Tucson has 100-year water supply, state finds

In a decision handed down this week, the Arizona Department of Water Resources determined that Tucson water would have at least 166,000 acre-feet of water available for annual use over the next century.

That's far more than the 130,000 acre-feet the state projects people living in Tucson Water's service area will use by 2034, when the newly approved state designation expires. One acre-foot is enough to serve four typical Tucson households for a year.

But since the city's assured supply partly consists of Central Arizona Project water, which could be cut back in the next few years due to dwindling Colorado River flows, an outside water expert, former ADWR chief Kathleen Ferris, warns that the state water agency may have to revisit Tucson's designation before the 10-year period runs out.

A top Tucson assistant city attorney, Chris Avery, however, said he foresees no serious setback to the city's designation due to project water shortages, noting that the utility has other supplies that the new state designation didn't mention.

Under the state's assured water supply law and rules, city water providers such as Tucson Water must prove they have enough water to meet expected demands for 100 years for the city to continue allowing new homes to be built.  But the state must renew its designations of assured supply every decade. In this case, Tucson Water's new designation runs out in 2034, meaning it will have to seek another designation at that time.

The state rules are set up that way because "it's difficult to project all possible uses out 100 years," Ferris said. "You can do that for a subdivision but not a city."

ADWR's decision found the city had assured supplies to serve all of its customers in the immediate Tucson environs, along with customers in the Corona de Tucson area. That's an isolated suburban area lying far southeast of the city of Tucson along Houghton Road and north of the Santa Rita Mountains.

The state designation cites the city's annual supplies of 144,000 acre-feet of Central Arizona Project water, 37,000 acre-feet of groundwater and about 15,000 acre-feet of treated sewage effluent. It found those supplies will be "physically, continuously and legally available" for the next 100 years.

Tucson Water has demonstrated that it will have "adequate delivery, storage and treatment works" in time to satisfy an annual estimated demand of 130,794.81 acre-feet per year for at least 100 years, the state decision said.

But the possible cuts to Tucson's CAP allotment raises questions as to how much of the supply will actually be available in the future, some outside experts have said.

The U.S. Bureau of Reclamation has said that over time, it wants to cut at least three million acre-feet of river water use. The proposals it floated in its draft environmental impact statement on the river last winter called for cuts of possibly 77% or more of Arizona's total CAP supplies.

Arizona and other states in the lower Colorado River basin are proposing to cut a much smaller amount of CAP water over the next two years

But the state officials acknowledge the cuts could grow larger in the future.

If CAP supplies are ever cut substantially to Tucson, the new ADWR designation of an assured water supply for Tucson makes it clear that  ADWR can "go back and look at this again" to see if it remains warranted, said Ferris. She's a water lawyer who helped draft the 1980 state Groundwater Management Act that contains the assured water supply provision.

"It depends on how deep the shortage is and what other water supplies are available to Tucson," said Ferris, who today is a researcher for Arizona State University's Kyl Center for Water Policy.

Avery, an assistant city attorney for Tucson Water, said he's confident that the city's assured water supply designation will remain intact if CAP supplies are cut. First, he said that a substantial portion of CAP supplies aren't included in the designation. Second, he said he expected customer demand here will not rise as much as the ADWR decision projects that it will.

Following the last two assured supply designations that Tucson received, it turned out that its customers' water use was less than what the designations had projected, Avery said.

"I am not concerned, given the portfolio of supplies Tucson water has, that ADWR will revisit our designation," Avery said.  "We have other supplies that are not included in this thing."