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Many Idaho basins need only a fraction of average spring runoff

Asan example, the Owyhee River basin needs only 14 percent of average runoff next spring to ensure an adequate irrigation supply next year.
Sean Ellis

Capital Press

Published on December 18, 2017 10:23AM

Sean Ellis/Capital Press File
Water is released for flood control purposes from the Owyhee Reservoir dam last spring. The large carryover in many Idaho reservoirs means that in 2018 irrigators will need only a portion of the normal runoff that feeds reservoirs and rivers.

Sean Ellis/Capital Press File Water is released for flood control purposes from the Owyhee Reservoir dam last spring. The large carryover in many Idaho reservoirs means that in 2018 irrigators will need only a portion of the normal runoff that feeds reservoirs and rivers.

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Because of high reservoir levels, many basins in Idaho and Eastern Oregon only need a portion of their average spring runoff in 2018 to ensure adequate water supplies for irrigators next year.

Ron Abramovich, a water supply specialist for the Natural Resources Conservation Service, projected spring reservoir storage levels based on current conditions and how much water remained in reservoirs this fall.

He then used those projections to estimate how much stream flow, or runoff, is needed next year to ensure an adequate irrigation supply in certain basin in 2018.

As an example of how the calculations are made, annual irrigation demand in the Boise River basin is 1.5 million acre-feet. The basin’s reservoir system is projected to have 800,000 acre-feet of water by next spring.

That means 700,000 acre-feet of runoff in the form of stream flow is needed next spring, which amounts to 51 percent of the average stream flow in that basin.

For many basins, “It should be no sweat,” Abramovich said Dec. 14 during the Treasure Valley Irrigation Conference in Ontario, Ore. “Shortages are not expected at this time.”

The Owyhee River basin, which provides water to 118,000 acres of irrigated land in Eastern Oregon and part of Idaho, needs only 14 percent of average runoff next spring to ensure an adequate irrigation supply in 2018.

Based on the basin’s average runoff, that shouldn’t be much of a problem, said Oregon farmer Bruce Corn, a member of the Owyhee Irrigation District’s board of directors.

“Even with this (December) dry spell that we’re in, we’re close to having an adequate irrigation supply for next year,” he said. “It won’t take that much more to fill the reservoir.”

According to the NRCS projections, the Upper Snake River basin needs only 66 percent of its average stream flow next year.

The projections are a good way to distill the current water supply situation down to something everybody can understand, said Lyle Swank, watermaster for Water District 1 in East Idaho, which is the state’s largest and provides water for more than 1 million acres of irrigated farm land.

But the actual water situation in the Upper Snake is more complex than that 66 percent number and the basin probably needs to at least hit that total to ensure there are no water disruptions next year, Swank said.

“I think there’s a very good chance we’ll get there,” he said of that 66 percent amount. “But I really think that’s the bare minimum we need to get through the year.”

Abramovich told Capital Press the calculations aren’t rocket science but they are meant to “give irrigators an idea of what’s needed for next year.”

What they show, he said, is that while the region may be in a bit of a dry spell right now, “We’re still in good shape.”

To view the projections online, visit http://www.id.nrcs.usda.gov/snow/ and click on “Surface Water Supply Index” and then on “2018 Streamflow Needed for Adequate Irrigation Supply.”



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