TWIN FALLS, Idaho — Changes to groundwater use can impact the Snake River and spring flows and the water rights of surface water users.
There’s been a moratorium on new appropriations of water rights within the Eastern Snake Plain Aquifer since 1992. The only way to acquire a water right to divert from the ESPA is through a transfer approved by the Idaho Department of Water Resources, Shelley Keen, IDWR water allocation bureau chief, said during a meeting of the Idaho Legislature Natural Resource Interim Committee.
The challenge there is that the aquifer and Snake River are connected hydraulically, and groundwater transfers can have an impact on senior rights from streams or rivers, he said.
IDWR’s policy generally is to allow some transfers to occur while preventing injury to senior water rights, he said.
The department recently completed a review of the cumulative impacts of its approved groundwater transfers from 2012 to 2018. The review included all 426 transfers involving pumping from the ESPA, with transfers from 679 wells to 685 wells.
A map shows a pretty even distribution of those “from” and “to” wells — “a hint that our policy is probably getting us where we need to be,” he said.
The review showed reduced pumping led to a positive impact on the aquifer of about 412,728 acre-feet and increased pumping resulted in an approximate 412,210 acre-feet depletion.
The question is whether the transfers are neutral, and it’s a pretty good offsetting of impacts, he said.
It indicates the 426 transfers don’t inadvertently result in aquifer depletion as a whole, according to IDWR.
“Most of the transfers allowed under this policy involved small amounts of water,” he said.
Half of the transfers involved 100 acre-feet of water or less. The big transfers were mostly cities consolidating water supplies, he said.
Most of the transfers were within the same grid, 1 square mile, and about 75% were within 5 miles. Only five transfers were 50 to 103 miles.
The department’s review also included the cumulative impact on the 11 river reaches, five above Milner Dam and six below it.
The range in the annual change in water the aquifer is contributing to river reaches was less than 0.5%. The total change for all reaches combined was 0.01%.
“Transfers that move water farther have a bigger impact, but it’s still very, very small,” he said.
The question, however, is whether future transfers will have different impacts, he said.
IDWR will collect and update data on the impacts of groundwater transfers annually.
Anyone applying for a transfer that involves relocating water more than a mile or two must submit an evaluation of the potential effects on a hydraulically connected river reach. If depletions exceed policy thresholds, the applicant can adjust the transfer proposal or mitigate the effects.