Explosive population growth will bring challenges to metro Phoenix’s water future

Editor’s Note: This is the first of a five-part series called “KTAR Water Watch,” which will explore the present and future of the water supply across Arizona and metro Phoenix.

PHOENIX — The images are shocking. Areas submerged for decades are now islands.

Lake Mead and Lake Powell, two reservoirs Arizona relies on for its water, are historically low, with each about a third full.

Arizona State University climatologist Randy Cerveny says in addition to climate change, it’s explosive population growth in Arizona and other states that rely on water from the Colorado River via Lake Mead and Lake Powell that will present challenges in the future in the Valley.

Cerveny says most of the water that fills the Colorado River is due to melting snowpack in Colorado. The state is going through a historically dry period and the Rockies haven’t experienced consistent, above-normal precipitation.

“The river is so severely allocated that just south of Yuma, every remaining drop is sent to California,” Cerveny said. “It doesn’t reach the Gulf of California anymore. Even if there were to be more extra water that fell on the Colorado Rockies and into the Colorado River, it has already been allocated.

“We will never see those lakes ever go back to the days prior to the drought that we’re in right now.”

The Valley gets about 30% of its water from the Colorado River via the Central Arizona Project canal.

Salt River Project provides about half of the water for the Phoenix metropolitan area — approximately 2.5 million customers — from its chain of reservoirs on the Salt River and Verde River. The rest of the Valley water supply comes from groundwater.

The region’s ongoing population and business growth could strain the overall system, but Cerveny says the water sources won’t all dry up at the same time.

But is the business growth sustainable given the available sources?

“It will depend on which community they’re going to be affiliated with because different organizations will have to use different sources of water,” Cerveny said. “SRP is pretty picky about who they allow to have access to their water. There are other sources that charge a lot for their particular water.”

Storage in SRP’s reservoirs has dropped very little since 1996 thanks in part to some recent wet winters in 2017, 2019 and 2020.

The winters of 2005 and 2010 were also solid, while last summer’s powerful monsoon also added water to the system.

SRP’s chain of six reservoirs on the Salt River and Verde River provide about half of the Valley water supply and collectively are at about 70% capacity.

Central Arizona Project saw the first and so far only water cuts because of the declining reservoir levels, but the reduction only impacts agriculture at this time.

Bo Svoma, staff scientist and meteorologist with SRP, says the Colorado River basin stream flows into Lake Powell are very sensitive to warming, whereas the Salt and Verde rivers are about five times less sensitive to warming.

“What warming does is it increases the evaporative losses from the landscape and results in less water for stream flow,” Svoma said. “The Colorado River is more sensitive to that than the Salt and Verde.

“That one reason is why things aren’t as dire on the Salt and Verde.”

Even with the Valley’s explosive population growth since the mid 1990s, Svoma says demand has actually gone down.

“SRP has been delivering less water because of better efficiencies and water use and people being more conservation minded with their water use, which is really important,” Svoma said.

“That brings SRP into a perfect balance with the long term median inflow into the system is equal to our water deliveries, which is not the case on the Colorado River, which is over-allocated.”

Svoma doesn’t see Lakes Mead and Powell dropping to the point that they can no longer supply water to the Valley.

“I think it will be challenging to address the over allocation on the Colorado River, but people are working on that,” Svoma said.

“What happens with future precipitation is very uncertain, but from a western U.S. perspective the climate model projections are leaning toward increased winter precipitation for the Colorado River is a positive.”

Will the water situation impact the ability to lure business, specifically semiconductor outfits? Svoma says yes.

“Certainty in the water supply is attractive to businesses and there are several current projects that could increase the resiliency and sustainability of Arizona’s water supply,” Svoma said.

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