HOLLISTER —Irrigators in the south-side basins can’t talk about this year’s water supply without looking back at last year — and being grateful.
From Bear River in southeastern Idaho to the Owyhee in southwestern Idaho, nearly full reservoirs to begin the 2018 irrigation season will offset low snowpacks.
Salmon Falls Dam, south of Twin Falls, had 98,800 acre-feet in storage on April 1. The reservoir can hold approximately 182,000 acre-feet but usually has just 56,000 acre-feet on April 1.
Irrigators on the Salmon Tract need approximately 110,000 acre-feet for adequate irrigation water supplies. With the reservoir nearly at that level at the end of last year’s irrigation season, irrigators were fairly confident that they’d be OK in 2018 even if Salmon Falls Creek only flowed at 15 percent of normal this summer.
The forecast is for a bit higher than that, but Salmon Falls Creek will look a lot more like it did from 2013 to 2015 when irrigators received just a third of their normal irrigation allocation. Ron Abramovich, a hydrologist with the U.S. Department of Agriculture’s Natural Resources Conservation Service, estimates a streamflow of approximately 44,000 acre-feet during April to September.
Abramovich spoke at the 65th annual Twin Falls Soil and Water Conservation District water forecast meeting but the message is the same for the smaller watersheds located south of the Snake River.
“It’s a mixed bag,” Abramovich said. “You’ve got good carryover but low snowpack.”
He is forecasting streamflows of 55 to 75 percent of normal across the south-side basins with the exception of the Owyhee River which is expected to flow at just 15 to 45 percent and has already peaked for the year. However, Abramovich expects Salmon Falls Creek will see multiple peaks this spring thanks to wildly fluctuating temperatures and storms in March.
Those storms during March finally produced the first above normal precipitation month for the region since November. But although the south-side basins have received about 30 percent of normal precipitation for the first 10 days of April, the Upper Snake has gotten 70 percent with 20 days left to go.
That’s been the story all winter, Abramovich said. Unlike last winter where storms were evenly distributed, the Upper Snake Basin has seen much more snow than the south-side basins at 130 percent of average. Some sites in Montana are at record highs. As a result, the Snake River at Heise is expected to flow at 109 percent of normal this summer.
Even Magic Reservoir, north of Shoshone, has benefited from this year’s storm track coupled with last year’s snowpack. Magic Reservoir is nearly full, passing inflow and storing 184,906 acre-feet at the end of March, its fourth highest amount since it was built 1917.
To understand this year’s forecast, irrigators should remember last year, Abramovich said.
During the snowmaggedon of 2016-17, ocean conditions pushed 45 atmospheric river events into the southwestern U.S. and Canada. Most years only about 10 of those events form.
While the rain-on-snow event in February 2017 is best known for the flooding it caused, the impacts are still helping irrigators. Streamflows jumped immediately as floodwaters reached them but those streams are still flowing at above normal levels more than a year later. Salmon Falls Creek flowed at 189 percent of normal in 2017 (153,000 acre-feet between April and September). Average flow is 68,900 acre-feet during that period.
That’s helping offset the low snowpack this winter. The Salmon Falls Basin, which includes Magic Mountain received just 67 percent of normal snow. At 53.5 inches, it was the 12th lowest level ever recorded. The lowest snowpack recorded was in 1977 when just 34.5 inches fell.
Soil moisture also factors into streamflow forecasts because wet soils allow more snow water to run off into streams and reservoirs. Soil moisture is okay across the south-side basins but is relatively dry compared to the Upper Snake basins.
A storm near Targhee in September 2017 abruptly ended irrigation demand in eastern Idaho. As that snow melted, it filled the soil profile just ahead of another storm in October that sealed the moisture in.
Still, the long-term forecast is for cooler than normal temperatures and a chance of storms that could boost the streamflow forecasts across southern Idaho and provide a bit more cushion for the 2019 irrigation season.
“Runoff from this year will provide the carryover for next year,” Abramovich told irrigators on the Salmon Tract. “You will use what you have in the reservoir this year.”
“It’s a mixed bag. You’ve got good carryover but low snowpack.” Ron Abramovich, hydrologist with the U.S. Department of Agriculture’s Natural Resources Conservation Service