Low soil moisture in the Upper Snake River Basin is a concern for Idaho water researchers and managers.

Multiple data sources show “very low soil moisture in the Upper Snake … from the surface down into the root zone,” said Erin Whorton, an Idaho Snow Survey hydrologist with USDA’s Natural Resources Conservation Service in Boise. The lack of moisture is evident in the top three feet of the soil column.

“We are below the 10th percentile,” she said, citing two NASA satellite-based tools.

Dry soils “act like a sponge and soak up runoff,” Whorton said.

Soils that are dry take a portion of snowpack and runoff as it heads toward rivers. Once snow blankets the ground and accumulation season ramps up, improvements are hard to come by, she said.

Frequently updated NRCS forecasts look at how expected streamflows compare to long-term medians under different scenarios. The reports are tracked by irrigators, recreationists, fish and wildlife biologists and power companies among others.

If Upper Snake soil moisture remains low, “we’re going to steer our water users toward our lower-volume streamflow forecasts,” Whorton said.

A year ago, soil moisture was slightly above current levels, she said. Snowpack in the 2023-24 water year was minimal until a snow cycle started in mid-January. Steady storm activity occurred into early March, contributing to mostly normal snowpack.

Soil moisture in the basin is about 70% of the long-term median, said Brian Stevens, water operations supervisor with the U.S. Bureau of Reclamation’s Upper Snake Field Office in Heyburn.

And base flows entering the river are dropping, he said. Base flows in the past month dropped from about 130% of the median to around 100% in the Ririe sub-basin, and from about 92% above the Heise gauge near Idaho Falls to the current 86%.

Recent dry conditions contribute to low soil moisture and low base flows, Stevens said.

Going into winter with low base flows and soil moisture means above-average snowpack would be needed to bring base flows above average later in the water year. Some water districts that deliver irrigation water rely heavily on reservoir storage while others rely more on natural flow, he said.

“If we get low base flows, natural flow would be low and it will add to groundwater concerns in the basin,” Stevens said.

Upper Snake reservoir system volume is about 101% of median compared to 113% a year ago. The system is about 54% full, down from 60%.

Idaho Falls-based Water District 1 delivers irrigation water and accounts for it based on water rights and storage contracts. Staff do not forecast runoff or contributing factors, but “we do track the hydrologic conditions of the watershed such as moisture to help irrigators plan and prepare for how the water right priorities may be affected by those conditions,” said watermaster Craig Chandler.

“Low soil moisture can create an increased demand for irrigation water, which can limit the water right priorities that are deliverable and increase storage usage.”

Idaho Power watches soil moisture as an indicator of runoff efficiency, spokesman Brad Bowlin said.

Soil moisture is among several factors the company considers in trying to model its system around anticipated runoff throughout the spring and summer, he said. Water supply is the company’s most important source of energy, so “we keep a close eye on trends and conditions across the basin and leverage our forecasting system to get a handle on the likely range of future conditions.”