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A potential gap exists between the amount of water needed to serve a big proposed state-land development north of Oro Valley and the volume available.

The exact amount of water needed for the 15,900-home project is unknown today. That's because water use and conservation patterns could change by the time the Arroyo Grande development gets under way in a number of years.

But based on Oro Valley's current water use and supplies, the town could need up to 3,715 acre-feet of outside water supplies annually for this development — whose land Oro Valley officials are considering annexing. That volume is needed for about 16,580 of the 38,000 people planned for the new project, according to calculations made by the Arizona Daily Star. The town has rights to enough Central Arizona Project water to serve the rest.

More CAP water may be available to this development from the State Land Department. It has rights to 14,000 acre-feet of water for all Pima County. But the department doesn't know how much it would give to the Oro Valley project, out of the 1 million acres it owns in Pima County.

An acre-foot of water contains 325,851 gallons — the amount of water that two to three families of four use in a year.

State Land Department officials unveiled a draft plan for this 9,100-acre development last week. They said they're in the early stages of working on such issues as water and traffic.

The site is bounded by Oracle Road on the east, the Tortolita Mountains on the west, Oro Valley on the south and Pinal County on the north.

Groundwater also lies under the development site, perhaps enough to serve the project for 100 years. But the town would have to pay another government agency to offset that pumping by recharging renewable water supplies elsewhere — at a high cost.

Oro Valley's water utility director said he recognizes that a water shortfall could exist for the project, "depending on the level of growth up there."

"Regardless of what that number is, it is something we would continue to discuss with state lands (department), so Oro Valley will get a proportionate share of CAP water" from the state, said Water Utility Director Philip Saletta.

Town officials believe they can deal effectively with the water issue for this project because they have succeeded in getting treated effluent to most local golf courses and are well on their way toward bringing CAP water to the town, he said. The CAP allocation is due in Oro Valley by 2012.

The state will have better information in a year or two about CAP water for this project after completing a study on the use of its entire 14,000 acre-foot CAP supply in Pima County, said Jamie Hogue, a deputy state land commissioner.

"We're looking at the most beneficial ways to use that water. We face this issue every single time, something we begin to address when we deal with annexation," Hogue said.

But a retired water consultant said the state is "putting the cart before the horse," by bringing this new development to the forefront before deciding what to do with its CAP water.

"I don't think the citizens of Oro Valley should be having to consider it or make up their minds about what they think when most aren't even aware that the state lands have any CAP water, let alone know whether it will be used there," Priscilla Robinson said.

Hogue said officials believe that going to the public early is a better way to get comments.

The state won't sell land to a developer until the state plans the project, including the water issues, she said.

"We appreciate and understand that water will be a limit on any potential development," Hogue said.

Under state law, developments inside a state-established water management area — such as one in the Tucson area — must establish a 100-year water supply before selling home lots.

Typically, developers and cities meet this requirement with groundwater and CAP.

If they rely on groundwater, they must join the three-county Central Arizona Groundwater Replenishment District and pay it to buy renewable supplies such as unused CAP water and recharge them. Cities and developers pay the district $285 an acre-foot for water, compared to $112 an acre foot for CAP.

How much CAP water comes to the Arroyo Grande project from the State Land Department makes a huge difference to the people of Oro Valley, Robinson said, because the town will pay more for additional water it gets the district to recharge.

"You get in hock in this thing, and it goes up and up," she said.

Oro Valley already belongs to the district for its existing customers. The high cost of that water is one reason officials are bringing in renewable supplies such as CAP and reclaimed water, utility director Saletta said.