Most Idaho reservoirs likely will fill despite a mostly dry end to winter, USDA Natural Resources Conservation Service hydrologists say.
But low snowpack and expected runoff in the Wood River and Lost River basins, in the east-central mountains, likely mean water-supply concerns for irrigators, dryland grazing operations and people who rely on natural streamflow, the NRCS Water Supply Outlook Report said.
Agricultural water shortages in these basins this summer are likely. The amount of water stored in Magic Reservoir is well below normal.
NRCS said it expects sufficient reservoir supply for irrigators across all Snake River basins except Oakley and Salmon Falls, where agricultural water shortages are predicted. Drought conditions likely will persist during spring in central and southern Idaho.
Dry soils around Idaho have concerned water managers almost since the snow-accumulation season began Oct. 1. Runoff saturates soil on its way to reservoirs. Unusually dry ground reduces inflow.
“Across much of the state, folks can attest that soil moisture is a little shy of adequate, with the forecast looking to get drier,” State Soil Scientist Shawn Nield of NRCS Idaho told Capital Press. “Should that trend continue, the soil profile will take extra water in the early season.”
He said ground with a standing cover crop or good residual cover over winter tends to offer better moisture conditions than bare ground by better trapping snow, preventing pore packing and plugging during rains, encouraging infiltration and reducing evaporation.
NRCS reported the La Nina winter was drier than hoped across much of the state. Though this year is slightly drier than 2020, water stored in most reservoirs is near 30-year averages and many basins’ snowpack is nearly normal.
NRCS April 8 said snow-water equivalent was 89-107% in northern basins, 83% in the large Salmon basin, 76-83% in the west-central basins, and 77-87% in the east. East-central mountain basins had snowpack 51-81% of average. Southern Snake River basin snowpacks ranged from 78% on the west side (Owyhee) to 87-94% to the east.
In the state’s southeast corner, Bear River Basin snowpack was 68%.
NRCS said in the report that reservoir storage looks sufficient for Bear River Basin irrigators, but dry conditions are a concern for ranchers using non-irrigated pasture.