April showers brought a brighter outlook to Idaho’s water supply forecast according to the May Water Supply Outlook Report just released by the Natural Resources Conservation Service (NRCS). Additional spring rains would assure adequate water for Idaho’s numerous users.
“So far, so good for the water supply forecast,” said Ron Abramovich, NRCS water supply specialist. “Good spring precipitation is a wildcard that can turn a near average snowpack into a winning hand by producing better runoff. That would really help in the central and southern basins where drought conditions still linger.”
April precipitation amounts were 115 to 130 percent of average across southern Idaho that includes the basins south of the Snake River and Little Lost, Bear River, and Upper Snake basins. From the Big Lost basin to the Payette and north to the Panhandle, April precipitation ranged from 60 to 90 percent of average.
Snowmelt season is nearly here and snowpacks across Idaho vary widely from 60 percent to over 120 percent of average. Most basins are near average. So far, only lower elevation drainages of the Owyhee and Weiser Rivers and have had their season snowmelt peak flow.
Although the warm days in mid-April kick-started the snowpack melt, cooler end-of-month temperatures slowed the melt which pays off later in the season by stretching out the water supply. The timing and magnitude of snowmelt streamflow peaks will depend on the precipitation and air temperatures over the next few weeks.
“These near average numbers sound encouraging, but the snow levels are less than last year,” said Abramovich. “This means the duration of high flows will be less than last year.”
Streams in the Clearwater, Salmon, Upper Snake Rivers and part of the Bear River basin are forecast to have average or better streamflow. While the Boise, Weiser, Payette and rivers south of the Snake are forecast at below average volumes. The lowest forecasts in the state are 65 percent of average in the Big Wood River, which will affect inflow to Magic Reservoir, and streams in the Owyhee basin.
Overall, most of Idaho’s major reservoirs will fill and water supplies should be adequate for most users, but supplies could be tight for users in Magic Reservoir drainage, Big Lost, Little Lost, Oakley and Salmon Falls basins.
Above average precipitation in May and June would nearly guarantee a good water supply for the numerous users and provide better reservoir carryover storage for next year.
For more information about snowpack, precipitation, runoff and water supplies for specific basins, view the complete May 2009 Water Supply Outlook Report online at www.id.nrcs.usda.gov/snow and click on the ‘Water Supply’ link.