How Water Curtailment Works: Priority Calls, Drought Stages, and What Your Right Actually Delivers

On a stream administered under prior appropriation, a senior rightSenior rightA water right with an early priority date, served first in shortage. Senior rights are the premium asset in every western water market. holder who is not receiving their decreed water places a CallCall (river call)A senior right holder’s demand that upstream junior rights stop diverting until the senior right is satisfied. The mechanism that turns priority dates into real water.. The state engineer or watermaster then curtails diversions upstream in reverse order of seniority, newest first, until the senior right is satisfied. The curtailment date is the priority date of the most junior right still allowed to divert.

That single date is the whole story for an owner. In Idaho’s 2026 delivery call, the Director curtailed Eastern Snake Plain groundwater rights junior to October 11, 1900, against a predicted shortfall of 181,600 acre feet. A right with an 1899 priority was untouched. A 1901 right was shut off unless it sat under an approved mitigation plan.

Some systems, especially managed aquifers, do not curtail by date. They curtail by stage: an index reading crosses a threshold and every permit holder takes the same percentage reduction. The Edwards Aquifer Authority is the clearest example. Stage 2 for the San Antonio Pool is triggered when the J-17 index well’s ten-day average falls below 650 feet and imposes a 30 percent reduction; Stage 3 below 640 feet imposes 35 percent; Stage 5 below 625 feet imposes 44 percent.

Two mechanics matter for pricing. Tightening is asymmetric: any single indicator crossing its threshold moves the pool to a more restrictive stage, but relaxing requires every indicator to recover. And reductions are prorated by the number of days a stage is in force, so a lessee’s delivered volume is a blended number across the year, not a flat annual haircut.

A lease transfers delivered water, and curtailment reduces delivered water, so every lease needs a curtailment clause. The clause should say who bears the shortfall, whether payment abates, whether makeup water is owed, and how the reduction is measured (by date on a stream, by stage on an aquifer).

The pricing consequence is direct. A San Antonio Pool permit for 100 acre feet delivered 70 acre feet during Stage 2 in August 2026 and would deliver 65 under Stage 3. A lessee who paid for 100 was overpaying by the reduction. Owners who understand this negotiate rate per delivered acre foot, or a base rate with a stage adjustment, instead of a flat number on the permit volume.

Curtailment orders almost always exempt juniors who have arranged to make the seniors whole. In Idaho, groundwater districts operating under the 2024 Stipulated Mitigation Plan, with its 205,000 acre foot annual conservation obligation, received safe harbor from the 2026 call; the ~930 rights curtailed were the ones not under a plan. In Colorado, a junior well operates under an augmentation planAugmentation planA Colorado court-approved plan that replaces the depletions of an out-of-priority use with other water, keeping senior rights whole. that replaces its depletions with other water, often leased.

That is why curtailment creates a lease market. Every mitigation and augmentation plan is denominated in acre feet of water someone has to supply, and the suppliers are senior right holders and storage owners. The 2026 Water District 1 ledger shows leases of 40,000 and 22,000 acre feet into Idaho mitigation alone.

Three questions, in order: