feet, or more than 25 percent of its supply, and 10 percent that the project would lose 480,000 acre feet.

By 2020, the odds rise to 14 percent that the project would lose 480,000 acre feet and that Lake Mead would drop below 1,025 feet in elevation. Then, the seven river-basin states would have to decide how the river would be managed when and if the lake dropped to the once-unthinkable level of 1,000 feet.

That’s the level at which cuts in CAP deliveries to cities and tribes could occur — cuts once considered unlikely until the 2030s.

Contact reporter Tony Davis at tdavis@tucson.com or 806-7746. On Twitter: tonydavis987