Arizona water law primer, and the hole in the middle of it

Why this matters. Arizona regulates the same physical water 2 completely different ways depending on whether a court calls it “surface water” or “groundwater.” The line between those 2 categories is a legal fiction that the Arizona Supreme Court has admitted rests on a hydrological misconception, and it is the reason a Saudi dairy company could pump for free.

Everything in the export story flows from this one structural fact. Read this before the mechanism doc.

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1. The split system

Arizona runs 2 separate bodies of water law.

Surface water follows prior appropriationprior appropriationThe western US rule. Whoever first took water and put it to use has the strongest claim, no matter where their land sits. The shorthand is "first in time, first in right.": first in time, first in right. You get a right by putting water to beneficial usebeneficial useActually using water for something productive: farming, drinking, industry, and in most western states now, keeping fish alive. In the West it is what earns you a water right and what caps how much you get., and your priority datepriority dateThe date your water right got in line. In a western drought, older dates get their full amount and newer dates get nothing. It is the single most valuable number in a western water deal. determines who gets cut first in a shortage. This covers rivers, streams, and their “subflowsubflowGroundwater that is really part of a nearby river. Whether a well is pumping subflow or true groundwater decides which set of Arizona rules applies, and the fight over it has run for decades.” (defined below). See ../00-primer/02-surface-water.md.

Groundwater follows a statutory code enacted in 1980, layered on top of a common law “reasonable usereasonable useYou can use the water however you like until it unreasonably harms someone else's use. This is Wisconsin's groundwater rule. The catch: it is enforced by a lawsuit after the harm, not by a permit beforehand.” rule. Groundwater is not appropriable. It belongs, in a limited usufructuaryusufructuaryA right to use something you do not own. Your water right lets you take and use water. The water itself belongs to the public or the state. sense, to whoever overlies it and pumps it.

The categories are legal, not hydrological. Pumping a well 500 feet from a river depletes that river. Arizona law mostly pretends otherwise.

Subflow, and the 50 year adjudication

“SubflowsubflowGroundwater that is really part of a nearby river. Whether a well is pumping subflow or true groundwater decides which set of Arizona rules applies, and the fight over it has run for decades.” is the narrow band of water moving through the saturated alluvium immediately beneath and beside a stream. Arizona treats subflow as surface water subject to prior appropriationprior appropriationThe western US rule. Whoever first took water and put it to use has the strongest claim, no matter where their land sits. The shorthand is "first in time, first in right.", and treats everything else beneath the ground as groundwater.

The Arizona Supreme Court has reaffirmed this narrow concept repeatedly, most importantly in Gila II (1993) and Gila IV (2000), the latter of which created a “subflow zone” test for whether a given well is pumping subflow. The Ninth Circuit summarized the doctrine in 2025 and noted the court retained it “even though it rests on a hydrological misconception” (9th Cir. opinion 23-2743, July 24, 2025).

Sorting out who owns what is the job of the Gila River General Stream AdjudicationadjudicationA court proceeding that sorts out every water claim on a river system at once. Arizona's has been running since 1974 and still is not finished., filed in the 1970s, consolidated into Maricopa County Superior Court in November 1981 as cases W-1 through W-4. Over 89,600 statements of claimant have been filed in the Gila adjudicationadjudicationA court proceeding that sorts out every water claim on a river system at once. Arizona's has been running since 1974 and still is not finished. alone (ADWR Adjudications).

It is still running. The University of Arizona Water Resources Research Center marked 51 years of adjudication litigation in 2025 (WRRC). As of July 2026 there is no final decreedecreeThe court order at the end of an adjudication, spelling out exactly who gets how much water and in what order., subflow zone delineation continues under a Special Master, and ADWR is still building groundwater flow models for contested watersheds.

For a buyer, the lesson is that 50 years of litigation has not produced a definitive answer to “whose water is this.” Assume the same ambiguity exists wherever surface and ground water meet.

The pre-1980 groundwater rule

Before 1980, Arizona applied the “reasonable usereasonable useYou can use the water however you like until it unreasonably harms someone else's use. This is Wisconsin's groundwater rule. The catch: it is enforced by a lawsuit after the harm, not by a permit beforehand.” doctrine, sometimes called the American rule. A landowner could withdraw as much groundwater as desired in conjunction with reasonable use of the overlying land, regardless of the effect on neighbors (Ariz. L. Rev. 49:321).

The one real limit was transportation. In Farmers Investment Co. v. Bettwy (“FICO”), 113 Ariz. 230 (1976), the Arizona Supreme Court held that water may not be pumped from one parcel and transported to another just because both overlie a common source, if the plaintiff’s land or wells suffer injury (Justia).

FICO enjoined mining companies and the City of Tucson. It threatened the mining economy and municipal water delivery at the same time, and that is what forced the 1980 Act.

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2. The 1980 Groundwater Management Act

What triggered it

Two pressures converged.

Legal pressure came from FICO, which broke the water supply arrangements of cities and mines simultaneously and created what one contemporaneous account called enormous controversy.

Federal pressure came from the Central Arizona ProjectCentral Arizona ProjectThe 336 mile canal carrying Colorado River water to Phoenix and Tucson. Arizona agreed to stand last in line for that water, so this is what gets cut first in a shortage.. The CAPCentral Arizona ProjectThe 336 mile canal carrying Colorado River water to Phoenix and Tucson. Arizona agreed to stand last in line for that water, so this is what gets cut first in a shortage. is the 336 mile canal that carries Colorado River water from Lake Havasu to Phoenix and Tucson. The authorizing legislation contained language about Arizona controlling its groundwater overdraft, and Interior Secretary Cecil Andrus threatened CAP funding and non-Indian CAP allocations to force the issue (Univ. of Colorado law archive).

Governor Bruce Babbitt later admitted he engineered that threat. In a 2023 interview he described telling Andrus: “Cec, you have to threaten to cancel the Central Arizona Project in order to motivate people to get going. Of course, I will deny that I ever asked you to do it, and I will condemn you for over-arching federal interference” (Arizona Daily Star).

Statewide overdraft at the time ran about 2.2 million acre-feetacre-footEnough water to cover 1 acre a foot deep: 325,851 gallons. Roughly what 2 or 3 suburban households use in a year. per year, close to half of consumption. Note the mechanism: Arizona’s landmark groundwater law passed because a governor borrowed a federal threat and aimed it at his own legislature. That pattern repeats in the Fondomonte story.

What it created

The original 4 AMAsActive Management AreaAn Arizona zone where groundwater pumping is actually regulated: metered, reported, and capped. Outside these zones there historically were no limits at all. were Phoenix, Pinal, Prescott and Tucson. Santa Cruz split off from Tucson in the 1990s, making 5. Douglas was added in 2022, Willcox in December 2024, and Ranegras Plain in January 2026, for 8 as of July 2026 (ASU AZ Water Innovation Initiative, ADWR press release Jan 12, 2026).

Two INAsIrrigation Non-Expansion AreaA lighter Arizona designation that freezes how much land can be irrigated without imposing full pumping regulation. came with the code (Joseph City and Douglas), and ADWR added the Harquahala INAIrrigation Non-Expansion AreaA lighter Arizona designation that freezes how much land can be irrigated without imposing full pumping regulation. in 1982. Douglas INA became the Douglas AMAActive Management AreaAn Arizona zone where groundwater pumping is actually regulated: metered, reported, and capped. Outside these zones there historically were no limits at all. in 2022.

The Act survived a takings challenge in Town of Chino Valley v. City of Prescott, where the Arizona Supreme Court held that landowners never owned the groundwater molecules, so regulating withdrawal took nothing from them.

The threshold that matters most

An “exempt wellexempt wellA well small enough to skip the permit process. Where a state draws that line is where most of the unregulated pumping happens.” is a well with a pump capacity of 35 gallons per minute or less, A.R.S. § 45-402(8) (statute). Exempt wellsexempt wellA well small enough to skip the permit process. Where a state draws that line is where most of the unregulated pumping happens. escape metering and reporting even inside AMAs.

“Irrigate” means applying water to 2 or more acres to produce plants for sale, human consumption, or livestock feed, A.R.S. § 45-402(18). Below 2 acres, you are not irrigating as a legal matter.

Inside an AMA, non-exempt wells may be drilled, deepened or replaced only with Director approval, subject to spacing rules meant to “prevent unreasonably increasing damage to surrounding land or to other users from the concentration of wells,” A.R.S. § 45-598(A) (ADWR, Willcox AMA).

Hold onto the 35 gpm number. Wisconsin has an equivalent threshold, set roughly twice as high. See 04-lessons-for-a-wisconsin-buyer.md.

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3. The critical gap

The GMA regulates inside AMAs and INAs. Outside them, the pre-1980 reasonable use rule survives, legislatively confirmed.

Estimates of how much of Arizona that leaves unregulated vary by source and by how many AMAs existed when the estimate was made:

So “roughly 80 percent” is a fair statement, and the honest range is 75 to 87 percent. The 3 AMAs added since 2022 moved the number by a few points, not by an order of magnitude.

The inverse is what makes the politics hard. Those same AMAs hold over 75 percent of Arizona’s population. The regulated minority of the land contains the voting majority of the people, and rural legislators have blocked pumping limits on the unregulated majority of the land for 45 years.

In the unregulated 80 percent, before 2026, a large agricultural operation faced:

• No permit to pump.
• No meter.
• No annual reporting of volume.
• No volumetric charge for the water itself.
• No cap tied to recharge.

The marginal cost of an acre-footacre-footEnough water to cover 1 acre a foot deep: 325,851 gallons. Roughly what 2 or 3 suburban households use in a year. was the electricity to lift it. That is the arbitrage.

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4. Transporting groundwater between basins

Arizona liberalized the FICO transportation rule as part of the 1980 compromise, then re-tightened it in 1991.

Outside AMAs today, under A.R.S. § 45-544 (statute):

• Within a sub-basin, or within a basin that has no sub-basins, groundwater moves freely with no damages owed.
• Between sub-basins of the same basin, groundwater moves subject to payment of damages, and injury is not presumed from the fact of transportation.
• Groundwater generally may not move away from a basin entirely, with carve-outs for mineral extraction and a few named basins.

The 1991 Groundwater Transportation Act added Article 8.1, which bars moving groundwater from outside an initial AMA into an initial AMA unless specifically authorized, A.R.S. § 45-551 (statute).

The statutory carve-outs, the “transportation basinsgroundwater transportation basinSpecific Arizona basins that the law allows water to be pumped out of and moved to a city. Butler Valley is one, which is why leasing it to a foreign hay grower caused an uproar.,” are:

Source: A.R.S. Title 45 index and ASU AZ Water Innovation Initiative.

Count carefully. The 1991 Act carved out 4 basins, and the Governor’s office in 2023 described Butler Valley as one of Arizona’s “5 water transportation basinsgroundwater transportation basinSpecific Arizona basins that the law allows water to be pumped out of and moved to a city. Butler Valley is one, which is why leasing it to a foreign hay grower caused an uproar.,” which reflects the later upper San Pedro addition (Governor’s office, Oct 2, 2023). Both counts appear in credible sources. Article 8.1 currently contains 5 transportation provisions.

Butler Valley matters to the Fondomonte story precisely because it is state-controlled groundwater reserve land. Arizona set it aside for Phoenix and Tucson to draw on someday, then leased the surface to an alfalfa operation that pumped the reserve for free.

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5. The Colorado River overlay

Groundwater is the loophole, and the reason Arizona cares so much about the loophole is that its surface water is shrinking.

Why Arizona is junior

The Boulder Canyon Project Act of 1928 authorized Hoover Dam and apportioned the Lower Basin: 4.4 million acre-feet per year to California, 2.8 to Arizona, 0.3 to Nevada. Arizona refused to ratify the Colorado River CompactcompactA binding agreement between states that Congress approves. Once approved it becomes federal law and overrides conflicting state rules. The Great Lakes Compact is one. for decades and litigated instead.

Arizona v. California, 373 U.S. 546 (1963), confirmed Arizona’s 2.8 million acre-feet and, critically, held that Arizona’s tributary use did not count against that apportionment.

The price came in 1968. To get California’s support for the Colorado River Basin Project Act authorizing the CAP, Arizona accepted that CAP water would be junior to California’s 4.4 million acre-feet in a shortage. Arizona’s biggest water project sits at the bottom of the priority stack.

The shortage machinery

The 2007 Interim Guidelines set shortage tiers based on projected January 1 elevation at Lake Mead. The 2019 Lower Basin Drought ContingencycontingencyA condition written into a purchase contract that lets you walk away if something checks out badly. Drafting these well costs an hour of attorney time and is the cheapest protection in the whole process. Plan (signed into federal law April 16, 2019) layered additional contributions on top. Both expire at the end of 2026.

Combined reductions to Arizona:

Source: CAP briefing materials, CSG West presentation.

Nearly all of it lands on CAP, in CAP’s internal priority order. The 2023 Tier 2a shortage of 592,000 acre-feet was about 34 percent of CAP’s normal supply, 21 percent of Arizona’s Colorado River supply, and 9 percent of Arizona’s total water use (CRWUA).

The first users cut are the CAP Excess and Agricultural pools, which is why Pinal County farmers went back to groundwater. Cutting the river pushes demand onto the aquiferaquiferUnderground rock or sand with water in the spaces between the grains. It is not an underground lake. Water seeps through it slowly, often only feet per year..

Where the post-2026 rules stand, as of July 2026

The 7 basin states have failed to agree. The Upper Basin (Colorado, Utah, Wyoming, New Mexico) and the Lower Basin (California, Arizona, Nevada) remain deadlocked after 5 years of negotiation, mainly over whether the Upper Basin accepts mandatory cuts (Water Desk, July 2026).

In May 2026, Reclamation announced it would proceed with its own plan. The preferred alternative is a 10 year framework under which Interior issues new operating guidelines every 2 years, with the first covering 2027 and 2028 (Colorado Springs Gazette, June 6, 2026).

Acting Reclamation Commissioner Scott Cameron said in June 2026 that the final environmental impact statement and record of decision would issue in mid to late summer 2026, and that Interior would gladly adopt a 7-state deal later if one materializes. The Lake Powell water year begins October 1 and Lake Mead’s begins in December.

Interior has signaled that the Secretary could cut Lower Basin deliveries by up to 3 million acre-feet, about 40 percent of the Lower Basin’s CompactcompactA binding agreement between states that Congress approves. Once approved it becomes federal law and overrides conflicting state rules. The Great Lakes Compact is one. apportionment. At a May 2026 briefing for Arizona water users, CAP head Brenda Burman presented modeling and noted that given diminished Powell releases, the Upper Basin is “in a definite breach of the Compact by Sept. 30 of 2026” (Water Desk, July 2026).

As of July 24, 2026, no record of decision has issued and no 7-state agreement exists. Treat every number in this section as live.

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6. What to carry forward

1. Arizona regulates the river hard and regulated most of the aquiferaquiferUnderground rock or sand with water in the spaces between the grains. It is not an underground lake. Water seeps through it slowly, often only feet per year. not at all. The gap is jurisdictional, not hydrological.
2. The unregulated share of Arizona’s land area is roughly 80 percent, and the credible range is 75 to 87 percent.
3. The binding threshold is the exempt well definition, 35 gpm, plus the 2 acre definition of irrigation.
4. Reform arrived in 1980 only when a governor imported a federal threat. It arrived again in 2024 to 2026 only through agency action, because the legislature would not act.
5. Colorado River cuts push demand onto groundwater. The 2 systems are connected through economics even where the law says they are not.

Next: 02-how-water-export-actually-works.md.