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Morrison Institute report

Water battle brewing in Az's 'Sun Corridor'

State's population centers face decisions on limited water

Arizona’s water future will depend more on how we manage the dwindling resource than on how much we have, says the lead author of a report from Arizona State University.

The state’s age-old struggle among agriculture, mines and cities is likely to heat up in coming decades as global warming saps an estimated 15 percent of our water supply and growth further stresses our metro areas.

“It grows every year and it‘s still a ways off, but there is an impending need to sort out those competing uses,” said Grady Gammage, Jr. of the Morrison Institute for Public Policy.

'Do you want growth, or do you want a lifestyle?'

The institute’s report - Watering the Sun Corridor - takes a look at the Tucson-Phoenix water situation in light of some new research and new ways to measure demand.

The intricate and sometimes delicate system - both physical and political - by which we get our water will be tested as the Arizona’s population centers continue to grow into an economically (but probably not physically) linked “megapolitan area” stretching from Tucson to the Phoenix valley.

The state will continue to shift from agriculture to urban use, the report said.

About half of the water in the Tucson-Phoenix corridor goes to crops. In Maricopa County, it’s just less than half, and in Pima County it’s about a third. But in Pinal County, where groundwater pumping has depleted some portions of the aquifer and caused the ground to sink 10 feet or more, it’s 96 percent.

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Native American tribes - most notably the Gila River Indian Community - have said they plan to use much of their share of Colorado River water to expand farming.

“There are going to be pressures to put more of that water to urban uses,” Gammage said.

The Morrison Institute questions the wisdom of stripping away funding and staff at the state Department of Water Resources at a time when the importance of water planning is rising.

The state has long served as an example for water planners across the nation. The 1980 Groundwater Management Act, which created the Active Management Areas to guide the flow of water across political boundaries, has served to soften the political head-butting that is a hallmark of Western water negotiation.

The Department of Water Resources was created to manage the use of our limited and shrinking supply. But since 2008, the state has cut more than 70 percent of the department’s budget. The department’s staff shrunk from almost 300 to fewer than 100.

“Shrinking ADWR and potentially jeopardizing our history of careful water management do not seem the best way to celebrate Arizona’s centennial,” the report said.

The report teases out a few details of water use that other demand estimates have traditionally not examined, Gammage said.

The traditional measure of demand for a community is expressed in gallons per capita per day, or GPCD. But those figures do not include significant urban uses, including factories, dairies (which use a lot of water), golf courses using wells, mines, gravel pits and quarries.

“It understates the demand,” Grammage said.

The corridor uses a combined 3 million acre-feet of water each year - a number the report warns is not sustainable. A combined water “budget” of 2.2 million acre-feet would be a sustainable average use, based on supply and population projections, the report said.

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The report asserts that ultimately, assuming that commercial agriculture is replaced by urban growth and a freeze on such uses as golf courses, mines and other non-residential urban uses, the Tucson-Phoenix corridor will have enough water to support about 8 million people. Other estimates have concluded the region could support 10-15 million.

The report does not examine how the water would be distributed among the players in the three counties of the Sun Corridor - it recognizes only that the Sun Corridor will be an inextricably linked economic unit. 

In the end, residents of Arizona’s population centers will have to collectively decide whether they want to continue along the current water-use path, which would eventually stifle growth, or continue to grow and stave off water use.

“Do you want growth, or do you want a lifestyle?” Gammage asked.

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10 things residents of the Sun Corridor should understand about water

Recent national media reports echo a number of popular misconceptions about Arizona's water and water future. In brief, here are 10 things every Sun Corridor resident should understand:

  1. Rainfall in the Sun Corridor has little to do with water supply. Water is brought to this desert from the mountains, where it rains and snows a lot more. Rainfall does directly impact demand for water use for landscaping.
  2. The renewable water supplies to the Sun Corridor provide "on average" 2.5-3 million acre feet (an acre foot is 325,851 gallons) of water which could theoretically support a population of 8-10 million people. But "average" in the context of water supply does not mean "reliable." Water supply in an arid region is highly variable, which is why water management has been so important.
  3. The Sun Corridor's plumbing systems include reservoirs in Arizona, bigger reservoirs on the Colorado River and groundwater banking. Together, these can typically store 4 to 5 years' worth of urban Arizona's water demands.
  4. Climate change will probably increase variability of supply, and may reduce the "average" number by as much as 15%. One bright spot is that our watering systems are designed to handle high variability.
  5. More than half of Sun Corridor water is still used to grow crops. Agricultural use has provided a buffer during droughts, when water for farming can be cut back to protect urban use.
  6. Groundwater is subject to far more regulation in urban Arizona than in most states. We have purposefully put significant amounts of water back underground for the last decade. Even so, the long-term goal of "safe yield" is a challenge to achieve and sustain.
  7. Per capita use of water has been declining since the 1980s. The Phoenix area uses much more water for landscaping than Tucson. This reflects historical and climate differences in the two cities. But both urban areas have been consistently reducing consumption.
  8. Reuse of urban water will be an important means of stretching water supplies in the future. Cities in the metro Phoenix area are among world leaders in reusing effluent, both for landscaping and for cooling water at the Palo Verde Generating Station. 9. 2.4 million acre feet of average annual water supply appears to be a reasonable estimate for planning. At the current rates of consumption, 2.4 million acre feet of annual water could support about 9.5 million residents in the Sun Corridor. That level includes no commercial agriculture.
  9. The Sun Corridor won't run out of water, but it faces serious challenges about how to strike the right balance between population growth and lifestyle.

—from 'Watering the Sun Corridor'