A healthy snowpack in much of southern Idaho still might not be enough to meet all the irrigation demands along the Snake River Plain in eastern and south central Idaho this summer.
The Idaho Department of Water Resources is predicting a 75,000-acre foot shortfall there this irrigation season.
Though all basins in southern Idaho have above-average snowpack, those in the eastern part of the state – like Henry’s Fork – are closer to the average amount, and those are the basins that fill the big reservoirs in the Upper Snake River.
Current conditions, plus two previous years of significant drought that depleted reservoirs and led to excess groundwater pumping, will make it tough for the system to fully recover this year, according to Idaho Department of Water Resources Deputy Director Mat Weaver. The reservoirs could once again not fill.
“We've had two really hard years on the aquifer and that aquifer is continuing to decline and is getting to very low conditions – some of the lowest conditions we've ever recorded or measured,” he said.
IDWR estimates about 900 groundwater rights dating back as far as 1953 could be shut off this year if the predicted shortfall happens. Previously, the anticipated gaps in supply and demand have implicated a much smaller number of water users – only those with rights dating back to 1978.
The change this year is due to a new proposed methodology announced in April by IDWR for figuring out whose water will get curtailed.
Under Idaho law, surface water users, often with more senior water rights, have priority over groundwater users with junior rights on the Snake River and within the Eastern Snake Plain Aquifer. The Department of Water Resources is charged with trying to make sure the senior users get all their water. That involves figuring out the extent to which groundwater pumping is affecting the amount of water the senior users can get from the river.
Weaver said the latest update to the methodology, which incorporates climate and hydrologic data through 2021, errs on the side of protecting the senior users even more.
“What we found is that because the data has been changing and getting largely more drier, warmer and greater irrigation demand, is that some of the processes in the previous methodology weren't providing enough protection for the senior [users],” he said.
Still, Weaver emphasized that the vast majority of groundwater pumpers in the region are part of approved mitigation plans negotiated in the big water rights settlement between surface and groundwater users in 2015.
Those users won’t be subject to water shut-offs, so long as they continue to meet the demands in the mitigation plans.
Meeting the demands wasn’t a problem for the first several years after 2015, which happened to be good water years. But when drought hit the region in 2021, IDWR found IGWA had breached the agreement because it didn’t reduce groundwater pumping by the 240,000 acre-feet per year target.
Because of the breach, IGWA needs to deliver 30,000 acre-feet of water this year and 15,000 acre-feet next year to surface water users. IDWR is also in the process of evaluating whether IGWA met its obligations in 2022, another drought year.
The week of June 6, IDWR Director Gary Spackman will hold hearings on the proposed updates to the methodology. Spackman will wait to issue official curtailment notices until after the hearings.
Find reporter Rachel Cohen on Twitter @racheld_cohen
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