Improved snowpack and favorable runoff conditions are needed through June to ensure reservoir-supplied irrigators in the Upper Snake River Basin get full allocations.

Conditions in the nine-reservoir system are mixed, said Jeremy Dalling, water operations civil engineer with the U.S. Bureau of Reclamation’s Upper Snake Field Office in Heyburn, Idaho.

Current forecasts call for future natural flow of around 80% of average through June above Milner, he said. Milner is the farthest reservoir downstream in the Upper Snake system.

“More snowpack, and favorable water supply conditions, are needed,” Dalling said. Forecasts will change as snow-accumulation and runoff seasons progress.

The system is 66% full or 104% of the long-term median, helped by good carryover storage following last irrigation season, he said.

Stored volume is about 6% below the year-earlier total after declining recently, Dalling said. At the end of November, volume was 4% below the year-ago level.

“Dry conditions in the summer and fall have resulted in lower natural flow” into the reservoirs, he said. “The lower natural flow expresses itself in lower inflow to the reservoirs and slower refill.”

Refill was slower since the end of November, partly due to weather including dry conditions, Dalling said.

Jackson Lake, Wyo., is the farthest upstream of the Upper Snake reservoirs. The next downstream is Palisades, in Idaho.

“Jackson Lake storage is maximized for this time of year, whereas Palisades has a lot of remaining space to fill before the spring,” Dalling said.

Factors contributing to current volume in Palisades include lower carryover storage, “also a function of the dry summer and fall,” he said.

To the west in the Boise River Basin, inflows to the three-reservoir system are forecast at 105% of average through July, said Ryan Hedrick, water operations hydrologist with Reclamation’s Middle Snake Field Office in Boise. The Boise is a Snake tributary.

The forecast will be updated, and “due to the extremely dry late January, it likely will come in lower Feb. 1,” Hedrick said.

Reservoir system storage volume is average in the Boise basin and 97% of average in the Payette River Basin to the northwest, he said.

The outlook for full irrigation supplies is “good at this point, but things could change,” Hedrick said. “Things look good for now, but it’s too early to make a final determination.”

Below-normal temperatures and above-normal precipitation are expected in Idaho’s northern two-thirds from February through April, Troy Lindquist, a National Weather Service meteorologist based in Boise, said during an Idaho Water Supply Committee meeting that the state Department of Water Resources hosted Jan. 22.

Some of the same conditions are expected in the March-May period, “although La Nina will be weakening,” he said. Only the state’s far north is expected to see below-normal temperatures and above-normal rain and snow.

La Nina is associated with northern U.S. conditions that are cooler and wetter than normal.