Arizona needs tens of thousands of new housing units to meet demand, but first, developers will need to find enough water.

The state’s water woes have been on full display this month as it lost 21% of its Colorado River supply to cuts, homes outside Scottsdale, Arizona, had their water cut off by the city, and a recently released model found planned housing units for more than 800,000 people west of Phoenix will have to find new water sources.

Arizona is one of the fastest-growing states and short 100,000 housing units, a state Department of Housing report released last year found, but depending on where they’re located, some homes will be more easily built than others. A law requiring developers in parts of the state to demonstrate the availability of 100 years of water supply means proposed communities that don’t have enough groundwater — which accounts for 41% of statewide use — have to find other sources.

“It’s really a consumer protection program,” said Tom Buschatzke, director of Arizona Department of Water Resources. “It’s really important that we ensure folks who are buying their homes, that they do have this hundred year approval.”

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Housing units built in towns and cities with multiple water sources are safe, but it’s more complicated in communities outside of active management areas, like the Rio Verde Foothills, which relied on water from Scottsdale. Scottsdale is no longer providing it to the community as it prioritizes its own residents.

“Water is NOT a compassion game,” Scottsdale Mayor David Ortega said in a statement last year.

“People who are in homes approved on the (100-year) program should feel confident about their future,” Buschatzke said, but home buyers should do their due diligence to ensure their homes will have enough water supply. “They should investigate, they should make sure they’re comfortable with the situation if they choose to live in a place that’s outside of the program.”

The homes proposed west of Phoenix wouldn’t have enough groundwater for 100 years, his department’s Lower Hassayampa Sub-basin Groundwater Model found, while southeast of Phoenix, a planned Superstition Vistas development for as many as 900,000 people will have to piece together water sources. Homes planned inside Apache Junction can use the city’s water, but other water sources would have to be identified for homes built outside the city.

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The Arizona Department of Water Resources plans to release a model on the Phoenix area by the end of March, Buschatzke said.

“It’s our role to provide (policymakers and elected officials), as robustly as we can, the data and the projections out 100 years that these models enable us to provide,” he said. “They’re not crystal balls, but they’re pretty accurate.”

Finding new water sources

For developers who can’t tap enough groundwater, options include leasing water from Colorado River Indian tribes or getting it from one of three basins in the state set aside for groundwater transportation.

“What we expect with additional Colorado River cuts really do fall to the fast-growth, far-flug suburbs much more than to the bigger, older cities that have made substantial investments in resilient and diverse water supplies and still have the capacity to manage demand so that they can stretch the available supplies over their customer base,” said Sarah Porter, director of the Kyl Center for Water Policy.

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These alternative water sources require investments and infrastructure, which adds to the costs of building.

“To just develop groundwater under a subdivision is typically a lot less expensive than acquiring water supplies and developing the infrastructure to move it, and so that of course translates into the profits of the costs of new homes,” she said. “That’s an element that, no doubt, is troublesome for homebuilders and developers.”

Still, ensuring homes have enough water is critical to ensure new communities are sustainable.

“We just can’t have people move wherever they want and count on the nearest city to provide them water,” Porter said. “That’s one thing we’ve learned that shouldn’t be terribly surprising for people who are moving to a desert.”

Towns and cities with established water sources could still in the future ask or mandate residents to use less water, with measures like alternate water days, prohibiting washing cars outside of a car wash, or getting rid of grass. Last year, Scottsdale asked residents to use 5% less water, suggesting adjusting irrigation timers and converting grass areas. In the first quarter of last year, residents reduced water use by 2.5%, according to Arizona public radio station KJZZ.