Rules for groundwater users
IDWR finds 181,600 acre-foot shortfall of water in April 2026 Methodology Order; Junior groundwater users encouraged to join mitigation plans, if they haven’t done so already, to avoid curtailment
In the ongoing Surface Water Coalition’s conjunctive administration delivery call, Director Mathew Weaver of the Idaho Department of Water Resources (IDWR) has found that senior Snake River surface water users may face a shortfall of 181,600 acre-feet of water in the 2026 irrigation season in an order signed today and sent to water users.
The Methodology Order is the court-approved process IDWR uses to evaluate water supply conditions, aquifer conditions, and irrigation demand. From that calculation, IDWR determines the impacts or injury to surface water users with senior water rights caused by junior water users pumping from the Eastern Snake Plain Aquifer (ESPA).
Under Idaho water law, surface water users with senior water rights have priority over water users with junior rights on the Snake River and the ESPA. The rule of law is “first in time, first in right.” On the Snake River, IDWR manages both surface and groundwater resources together as one whole, or “conjunctively,” in calculating impacts each year.
The Director’s April order means that certain groundwater users who draw water from the ESPA with a priority date junior to Oct. 11, 1900, must be covered by an IDWR-approved mitigation plan by May 1, 2026 or they could face curtailment. This includes groundwater users in the ESPA Area of Common Groundwater Supply (ACGWS), as well as portions of the Big and Little Lost River Basins, which were added to the ACGWS in 2024 upon passage of Senate Bill 1341 and phased into administration on November 1, 2025 (see map on page 2).
All groundwater users who participate in an approved mitigation plan, including the 2024 Water Settlement, will not face curtailment so long as they are in compliance with their plan. In addition, the A&B Irrigation District, Coalition of Cities, Southwest Irrigation District, and the Water Mitigation Coalition have approved mitigation plans.
The Director’s April 2026 Methodology Order found that that 181,600 shortfall would occur to the American Falls Reservoir District No. 2 (43,900 a.f.), and the Twin Falls Canal Co. (137,700 a.f.).
The shortfall estimate is based on a water flow forecast for the Snake River and ESPA discharge conditions. The joint forecast issued by the U.S. Bureau of Reclamation and the U.S. Army Corps of Engineers on April 2, 2026, predicted an unregulated inflow of 2,300,000 acre-feet on the Snake River at the Heise gage for the period of April-July 2026. The forecasted flow volume equates to 70 percent of average. IDWR applied the latest groundwater model for the ESPA, ESPAM 2.2, the best available scientific tool, to determine which junior groundwater users could be affected by the Director’s order.
