April 1 water outlook

Heavy snowfall occurs along Highway 21 near the Bad Bear Snow Course on March 31.

All of Idaho had received below-normal precipitation for the water year through April 1, with the Wood and Lost River basins experiencing the most notable shortfalls, according to a recently released water supply outlook report.

Nonetheless, the state’s irrigators should have an ample storage supply thanks to strong reservoir carryover, USDA’s Natural Resources Conservation Service concluded in the report.

According to the report, most of Idaho received below-normal precipitation during March, except for the Upper Snake and Lost River basins, which received between 110% to 120% of their usual precipitation for the month.

Despite the below-normal precipitation throughout most of the state, modest snowpack gains were reported, nonetheless, in many basins, thanks to colder weather.

“Even with below-normal precipitation in March, slight snowpack improvements were observed across most of Idaho since March 1,” Daniel Tappa, supervisory hydrologist with the Idaho NRCS, said in the report. “While this seems paradoxical, it implies that little snowmelt occurred and the precipitation that we did receive came in the form of snow.”

April 1 snowpack levels range from 90 to 110 percent of normal throughout most of Idaho. Shortfalls have persisted in the Wood and Lost basins, which had between 70% and 80% of their usual snowpack through the end of March.

Northern Idaho had above-average snowpack, at 110 to 120 percent of normal.

Streamflow forecasts predict the majority of Idaho runoff will be between 80% and 120% of normal. The greatest areas of concern for streamflow are the Wood and Lost basins, where streamflow forecasts range from 20% to 60% of normal, according to the report.

For the month of March, the Upper Snake Basin averaged 110% of its normal precipitation. Furthermore, the report concluded that a wet period from January through March has improved precipitation totals for the water year but has not entirely offset the deficit from an extremely dry November through December. Precipitation for the water year for the Snake above American Falls was 95% of normal, though the snowpack was 105% of normal through April 1.

Overall reservoir storage in the Upper Snake reservoir system is at 125% of normal. Streamflow forecasts in the Upper Snake mostly range from 95% to 125% of normal, with the lowest forecast in the system for the Portneuf River at Topaz, which is 78% of normal.