“Soil moisture is just a memory creator,” Elise Osenga, community science manager at Aspen Global Change Institute (AGCI), said at a summit in Carbondale on May 11. “It’s this memory of what’s happened in our water cycle.”
Those memories are based on soil moisture entering winter, which determines how much water the soil can take up during spring snowmelt. Aspen Journalism’s analysis of soil-moisture data from three of the Roaring Fork Observation Network (also known as iRON) stations located across the Roaring Fork basin shows that soil moisture appears to be moderate this water year. In short, it’s neither the wettest year nor the driest.
“The good news is, we’re wetter than we were in the early winter of 2020,” which led to the poor runoff seen in spring of 2021, Osenga wrote in an email.
Soil moisture is measured in “volumetric water content,” which refers to the amount of water in a given volume of soil. Stations record soil moisture at three different depths: 2 inches, a depth that is more sensitive to weather events and air temperatures; 8 inches, a depth that captures changes across seasons, drying events, and moderate and heavy rains; and 20 inches, a depth that is less volatile and that records intense rain events. It’s also worth noting that all the years on record fall during an extended drought period.
If soils are dry entering winter, they’re going to take up a lot more water from snowmelt in the spring, and it’s going to influence the amount of runoff reaching rivers and streams. Osenga said water planners can look at soil moisture from November to January to figure out the trends for spring since soil moisture doesn’t move a lot in the winter months, especially at high-elevation sites.
“What happens is ground temperatures are very close to freezing,” Osenga said. “There’s a layer of snow on top. There’s just not a lot of water movement over the course of the winter.”
At Sky Mountain Park, near Snowmass Village, soil contained a two-week average of 0.16 cubic meters of water per cubic meter of soil at an 8-inch depth for the first half of November 2022 and 0.19 m3/m3 of water in early-mid-December. That is close to the 2016-20 average of 0.19 and 0.21, respectively. By comparison, soil contained about 0.22 m3/m3 of water in the first two weeks of November 2021 and 0.27 m3/m3 of water in early to mid-December 2021. In late fall of 2020, soil contained at this site a two-week average of 0.12 m3/m3 of water in early to mid-November 2020 and 0.16 m3/m3 in the first couple of weeks of December 2020.
The station at Independence Pass, which sits at 12,080 feet, recorded soil containing an average of 0.17 cubic meters of water per cubic meter of soil at an 8-inch depth in the first half of November 2022 and 0.16 m3/m3 in early to mid-December. That is above the 2016-20 average of 0.14 m3/m3 and 0.12 m3/m3, respectively. By comparison, soil contained about 0.08 m3/m3 in the first couple of weeks of November 2021 and December 2021 — and it held 0.12 m3/m3 of water in the first half of November 2020 and 0.1 m3/m3 in early to mid-December 2020.
Looking across the Roaring Fork Basin, soil moisture on Nov. 2, 2022, measured 70% of average for the higher elevations sites and 100% of average at lower elevations, according to a NOAA map using estimates of soil moisture based on temperature and precipitation values compared with 1981-2010 modeled soil-moisture values. That is wetter than in November 2020, when soil moisture was mostly between 50% and 70% of average.
When spring comes, snow starts melting out and soil saturates. It begins to dry out over the course of the spring, and only a major rain event can change soil moisture.
During the second half of May, the monitoring stations recorded 0.29 m3/m3 of water at an 8-inch depth at the Sky Mountain Park station, which is on par with last year’s 0.28 m3/m3 and the average of 0.29, and it measured 0.23 m3/m3 of water at Independence Pass, slightly down from 0.24 m3/m3 in 2021 and the 2016-20 average of 0.25 m3/m3. (Last year’s data wasn’t available for the Independence Pass station at time of writing.)
Summer rain storms impact fire risk and plant life cycles. They also influence how much has to be made up by late-fall precipitation going into winter.
Low-elevation sites provide a different window on soil moisture
Low-elevation sites don’t provide the same information about spring runoff as that provided by high-elevation sites, where snow persists all winter. Soil moisture in Glenwood Springs, for example, is much more influenced by storms, rain events and warm days. Low-elevation sites can tell us how hot and dry soils got in mid-summer or whether there were any early rainstorms or snowstorms in a given year.
Soil moisture data from Glenwood Springs indicates that soils weren’t as dry last summer as they were in 2021. That station also showed that soils were holding a monthly average of 0.15 cubic meters of water per cubic meter of soil in July 2022, up from 0.13 m3/m3 in July 2021 but down from 0.20 m3/m3 in July 2019 — the highest level of moisture recorded for the month of July in the 2016-22 period.
Osenga said soil moisture is a useful tool that helps researchers better understand specific sites and how plants may have been impacted in a given year. ACGI is also looking at precipitation, air and soil temperatures, and relative humidity to better understand mountain hydrology and ecology.
“Soil moisture does not operate in isolation,” said Tanya Petach, climate science fellow at ACGI, said.”[It’s] intimately involved in many parts of our water cycle.”
This story ran in the Glenwood Springs Post-Independent, The Aspen Times and Grand Junction Daily Sentinel.