Cities across metro Phoenix are still fleshing out how deep Colorado River cuts will go — with good reason.

Much uncertainty remains about who will lose water and when.

But we can’t overlook an equally important question:

Where will Arizona’s most populous region turn for water once these cuts are made?

In the short term, there’s only one real source.

Groundwater.

Demand for water outstrips supply

Arizona State University’s Groundwater CuRVE Project simulated what might happen when cities across Maricopa County tap this finite source to fill recurring needs.

It paints a sobering picture.

The study found that demand for tap water radically outstrips cities’ available supply as Colorado River shortages deepen.

And that despite decades of saving extra supplies underground, there simply isn’t enough water to meet those demands for long.

“It’s not sustainable,” said Cynthia Campbell, a former ASU researcher who completed much of the modeling.

“Hopefully, we put a nail in the coffin of the idea that pumping groundwater is a solution.”

It’s a city’s worth of water, every year

The need for additional water increases exponentially as Colorado River shortages deepen, the study found.

That ranges from about 30,000 acre-feet a year under the lightest cuts that cities could face over the next decade to nearly 300,000 acre-feet a year under the deepest cuts.

The smaller amount is enough to satisfy Buckeye and Avondale’s tap water demands today, with water left to spare, while the larger amount is roughly what Phoenix sends to taps each year.

So, yeah, it’s a lot of water.

And it’ll have to come from somewhere.

Groundwater alone can’t fill gaps

When Pinal County irrigation districts lost their Colorado River water a few years ago, they resumed pumping. 

Yet most now deliver far less water than they once did. Wells simply don’t have the capacity to meet previous needs.

Farmers adapted by leaving some fields barren while reworking how and what they grow on those still in production.

Cities have less flexibility. 

It’s much harder to idle homes or deliver drastically less water to taps.

And there’s another problem:

The study found there aren’t enough aquifer locations with enough water to meet every need over 50 years — even under the rosier shortage scenarios.

Rules don’t allow this much pumping

There are a few important caveats:

The study did not put any regulatory constraints on pumping, even if the current rules would likely forbid most of the withdrawals it contemplates.

Nor did it consider where stored water might be recovered, how cities might move supplies among themselves or whether they are making alternate plans to avoid groundwater pumping.

Assumptions were purposefully kept simple to get a clearer look at cities’ supplies and demands — and what might happen if the full gap must be filled from the ground.

Maybe that’s not how it’ll play out in real life. 

But it’s still important to consider the consequences if it does.

Irrigation districts could be hit first

Groundwater Savings Facilities would likely feel the effects first.

Cities have sent extra Colorado River water to a handful of Phoenix-area irrigation districts for years.

The practice allows these districts to use surface water for agriculture instead of pumping the groundwater to which they are entitled.

It props up the aquifer while cities earn credit to recover most of that water later.

But the study found those savings could rapidly and markedly decrease, leaving irrigation districts with little choice but to resume pumping groundwater.

Less Colorado River water to go around also raises questions about how cities will meet their Assured Water Supply designations, which were largely built around the availability of this supply.

Those designations certify that Phoenix-area water providers have acquired enough renewable water to meet current and some future demands for 100 years.

Groundwater has traditionally not met that definition.

Long term, the aquifer declines

The study also found that most of the Phoenix Active Management Area would experience aquifer declines over 50 years — even at less severe shortage levels.

At the most severe shortage levels, about a quarter of the area would face declines greater than 200 feet. 

That outpaces the current estimated rate of decline in Willcox, one of the state’s most rapidly depleting groundwater basins.

Cities won’t have extra supplies to replenish the additional water they pump, considering that most stabilized past groundwater declines with Colorado River water.

And, over time, that could collapse the layers of sediment that store water, permanently shrinking the aquifer’s storage capacity. 

Heavy, rapid pumping also could cause the ground to sink and crack at the expense of roads, building foundations, irrigation canals and other critical infrastructure that lie on top of it.

We all have a stake in solutions

Stopping growth and cutting off water use for agriculture won’t solve these problems, even if they are commonly suggested solutions.

Both scenarios were incorporated into the model.

The deepest drawdowns still occurred.

If nothing else, the study underlines how badly Arizona needs new water supplies — and how quickly we must work together to get them.

After all, Colorado River shortages begin next year.

“We will have to draw down our aquifers in the short term,” said Kathryn Sorensen, who directs research at ASU’s Kyl Center for Water Policy and developed much of the data in the model.

“But we also can collaborate to cause the least amount of harm to the aquifers — and to distribute the benefits and costs of doing so fairly.”


Joanna Allhands writes about water, land use and other issues important to the Arizona Farm Bureau. Reach her at joannaallhands@azfb.org.