Who Owns the Water That Doesn't Exist?

What Does Shortage Actually Look Like for Each Right Holder?

The Colorado River Compact of 1922 divided the basin into Upper and Lower Basins, allocating 7.5 MAF/yr to each. A subsequent 1944 treaty allocated 1.5 MAF/yr to Mexico. Total paper allocation: 16.5 MAF/yr. The 20th-century mean flow used to calibrate these allocations was approximately 16.4 MAF/yr — but that century included an anomalously wet period. The current best estimate of natural mean flow is 12.4 MAF/yr. The basin is 4.1 MAF over-allocated on paper relative to what the river actually delivers.

This investigation models what happens when the 4.1 MAF gap collides with the doctrine of prior appropriation. Priority date determines who gets cut first. Junior rights (many of Arizona's Central Arizona Project municipal deliveries, established post-1968) get curtailed before senior rights (California's Imperial Irrigation District, dating to pre-1900). The economic perversity: high-value municipal water serving millions of people loses access before low-value agricultural irrigation of alfalfa.

Price to Secure One Acre-Foot

The supply cost curve below ranks conservation and fallowing options by cost per acre-foot, stacked to show cumulative volume available. The shape of this curve determines how expensive different curtailment levels are. The first 0.4 MAF of supply (CAP agricultural fallowing in Arizona) is cheap. The next 3+ MAF escalates steeply. The curve reveals why water markets produce wildly different prices depending on where you are on the stack.

Strict Priority vs. Pro-Rata: $1.7B Premium

At a 2 MAF curtailment level, strict prior appropriation concentrates the entire cut on junior rightholders. For the Lower Basin, this means Arizona's CAP takes the full burden first. The agricultural sector there is less senior than California's Imperial Valley — so Arizona's cities bear the brunt before IID loses a single acre-foot of alfalfa water.

A pro-rata allocation (each right holder loses the same percentage of their entitlement) spreads the pain more evenly. The economic damage is lower because pro-rata cuts high-value users proportionally less and lower-value users proportionally more relative to strict priority. The difference at 2 MAF curtailment: $6.1B under strict priority vs. $4.4B under pro-rata — a $1.7B premium for maintaining priority order.

1.498 MAF of Unexercised Tribal Rights

The Winters doctrine (1908) established that Native American tribes hold federally reserved water rights with a priority date predating state water law — in many cases, the oldest rights on the river. Twenty-two federally recognized tribes have water rights in the Colorado Basin, with adjudicated and asserted claims totaling an estimated 3.0–4.0 MAF/yr. Of the 2.593 MAF that has been formally quantified through settlements, approximately 1.095 MAF is currently exercised — leaving a 1.498 MAF gap of senior rights that have not yet been developed.

This is not a theoretical concern. Tribal water development is accelerating. The Navajo Nation, Gila River Indian Community, and Colorado River Indian Tribes have active infrastructure development underway. As tribal economies grow and water projects come online, the 1.498 MAF gap will close. Because tribal rights are senior, each MAF of new tribal development displaces a junior right elsewhere in the system.

Phoenix Gets Cut Before IID Loses a Drop

Prior appropriation isn't going anywhere — it's too deeply embedded and too many legitimate claims depend on it. What the analysis does is identify precisely where the framework concentrates harm: high-value municipal users bear cuts before low-value agricultural users, because seniority date trumps economic value. Any voluntary alternative — market transactions, fallowing programs, interstate compacts — has to account for this inversion explicitly or it won't work.

The 1.498 MAF tribal sleeping giant adds a second layer: as senior tribal rights are exercised, they further constrain the junior supply that CAP and similar systems depend on, compounding the economic impact identified here.

What This Analysis Does Not Resolve

Supply cost curve uncertainty: The cost-per-AF estimates use midpoint values from a range of sources. Actual fallowing costs depend on crop type, lease duration, and market conditions. The step curve should be read as a structural picture, not a precise price schedule.

Pro-rata is not legal: The pro-rata scenario is a normative comparison, not an operationally available option. Implementing pro-rata curtailment would require renegotiating Compact obligations or federal legislation. The damage differential is a measure of the cost of prior appropriation relative to an alternative framework, not a policy recommendation that can be implemented administratively.

Tribal rights adjudication is ongoing: The 3.0–4.0 MAF total tribal claim range reflects partially adjudicated claims. Some tribal rights have been quantified through settlement; others remain in litigation. The 1.498 MAF unexercised gap carries significant uncertainty in both direction and timing.

Interact with This Data

Move the curtailment slider and toggle between Strict Prior Appropriation and Pro-Rata. Watch the priority waterfall fill in and the $1.7B efficiency premium appear in real time.