Water Outlook

Another dry month and warmer than normal spring temperatures are deteriorating Idaho’s water supplies. A shift to warmer temperatures caused the mountain snow water content levels to peak and start melting much earlier than normal. With more snowmelt water percolating into the ground and not showing up in the rivers, watermasters are operating under reduced irrigation allotments. Hence, surface irrigation shortages are expected across southern Idaho this season.

 

“The Snow Drought started back in January, when water-year-to-date precipitation was looking good,” said Ron Abramovich, NRCS Water Supply Specialist. “However, warm mid-winter temperatures from January to April allowed moisture to fall as rain and cause intermittent snowmelt at mid-elevation sites, which really took a toll on Idaho’s snowpack.”

 

The best way to think about the May 1 snowpack levels is to imagine it is really June 1. Current snowpacks are melted out or nearly melted out in the Owyhee, Weiser, Oakley, and Little Wood basins and lower elevations across eastern Idaho. Snowpacks are 10 to 15 percent of median in the Little Lost and Mud Lake area and increase to 20 to 40 percent across parts of southern, central and northern Idaho. The Salmon and Boise basins snowpacks are 40 to 50 percent of median, and the highest snowpacks are 50 to 65 percent in the Clearwater, Henrys Fork and Snake basins above Palisades Reservoir. 

 

April precipitation was below normal across the state. The highest April percentages were 90 to 100 percent of average in the Owyhee and lower eastern Idaho basins, including Willow, Blackfoot and Portneuf. The lowest percentages were 40 to 55 percent of average in the west central, Big Wood, Oakley and Salmon Falls basins. 

 

The story on reservoir storage remains the same as last month – many will not fill or be full for long. Reservoirs will be drafted early to meet irrigation demand, leaving them at minimal storage levels before summer’s end. 

 

Streamflows are influenced by how fast the water pours out of the mountainous snowpack. With warmer than normal March and April temperatures, the snow melted gradually thereby allowing the soil to absorb the available water before it reached many of our streams. This has resulted in streamflow forecasts decreasing again for April. The highest stream forecasts are 75 to 80 percent of average in the northern most part of Idaho. Streams across southern Idaho have peaked, and many are flowing at just above minimal levels or summer baseflow levels. Irrigation water users should consider using the lesser volume forecast based on the recent dry and warm weather trends we have experienced since January. 

 

Some good news from the early streamflow peaks is that many streams and rivers that are normally inaccessible to fishing in the spring and early summer months may open up much sooner this year, so be safe and enjoy Idaho’s backcountry.