Does Arizona have enough water? Why that’s such a tough question to answer

Maricopa County is the fastest growing in the nation. The allure of the Southwest remains strong, and demographers predict that the state’s population will climb from 6.8 million in 2017 to 8.2 million by 2030.

Finding enough water will challenge water managers.

Warmer temperatures mean more rain – and ironically, less water to store

The Earth is getting warmer. 2014 was the hottest on record – until 2015, which was the hottest until 2016 and 2017, which was even hotter. Climate change has already started to alter Arizona’s weather.

Climate scientists predict lower precipitation levels for the Southwest. Warmer temperatures mean that more of the precipitation will come in the form of rain rather than snow.

This shift may seem trivial. But consider this: Where does Arizona store its water? Surely, this is a trick question, as the answer is obviously in the lakes behind the dams on the Salt, Gila and Colorado rivers. That answer is correct, but incomplete.

Most water is stored as snowpack in the White Mountains and the Rocky Mountains. Warmer temperatures mean more rain, less snow, more runoff in the shoulder seasons and, most importantly, more evaporation, which will reduce the amount of water available for cities, industry and farmers.

Arizona has dammed and pumped just about every source it can find

Arizona’s water supply principally relies on three sources:

• Surface water diverted from the Salt, Verde and Upper Gila rivers;
• Groundwater pumped from wells around the state; and
• Colorado River water delivered to Maricopa, Pinal and Pima counties through the Central Arizona Project canal.

Virtually all the surface water is committed to existing users under the prior appropriation doctrine. Groundwater supplies have stabilized in the areas of the state governed by the Arizona Groundwater Management Act, though even in Active Management Areas, excessive groundwater pumping causes sinking of the land.

Outside AMAs, the so-called “reasonable use” doctrine governs. An oxymoron, this doctrine allows for the drilling of new wells with essentially no limits on pumping, setting the stage for uncontrolled pumping and plummeting water tables.

In Arizona, when water suppliers have needed more water, business-as-usual has meant increasing diversions from rivers, drilling new wells and/or building new dams.

In 2018, surface water is already committed, and groundwater pumping that lowers the water table is, by definition, unsustainable. As for new dams, the major rivers have already been dammed, most repeatedly. It’s possible a couple of small dams may yet be built on small tributaries.

But that will not significantly add to the state’s water supplies. In short, business-as-usual is not a viable option.

Central Arizona Project secured our water future – and also condemned it

Perhaps the largest project to augment Arizona’s water supply is the Central Arizona Project, a 330-mile canal that delivers Colorado River water from Parker, Ariz., across the desert to Phoenix, south to farmers in Pinal County and on to Tucson.

At a cost of more than $4 billion, paid mostly by the federal government, the CAP represents the ultimate in trans-basin diversions, as it moves water 2,900 feet up in elevation. Generations of elected officials in Arizona, starting with Sen. Carl Hayden in the 1920s, dreamed of a water project to fuel Central Arizona’s growth.

That dream turned to reality in 1968 when Congress enacted the Colorado River Basin Project Act, which not only authorized the CAP but also appropriated funds to build it. But the funding came with a hitch, at the insistence of California’s congressional delegation.

In a court concession, Arizona agreed to the lowest priority water

California was still smarting over the victory Arizona obtained from the U.S. Supreme Court in Arizona v. California in 1963. The court held that in the Boulder Canyon Project Act of 1928, Congress divided the Lower Colorado River Basin’s water to give California 4.4 million acre feet (MAF), Nevada, 0.3 MAF, and Arizona 2.8 MAF. (An acre-foot is about 325,000 gallons.)

The act also reserved to Arizona the exclusive beneficial use of the Gila River. The problem for California is that, between 1928 and 1963, the state entered contracts with the Secretary of the Interior to divert 5.3 MAF. Arizona v. California essentially turned the clock back and required California to stop diverting about 900,000 acre-feet of Colorado River water.

In exchange for its congressional delegation’s support for funding the CAP, California insisted that it receive the highest priority in the event of a shortage on the river.

The second priority was for Nevada’s 0.3 MAF, as well as Arizona’s 1.4 MAF for uses along the river. The lowest priority went to Arizona’s 1.4 MAF for the CAP.

So, now we have a math problem: More water rights than actual water

This concession didn’t trouble Arizona at the time because it was believed that the Colorado River annually carried 18 or 20 MAF. But subsequent tree-ring studies at the University of Arizona determined that the mean annual flow is only 14 MAF.

This creates a huge problem of math for Arizona. The Colorado River Compact of 1922 allocated 7.5 MAF to the Upper Basin and another 7.5 to the Lower Basin. The 1944 Mexican – United States Treaty allocated 1.5 MAF to Mexico.

That totals 16.5 MAF.

And things get worse for Arizona.

Evaporation losses off Lakes Mead and Powell annually amount to about 1.6 MAF, and climate change is predicted to lower Colorado River flows by 9 to 25 percent. In short, paper rights to water annually exceed wet water in the river by millions of acre-feet.

If water rights holders take their full allocation every year, Lakes Mead and Powell will eventually dry up.