Fall soil conditions across the Upper Colorado River Basin are not as dry as in the past few years, but the amount of moisture packed into the dirt heading into winter is still below long-term averages, according to new modeling from the Colorado Basin River Forecast Center.
“Across many of the major contributing areas, the higher elevation areas that generate the most runoff, our model soil moisture conditions are near to below normal,” said Cody Moser, a senior hydrologist at the forecast center.
Soil moisture this time of year is an important factor for water managers who study weather conditions for a sense for how much runoff to expect in the spring. After a particularly wet 2019, the past two years have been hotter and drier across the West; those conditions dried out the soil, which then sucked up valuable snow melt before it ever made its way into a river or stream.
“The past couple seasons have seen some pretty significant impacts from soil moisture conditions leading into the runoff efficiency that you can see for the following season,” Moser said.
This year’s fall soil moisture map published by the forecast center shows an improvement in soil conditions in southwest Colorado, which is still dry but generally better overall when compared to last year. Those gains are owed in part to a steady pattern of monsoonal rains this summer.
Large parts of the Western Slope around Grand Junction and Gunnison and near the headwaters of the Colorado River have between 70% and 100% of the moisture packed into the soil when compared to an average from 1981 to 2010, according to the forecast center. Parts of southwestern Wyoming and around the Green River in Utah are not as well off, according to the models, registering between 30% and 70% of the average moisture in the soil.
For every 1% change in average soil conditions, the forecast center estimates a 0.5% change in runoff. A forecast center analysis explained it this way: “For a watershed that produces approximately a million acre-feet of water under completely average conditions, soil moisture conditions that are 1% wetter than average could yield an additional 5,000 acre-feet of water.” (An acre-foot is about 325,000 gallons or enough to supply two or three households annually.)
Dave Kanzer, director of science and interstate matters at the Colorado River District, said the current soil conditions are not quite as good as some were hoping for given the fairly consistent rain around the Western Slope this summer.
“It’s a little bit of an eye-opener,” Kanzer said.
Rain in July and August helped soak western Colorado soils, Kanzer said, but that progress stalled during a drier September and October.
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“Because of those two drier months, some of those gains — they weren’t erased but they weren’t magnified either,” Kanzer said. “Things are maybe better than they have been in a few years, but not stellar except in a few isolated spots.”
There are other factors, too, that impact spring runoff. A dry, windy spring with low relative humidity can sponge off valuable moisture that was locked into the ground in the fall, Kanzer said.
“It’s a diverse, complex situation,” he said.
Still, the impact of soil moisture on runoff is significant — so much so that the river district recently emphasized the need to better understand dry soil conditions, Kanzer said.
In October, the district approved a $60,000 grant for the nonprofit Aspen Global Change Institute to help fund continued monitoring of soil conditions at 10 AGCI stations throughout the Roaring Fork Valley. The district helps pay for various local and regional water projects through its Community Funding Partnership, a program Western Colorado voters approved via a mill levy increase in 2020. The partnership so far has distributed more than $5.5 million to more than 60 projects.
The additional focus on soil moisture in recent years is expected, said John Tracy, director of the Colorado Water Center at Colorado State University. He said other western watersheds have been dealing with this challenge for decades, but that the impacts are more noticeable in difficult drought years.
“What I think has happened is that this has been going on and it’s just that all of a sudden we’re really seeing the impacts and that’s why it’s getting so much more attention right now,” Tracy said.
It could still take multiple years to erase the soil moisture deficits that have built up across the Colorado River Basin, Tracy said.
“We need well above average snowfall to hit that average streamflow going into the future,” he said.