Change in snowpack statistics ignores climate change effects
Farmers and river lovers depend on mountain snowpack reports to estimate how good summer streamflows will be, but a recent data change raises questions about the accuracy of that information, particularly as climate change worsens.
The U.S. Natural Resource Conservation Service announced last week that it would be shifting the 30-year timeframe used to calculate its snowpack “normals” to the most recent decades. Instead of using snowpack statistics calculated between 1980 and 2010, averages and medians will now be based on the snowpack that existed between 1990 and 2020.
For each month during the first half of the year, the Montana NRCS prints a report showing how the snow levels at that point compare to the 30-year average. For instance, the report published last January 1 showed that half of western Montana had less than 90% of its median snowpack for 1980 through 2010.
Now, however, we can’t really compare this January’s snowpack to last year’s because the NRCS is using different data. Is that a problem? Maybe not from one year to the next. But as climate change reduces mountain snowfall, comparing less snow to an average of less snow paints a picture that’s rosier than it should be.
It’s actually not the depth of snow that counts, but the amount of water in the snow, something called the “snow water equivalent” or SWE. Three feet of fluffy powder holds less water than 3 feet of packed snow, and crops and streams need lots of snowmelt to be released slowly in the summer.
University of Montana Environmental Studies professor Len Broberg documented how the amount of SWE along Montana’s northern Continental Divide has dropped since 1980. His peer-reviewed research paper, published in April, shows the mountains had more snowpack SWE in the 1980s, but the NRCS has just removed that decade of data from the calculation. The resulting average or median for the 1990-2020 snowpack is likely to be slightly lower than before, which could make the current year’s snowpack appear to be more than if it was compared to a higher median.
“My research does show the snowpack’s 5-year running average of snow water equivalent declining in the Crown of the Continent region from 1980 to 2013. So adjusting the period to 1990 to 2020 would lower the average for many snow basins and therefore understate the impact of climate change,” Broberg wrote in an email.
The numbers indicate that could hold true for several basins in Montana. For example, the peak SWE median for 1980-2010 in the lower Flathead River basin is about 27 inches, compared to about 12 inches for 1990-2020. Likewise, the Flathead Lake basin shows a big drop in the SWE median with the new data. However, in the Bitterroot basin, the two medians show little change between time periods, and the NRCS says the data doesn’t allow comparison for the Blackfoot basin.
The NRCS website says the shift was needed because the data underlying the newer period is being collected and processed differently. The NRCS made a similar data shift in 2011, moving from the years 1970-2000 to 1980-2010.
The NRCs began installing automated SNOTEL equipment that measures the moisture in snow and snow depth at high elevations in the 1970’s. Many locations didn’t have a SNOTEL station until later, although many had the older snow courses, where technicians measure snow amounts manually. For those sites that didn’t have SNOTEL equipment in the 1980s, the NRCS filled in data for the 1980s using estimation methods.
The NRCS says the new data includes new values for nearly 700 automated SNOTEL stations and more than 900 snow course measurements. So they don’t include the data estimates needed prior to 1990.
That’s also why the NRCS cautions against comparing averages and medians of the two time periods. The website cautions the change in “normals” could be the result of the change in calculation methods and the number and location of monitoring stations used in the calculation.
The only nod to climate change on the NRCS website is the caution that says differences may also reflect a “change in monitoring site conditions.”
A commenter on the NRCS blog – Bill Howland – suggested that the NRCS allow viewers to view plots comparing the older and newer data to “see how far the dates for peak SWE totals are shifting each decade due to climate change.” Climate change not only affects snowpack totals, but it also causes snowpack to peak earlier in the winter as warmer temperatures cause snow to melt earlier.
In response to Howland, NRCS communications officer Ben Weaver only reiterated the reasons why direct comparison between the two peaks is “not advised.”
Regardless of the reason for the differences, a new snowpack median that is only about half of the previous median – such as with Flathead Lake and the lower Flathead River basins – is going to produce a big change that can be misleading. Whereas last year’s January snowpack around Flathead Lake was 100% of the 1980-2010 median, that same snowpack would now be about 150% of the new median.
“It really depends on what you want to use the percent of average snowpack for. If you want to use it for agriculture and expected water availability, it may cause farmers and irrigators to think there will be more water, at least until they adjust to the use of the new average,” Broberg wrote.
This year’s snowpack got a late start and much of drought-stricken western Montana is in bad shape, even compared to the new medians. As of Monday, the upper Clark Fork and Bitterroot basins have only about half of the median for December, while farther north, the Flathead has 60%. The lower Clark Fork basin below Missoula is the worst off in the region with just 35% of what it should have for December.
Calls to the NRCS in Washington, D.C., and Bozeman to ask about the change were not returned Monday. On a side note, Bozeman water supply specialist Lucas Zukiewicz, who has published Montana’s monthly water supply reports for the past several years, is no longer listed as working for the NRCS and his position has yet to be filled.
Contact reporter Laura Lundquist at firstname.lastname@example.org.