Why a 100-year supply? How Arizona got its famous, yet arbitrarily numbered groundwater rule

Arizona’s 100-year water supply requirement came into sharp focus this year when Gov. Katie Hobbs announced news of a potential shortfall.

It came up again recently when state Senate President Warren Petersen publicly discussed why the requirement is 100 years and not some other number.

Petersen, R-Gilbert, said the number is arbitrary during a meeting about the state’s financial health in November. Petersen denied he's planning, or has heard of plans, for new legislation next year to change the number.

The longtime politician hailing from a family of homebuilders said in the aftermath of Hobbs’ announcement he wants the public to know Arizona has "plenty of water" to continue building homes. He stood by the position in a Dec. 13 interview on azcentral.com’s Gaggle podcast.

“Why is it 100 years?” he said on the podcast. “Why isn’t it 105 years — why isn’t it 95 years? California’s (rule) is 25 years … You don’t go to the gas station and buy 100 years of gas.”

What is the 100-year requirement?

The Indigenous Hohokam, forefathers of the Pima, Maricopa and other Native American tribes, thrived for centuries in what are now called the Phoenix and Pinal Active Management Areas.

These parts of the state are flush with surface water in certain areas, augmented by the Central Arizona Project canal that moves water from Colorado River reservoirs to communities including Tucson.

They also contain untold acre-feet of groundwater, which experts say is still being pumped out at unsustainable rates. An acre-foot of water is roughly enough to serve two to three households for a year.

Action urged: Governor's water council submits management proposals, already faces lawmaker opposition

The amount pumped from the Active Management Areas is regulated because of the Groundwater Management Act. The law, passed in 1980 by the Arizona Legislature and former Democratic Gov. Bruce Babbitt, is still praised as one of the most forward-thinking water laws in the country.

It requires developers of housing subdivisions in the Active Management Areas to prove a 100-year water supply actually exists on the land before they fire up the bulldozers.

One of its goals was to steer the state's fast-growing development into the Active Management Areas that have more water than other parts of Arizona. It also helped ensure the CAP canal would receive help from federal officials, who required a check on groundwater pumping.

The requirement has two major provisions. The first is that metro Phoenix developers must either obtain an agreement to build homes from a city or another “designated assured water supply,” which includes some water companies. These water-distributing entities use surface water to replenish the groundwater they use.

Developers outside of major city areas, but still in Active Management Areas, must obtain a certificate from the state Department of Water Resources showing that a property has a 100-year water supply.

The act doesn’t affect rural Arizona or parts outside of the management areas. It also doesn’t generally affect industrial, agricultural or commercial sites that weren’t built as part of subdivided lands.

Is 100 years the right number?

Fraudulent land sales in Arizona led the state to pass a law in 1973 forcing developers to disclose if there’s an “adequate” water supply on land they sell. Arizona officials determined a few years later that “adequate” meant water “continuously available” for at least 100 years.

Critics at the time argued for 30 to 50 years, saying that would be more in line with the 30-year mortgage typically used in borrowing money to buy a home. A former land commissioner called the 100-year requirement "unrealistic, arbitrary and capricious."

State officials ignored their concerns and stuck with 100 years. The number was soon codified in the 1980 Groundwater Management Act, which banned development in the Active Management Areas where at least a century's worth of water could not be proven.

Pipelines? Desalination? Turf removal? Arizona commits $1B to augment, conserve water supplies

Kathy Ferris, a lawyer and one of the architects of the 1980 law, said that she and the late Jack DeBolske, former executive director of the League of Arizona Cities and Towns, pushed for the “adequate water supply” rule of “at least 100 years” to be included in their sweeping new law.

“We really didn’t discuss the number of years,” said Ferris, now a senior researcher for the Kyl Center for Water Policy at Arizona State University’s Morrison Institute.

Water expert Sarah Porter, executive director for the Kyl Center for Water Policy at Arizona State University’s Morrison Institute, agrees with Petersen that the number “100” isn’t validated scientifically. But she doesn't think it should be lowered.

“In the minds of greatest water planners and industry leaders, 100 years was the right time frame,” Porter said. “New water-supply projects have very long timelines because of the vulnerability of cities and how devastating it could be for a city to have a serious water shortage.”

Considering the growth in Maricopa County over the past 40 years, “I’m very thankful it’s a 100-year timeline.”

If it were only 40 years, for example, it might be tougher to convince people that buying a home in metro Phoenix would still be a good investment decades from now, she said.

Arizona's water supply is well-managed

Porter pointed out that in most Phoenix-area cities, the 100-year rule gets extended every 15 years.

For now, scientific modeling shows the system can go on almost indefinitely in these better-watered areas. Yet outlying parts of metro Phoenix that require a 100-year certificate for development don’t provide the same assurance.

The latest modeling of the entire Phoenix Active Management Area shows a 4% deficit overall in the 100-year requirement, about 5 million acre-feet of water. That’s why in June, Hobbs put a halt to new subdivisions that can’t prove a 100-year water supply by means other than groundwater supplies.

Stopped: Arizona will halt new home approvals in parts of metro Phoenix as water supplies tighten

In Petersen’s view, the 4% deficit means that some areas “only have a 96-year supply.”

If Arizona's rule required only a 95-year supply, or 25-year supply like in California, "nobody would be talking about how Arizona is out of water,” Petersen said on the podcast.

Converting farmland to home developments saves water, he noted. He's also correct that Arizona uses roughly the same amount of water now as it did in the 1950s despite a much larger population and economy.

Yet the problem is that "some areas would be hit harder than others, especially in Buckeye," Ferris said. She added she believes Petersen is "in denial" about the water supply.

"We have a problem in some places. California has a problem in many places. There is not plenty of water for everyone to do just do as they please," she said.

With climate change, drought and fights over dwindling levels of Colorado River water available for all of the states that use it, water researchers want to see more regulation, not less.

“In 1980, 100 years was a big lift,” Ferris said. “Now I definitely think it’s not long enough.”

Reach the reporter at rstern@arizonarepublic.com or 480-276-3237. Follow him on X @raystern.

This article originally appeared on Arizona Republic: How Arizona got its arbitrarily numbered groundwater rule