First water supply outlook released for 2026
On Jan. 1, the U.S. Department of Agriculture’s Natural Resources Conservation Service (NRCS) released the first Wyoming Basin and Water Supply Outlook of the new year, analyzing snowpack, precipitation, streamflow and reservoir storage across the state of Wyoming.
The report assesses the water supply outlook for the entire Cowboy State, as well as individual basins including the Snake River, Madison Headwaters, Yellowstone River, Wind River, Big Horn River, Shoshone River, Powder River, Tongue River, Belle Fourche River, Cheyenne River, Upper North Platte River, Lower North Platte River, Laramie River, Sweetwater River, South Platte River, Little Snake River, Upper Green River, Lower Green River and Upper Bear River basins.
Making forecasts
When it comes to making forecasts for the Wyoming Basin and Water Supply Outlook, NRCS explains most annual streamflow in the western U.S. originates as snowfall, which accumulates in the mountains during winter and early spring.
As snowpack accumulates, hydrologists begin to estimate the runoff that will occur when it melts.
“Measurements of snow water equivalent (SWE) at selected manual snow courses and automated snowpack telemetry sites, along with precipitation, antecedent streamflow and indices of the El Niño/Southern Oscillation are used in computerized statistical and simulation models to prepare runoff forecasts,” reads the report. “Unless otherwise specified, all forecasts are for flows which would occur naturally without any upstream influences.”
NRCS notes forecasts are by no means perfect. In fact, the agency explains streamflow forecast uncertainty arises from three sources – uncertain knowledge of future weather conditions, uncertainty in forecasting procedures and errors in the data.
Therefore, it is important individuals interpret these forecasts as a range of values with specific probabilities of occurrence instead of a single value.
“The middle of the range is expressed by the 50 percent exceedance probability forecast, for which there is a 50 percent chance the actual flow will be above and a 50 percent chance the actual flow will be below this value,” NRCS explains. “To describe the expected range around this 50 percent value, four other forecasts are provided – two smaller values at 90 percent and 70 percent exceedance probability and two larger values at 30 percent and 10 percent exceedance probability, respectively.”
The report further notes the wider the spread among these values, the more uncertain the forecast will be. As the season progresses, forecasts will become more accurate, primarily because a greater portion of the future weather conditions become known.
Lastly, NRCS notes the median is the official normal for snowpack, precipitation, reservoir storage and streamflow calculations.
Snowpack and precipitation
According to NRCS, as of Jan. 1, SWE across Wyoming was reported at 84 percent of median.
The Yellowstone River Basin reported the highest SWE at 137 percent of median, while the South Platte River Basin had the lowest SWE at five percent of median.
NRCS notes the Belle Fourche, Cheyenne, Laramie, Little Snake, Lower North Platte, Powder, South Platte, Tongue, Upper Bear and Upper North Platte river basins posted SWEs below the median recorded from 1991 to 2020.
Additionally, the agency reports the highest amount of precipitation occurred in the Wind River Basin at 266 percent of median, while the Lower North Platte River Basin saw the least amount of precipitation at 71 percent of median.
Streamflow yields
For April through September, NRCS forecasts average streamflow yields from all basins in the state of Wyoming – except the Green, Little Snake and Cheyenne river basins – at 115 percent of median.
April through July median streamflow yields for these four exceptions are predicted at 70 percent of median in the Cheyenne River Basin, 80 percent of median in the Little Snake River Basin, 107 percent of median in the Lower Green River Basin and 110 percent of median in the Upper Green River Basin.
The NRCS report shows streamflow yields for individual basins across the state at 90 percent of median in the Lower North Platte River Basin, 91 percent of median in both the Powder River and Tongue River basins and 92 percent of median in both the Upper North Platte River and Laramie River basins.
Median streamflow yields for the Big Horn, Yellowstone, Wind, Snake, Shoshone and Sweetwater river basins are estimated at 117, 128, 129, 145, 146 and 167 percent, respectively.
Reservoir storage
Additionally, the NRCS report outlines reservoir storage across the state of Wyoming, which averaged 66 percent of median as of Jan. 1.
Reservoirs reporting numbers below median include those in the Upper Bear River Basin at 37 percent, the Laramie River Basin at 50 percent, the Upper North Platte River Basin at 59 percent, the Cheyenne River Basin at 79 percent, the Wind River Basin at 81 percent and the Buffalo Bill Reservoir on the Shoshone River at 85 percent.
Reservoirs in the Snake River Basin and Lower Green River Basin, as well as the Boyson Reservoir, posted near median at 96 percent.
Reservoirs in the Upper Green River Basin and the Lower North Platte River Basin also posted near median at 95 percent and 97 percent, respectively.
Reservoirs in the Belle Fourche River Basin were above median at 105 percent.
Hannah Bugas is the managing editor of the Wyoming Livestock Roundup. Send comments on this article to roundup@wylr.net.
