March Basin Outlook report is generally positive

Published on March 21, 2020

Natural Resource Conservation Service (NRCS) State Soil Scientist Jeff Goats publishes weekly and monthly reports on the state of basins across Wyoming. 

            These basins include Snake, Madison, Yellowstone, Wind, Bighorn, Shoshone, Powder, Tongue, Belle Fourche, Cheyenne, Upper North Platte, Lower North Platte, Sweetwater, Laramie, South Platte, Little Snake, Upper Green, Lower Green and Upper Bear. 

            Each Monday, Goats publishes his weekly snow report, which lays out the snow water equivalent (SWE) as a percent of the median. 

            According to Goats, the normal (median) is based on all reporting snow telemetry (SNOTEL) sites in the basin with calculated medians. He notes newer SNOTEL sites may not have medians figured and the figures do not include manually measured snow courses. 

            “The statewide SNOTEL percent of median is a percent of median using all SNOTEL sites in Wyoming with calculated medians,” Goats explains. “The weighted state average is figured using the area of basins in square miles. The reference period for computing medians is the 30-year period 1981 through 2010.”

            Goats notes of all the basins in Wyoming, Sweetwater is the most concerning. 

            “Sweetwater Basin has been consistently low for every report we’ve done this snow season,” he says. “I don’t really have an explanation for this other than the fact there are significantly less SNOTEL stations, less than half a dozen, whereas basins like Bighorn have over a dozen.”

            “Currently the state’s SNOTELs are reading 113 percent of median with a basin high of 129 percent and a basin low of 71 percent. Last year, the state was at 112 percent, and at 112 percent in 2018,” Goats explains.

            He notes while a handful of the basins are trending lower, a majority have a higer percent of the median

How forecasts are made

            “Most of the annual streamflow in the western United States originates as snowfall that has accumulated in the mountains during the winter and early spring,” Goats explains. “As the snowpack accumulates, hydrologists estimate the runoff that will occur when it melts.” 

            “Measurements of snow water equivalent at selected manual snow courses and automated SNOTEL sites, along with precipitation, antecedent stream flow and indices of the El Niño/Southern Oscillation are used in computerized statistical and simulation models to prepare runoff forecasts,” says Goats. “Unless otherwise specified, all forecasts are for flows that would occur naturally without any upstream influences.”

            According to NRCS, the use of a median is preferred over the use of an average when reporting SWE.

            “Although average is a commonly-used and well understood statistic, median is also a common descriptor used to express a middle value in a set of data,” says NRCS. “This middle value is also known as the central tendency.” 

            NRCS continues, “Median is determined by ranking the data from largest to smallest, and then identifying the middle so there are an equal number of data values larger and smaller than it is.  While the average and median can be the same or nearly the same, they are different if more of the data values are clustered toward one end of their range and/or if there are a few extreme values.” 

            “In statistical terminology, this is called skewness. In this case, the average can be significantly influenced by the few values, making it not very representative of the majority of the values in the data set,” says NRCS. “Under these circumstances, median gives a better representation of central tendency than average.”

            “In general, SWE for a given day over a historical period shows skewness. This is particularly evident at the onset of snow accumulation and near the time of melt out, when many years have very small or zero values and only a few have significant nonzero values,” NRCS explains. “Skewness may also be noticeable throughout the year due to the presence of a few large snow years.” 

            “In these cases, the median is typically different, usually smaller, than the average but better represents the central tendency of SWE than does the average,” NRCS explains.  

            Callie Hanson is the managing editor of the Wyoming Livestock Roundup. Send comments on this article to roundup@wylr.net.