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Burley flooding

An irrigation wheel line sits in several inches of water Jan. 11 near Burley.

BURLEY — Winter hasn’t officially begun, but water forecasters are feeling confident that irrigators across southern Idaho will have adequate supplies in 2018.

But on the question of whether this winter will be as cold and snowy as last winter, forecasters are not as confident.

As Kelly Olson pointed out during the University of Idaho Agriculture Outlook conference, the region is expecting a weak La Nina event this winter.

“Last year we had the same forecast and look what happened,” Olson said. She is the executive director of the Idaho Barley Commission and not a weather forecaster, but she knows how the winter affected grain producers.

La Nina is a climate phenomenon associated with cooler-than-average sea surface temperatures in the Pacific Ocean. La Ninas tend to bring wide-ranging weather conditions, which can make forecasting more difficult. But analyzing the 13 La Nina events that have occurred in Idaho since 1982, shows that 12 of those events brought above normal snowpack to the Boise River Basin and weak La Ninas appear to produce the most snow, said Ron Abramovich, water supply specialist with the USDA Natural Resources Conservation Service.

“Don’t believe the first weather forecast you hear,” Abramovich told participants at the ag outlook conference. “But when they are all favoring similar scenarios,” it’s time to pay attention.

Idaho water users are in good shape for 2018, he added. Thanks to primed soils in the mountains and high reservoir carryover, only minimal streamflow volumes are needed to provide adequate irrigation water supplies.

As of early December, the eight reservoirs that make up the Upper Snake system were at 85 percent of capacity. Palisades Reservoir is at the highest level for this time of year in 30 years, needing only about 40,000 acre-feet of water to fill. Projections now indicate the Snake River needs only to flow at 66 percent of average as measured at the Heise gauge west of Yellowstone to provide adequate irrigation supplies. An acre-foot is the amount of water needed to cover a football field with 12 inches of

Even Salmon Dam, which rarely fills, needs only 1 percent of average streamflow next summer to provide adequate irrigation water for the Salmon Tract south of Twin Falls. Projections now put reservoir storage at 97,000 acre-feet during January, February and March. Irrigators need 110,000 acre-feet to provide a full water supply.

With the possibility for adequate irrigation water supplies looking good right now, water managers are beginning to evaluate strategies to protect that stored water while minimizing potential flood events.

Terrell Sorensen, UI extension educator in Power County who previously managed an irrigation company, said that’s a real balancing act. He pointed to the Oroville Dam in northern California as an example of what water managers in southern Idaho do not want to see happen. The Oroville Dam is the nation’s tallest and provides irrigation water to 755,000 acres.

Through mid-December of 2016, the watershed was on pace with previous drought years. After a month or so of above normal precipitation, the reservoir reached flood control curves and remained above that level until May. Managers were forced to release water through a broken spillway. Nearly 200,000 people were forced to evacuate amid fears the spillway would fail. Yet by the end of the year, the reservoir was only about one-third full, lower than the previous drought year.

“It’s a balancing act,” Sorensen said. “You don’t want to use your emergency spillway but you still want to fill the reservoir.”

Idaho managers were able to carryover much of last year’s abundant snowpack, which is providing not just a cushion for next year but also allowing additional managed aquifer recharge.

Approximately 317,000 acre-feet of managed recharge was accomplished last winter.

This year efforts have begun even earlier with 150,000 acre-feet already done by December 6th. Milner Gooding is recharging approximately 800 cfs (cubic-feet per second) with the Twin Falls Canal Company adding another 100 cfs in the Magic Valley. Other projects are providing around 600 cfs of recharge.

As impressive as those numbers are, studies indicate just 7 to 10 percent of the water that reaches the Eastern Snake Plain Aquifer comes from managed recharge, Sorensen said. The rest comes from incidental recharge including all the runoff when the low elevation snow melted last February. As good as that was for the aquifer and groundwater pumpers, most farmers do not want to see a repeat of that.

Right now, the models do not indicate the valleys will see as much snow as last winter, Abramovich said. However, several models indicate elevated chances for very stormy periods, heavy mountain snow and an active Pacific jet stream.

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