A decade of work by water users and the state to replenish the Eastern Snake Plain Aquifer reduced its decline, according to Idaho officials.

Reports to the Idaho Water Resource Board’s aquifer stabilization committee Aug. 8 showed the aquifer, which lies underneath much of the state’s southeast and south-central regions, had a net gain of 800,000 acre-feet of water from spring 2023 to spring 2024, and 500,000 since 2015.

The 800,000-acre-foot year-to-year gain represents 5% of the long-term decline — 14 million acre-feet since 1952, said Steve Stuebner, a board spokesman. “We are at the lowest aquifer levels since farming began in the early 1900s.”

However, 2.6 million acre-feet has been returned to or retained in the aquifer since 2015 thanks to work by the Idaho Ground Water Appropriators, groundwater users’ private recharge efforts and reduced pumping, and the board’s managed-recharge program, he said.

“We need all of our farmers and water users to work together to restore the ESPA to sustainable levels,” Stuebner said.

Population and business growth, and more efficient irrigation systems that leak less, are factors in the long-term decline.

An annual water-rights call requires groundwater pumpers to ensure surface water users with senior rights receive amounts to which they are entitled. Impacts vary based on weather and other factors. The parties resolved conflicts for the 2024 irrigation season and, as part of an executive order that Gov. Brad Little signed June 26, must establish an improved mitigation plan by Oct. 1.

A goal to bring the aquifer back to 1990s levels is part of a 2015 settlement agreement that the state Department of Water Resources adopted in 2016 as the IGWA mitigation plan.

The board for a decade has conducted an annual recharge program between irrigation seasons, and in recent years has increased recharge capacity.

State-led recharge has averaged nearly 250,000 acre-feet a year, the target, “accomplished through the board’s partnership with canal operators throughout the ESPA,” said Wesley Hipke, IWRB recharge program manager. “We have been able to reach that average by adding increased capacity to take excess water and recharge it instead of it just leaving the state.”

Variables include weather and reservoir carryover storage. The program this year returned about 379,000 acre-feet, up from 146,000 a year earlier.

Snake River reach gains from Blackfoot southwest to Minidoka dropped from about 1.6 million acre-feet to 1.4 million between 2019 and 2023, according to a board news release. Natural spring flows from the aquifer also are dropping slightly after the big winter of 2017, based on measurements from 17 springs that flow into the river from Milner Dam southwest to King Hill.

Aquifer elevation as reflected in an index of 20 sentinel wells is five feet higher due to annual recharge and pumping reductions, said Alex Moody, a department hydrogeologist.

“What we are doing is working, and there’s still a lot more work to be done for the aquifer to be sustainable in the long term,” Hipke said.

“The ESPA leaks and aquifer-storage gains can be fleeting,” Mike McVay, a department hydrogeologist, said in the release. “Changes in weather are to be expected. Perseverance through dry times and drought will be vital to our success.”