Valley dwellers of 500 years ago were so sophisticated they built irrigation canals now said to rival the best in Renaissance Europe, developing a far-flung agricultural and trading society unmatched in the Southwest. In a short period, these "Hohokam" people vanished. Why?

The best guess is that the water ran out: A conclusion that has haunted Arizonans for more than a century. What if we faced such a drought?

Now, for the first time, scientific models of how much water the Valley could secure in such a 500-year drought show Phoenix and its growing neighbors could flourish, even doubling their growth on their supply. And those models led the Central Arizona Project board to sell water to thirsty Las Vegas, Nev.

The models that drove that decision show that metro Phoenix has enough available water to support between 5 and 6 million people.

But those predictions come with a big warning.

The models also show that even that water supply may not be enough to avoid serious problems farther down the road, if planning doesn't start now.

 Grady Gammage Jr., who recently left the CAP board, was one of the strongest advocates on the 15-member board for the proposal to sell Nevada 1.25 million acre feet of water. He said that the Phoenix water supply is secure for now. 

Gammage said CAP's new scientific modeling bolstered his confidence in the short-term future of Phoenix. 

The new models used tree ring data from the 16th century, a first for these types of models that help planners base future possibilities on past realities. With this new information, planners can consider what would happen if history repeated itself and Arizona experienced a drought similar in magnitude to the one that drove the Hohokam people out of the valley almost half a millennium ago.

The models show that even under worst-case scenarios, the Phoenix metro area has plenty of water to support almost twice its current population, as much as 5 or 6 million people without problems, Gammage said. 

 "Phoenix is in a better position than any other arid western city to deal with water.  There are very few cities in the United States that can point to the water supply that will allow them to double their population. Phoenix can do that," Gammage said. 

He said in the short run the benefits of helping out the people of Nevada and gaining a future ally against California outweighed the costs. 

California currently has senior priority to Colorado River water. If a drought reduces the flow of the Colorado, California will continue to take its full share, leaving Arizona to bear the loss. Gammage says that by gaining Nevada as an ally, Arizona may be able to renegotiate its secondary status, and gain insurance against future drought.

Joe Feller, who teaches water law at ASU, doubts that Nevada's help will do much good.

 "My guess is that Nevada's support isn't going to make much difference," he said.  "We can't change the priority scheme unless California goes along,"

The CAP models also show that if the Phoenix area continues growing at its current rate, water could become a problem in the not so distant future, said Gammage. 

Between 1990 and 2000, the Valley population grew over 45 percent.  If that rate of growth continues, the population of the Metro area will be well over 8 million people by the year 2025. Gammage said that a population of that size could force tough choices for Valley residents. 

"If you don't carefully plan how you're going to use water supply for future growth and you go on growing at the rate we've been growing in the way we've been growing, you wind up doing things like Las Vegas is doing," he said. "I don't want to see us do that without at least debating."

Over the last few years, Las Vegas has passed a host of water conservation laws, including restrictions against washing cars except in designated car washes and incentives for residents to tear up lawns.

 Advocates for allowing development to continue point to the immense economic benefit, with as much as a third of all money flowing through the metropolitan economy created by the housing industry.

Spencer Kamps, vice-president of legislative affairs for the Arizona Homebuilders association said that concern about growth is overblown. 

"We think there is plenty of water out there," Kamp said. "First of all, Arizona requires that all new developments secure a 100-year supply of water. California and Colorado both have 25-year requirements. So 100 years is a pretty extreme length of time."

 He also said that development in the west Valley is actually increasing, not decreasing, the available water supply locally. 

"Agriculture uses more water per acre than homes.  So anytime we retire (agricultural) property and develop it into homes we are reducing the water load on a per acre basis. Specifically in the west Valley, the bulk of the development is going in on former agricultural property." 

 "Because of that, we are starting to have significant water issues, not in terms of lack of water, but in terms of too much. The west valley water table is actually rising." 

Mike Pasqualetti, ASU geography professor and member of the ASU Operations Sustainability Committee, said whether or not these new developments cut into the valley's water supply, relying on development weakens the metro area. 

"The city can no longer operate like a shark," he said. "The shark has to move to stay alive, never stops moving, and the city, the way we've been operating around here, can't stop growing.  Growth is becoming a part of the economic structures of these communities, and if you want to see what happens when growth stops, go take a look at Flint, Michigan or Detroit or Cleveland." 

Pasqualetti said he sees conservation practices as important to buy Phoenix time to deal with the larger issues. 

"Most communities have a rebate system for those people who change to desert landscaping," he said. "Most of the communities around here have also recommended not planting winter lawns. The golf courses that get approved have to use secondarily treated water, or waste water. So we are doing those things, but there's a lot more that needs to be done and can be done."

Gammage said that the focus should shift from encouraging water conservation to the real issue, slowing Phoenix's explosive expansion.

"When you tell the citizenry that we are in such a severe drought that they can't run the faucet when they are brushing their teeth and then you headline in the paper the next day that you are approving 300,000 new lots on the west side of Phoenix and 5 golf courses, they get pissed off," he said. "There's no problem with you running the faucet.  It is a drop in the bucket, literally.

"What we ought to do is explain to them the big picture of our water supply, and we ought not to be afraid to have a debate about how much are you willing to compromise your lifestyle."

One influence he singled out as a cause of the area's continued explosive growth is an amendment to the Groundwater Management Act. 

The Groundwater Management Act states that new subdivisions must demonstrate that they have a hundred year supply of renewable water before being approved.  Originally this meant that new subdivisions had to purchase water from the CAP or a city unless another renewable source was readily available, Gammage said.  He said the amendment modified these rules to allow subdivisions to use water banked by the Groundwater Replenishment District.

Gammage said that many of the new developments in and around the city of Buckeye have relied on the GRD for water. 

"This mechanism was created, we thought for the exception, in terms of subdivisions. We thought it might have 20,000 or 30,000 lots that would be entitled.  Today it has more like 175,000 lots and it has another 500,000 lots in the pipeline." 

He said that he doesn't advocate doing away with the system altogether, but he does think that new developments should be held to higher standards of water conservation than established areas.