‘Snow water equivalent’ critical to moisture levelsWritten by Christy Hemken
Because of its importance, several methods are employed throughout the winter months to monitor snowfall trends and also that snow’s “snow water equivalent” (SWE).
“The SWE tells you how much water is there in the snow, and that means a lot more than how much snow there is,” says NRCS Water Supply Specialist D. Lee Hackleman. “You can figure out the density of the snow by figuring out the SWE and how much total water is held in the snow.”
Hackleman says a lot of times measurements will be taken at a site and the snow depth will have gone down but the moisture content will have actually gone up.
To track snow depth and water equivalents in Wyoming, Hackleman and his team take measurements from snow courses and SNOTEL sites throughout the state.
Snow courses are sites where periodic measurements are taken to track snowpack accumulation patterns. These sites, of which there are about 800 in the West, are each measured by hand. Sixty-six of them are located in Wyoming, and Hackleman says most of them are reached by snowmachine and they’re visited for five or six months during the winter.
SNOTEL sites, on the other hand, rely on meteor burst telemetry to relay their data to a master station in either Boise, Idaho or Ogden, Utah, from where it’s sent by telephone to Portland, Oregon.
According to NRCS, meteor burst telemetry relies on the physical phenomenon that enables radio signals to be reflected off ionized meteorite trails 50 to 75 miles above the Earth’s surface. Utilizing this principle, sites as far apart as 1,200 miles can communicate with one another for very short periods ranging from fractions of seconds up to several seconds.
Because it’s not dependent on radio waves, the usual interference from mountains and terrain is not an issue for a meteor burst system, which are able to transmit data up to several times per day.
“The SNOTELs give out a reading every hour year round, and along with snow they measure wind, solar and total precipitation data,” says Hackleman.
Beginning to modernize their snow surveys in 1977, the NRCS began the SNOTEL project, a name derived from SNOpack TELemetry, to measure and transmit snowpack, precipitation and temperature data on a daily basis throughout the West.
Currently over 700 SNOTEL sites are operating, with most of them powered by solar panels and only visited a few times each year. Of those, 83 are monitored by Wyoming in the state, plus two in South Dakota.
Once SNOTEL data arrives in Oregon at the National Water and Climate Center, it’s analyzed and interpreted by computer programs known collectively as the Centralized Forecasting System. From there it’s made available to NRCS offices and the general public through the Internet.
Of this winter’s data, Hackleman says, “We got up to a little over 100 percent of average for a little while, but we’re back down to 98 percent now. The areas that are doing the best are in Yellowstone and the Little Snake and Upper North Platte, which are all above 110 percent snowpack. The worst areas are still the Upper Green and the Wind River.”
“I think we’re supposed to get some moisture in the first two weeks of March, but then they’re talking dryer than normal after that,” he noted. “We’re supposed to be a little warmer than normal, too. We just hope it’s not like it was two years ago where it quit snowing the first of March.”
In the Feb. 25 snowpack and precipitation report, the NRCS reported snowpack remains much above average for most locations across the West. “This is a very unusual year where snow is abundant in almost all regions of the West,” the report reads. “In some areas, such as in the Cascades and Sierra mountain ranges, low elevation snowpack is even higher in percent of average, with concerns persisting about the vulnerability of the snowpack to rain on snow events.”