How climate change is increasing the unpredictability of Utah’s streams
Water is already a precious commodity in dry southern Utah.
But climate change may be making it even harder to come by — just by shifting when and how the region’s precipitation arrives.
A new study from the Desert Research Institute found that three big factors are hurting the reliability of streamflows in Utah and across the arid West.
Winter temperatures have increased over the past 40 years. During that same span, snowfall has decreased. And precipitation that used to come down as snow is falling as rain instead.
Abhinav Gupta, a Nevada-based researcher who led the study, said that all adds up to be bad news for Utah’s streams.
“Because of all these three factors combined,” Gupta said, “what we are seeing is that these fluctuations of streamflow become more wide.”
Even if Utah gets the same amount of total precipitation from one year to the next, the benefit the state derives from that precipitation can change based on how it falls.
When water arrives as winter snow, it gets stored for months and slowly trickles down as it melts. That creates a steady supply of water in streams and helps river levels rise and fall smoothly and predictably.
But if that water arrives in short bursts of intense rainfall, that makes it harder for the soil to absorb it, for reservoirs to capture it and for underground aquifers to refill with it.
The research published in the Journal of Hydrology examined climate and streamflow conditions nationwide from 1980-2013. In Utah, it focused on four watersheds: the White River and Salina Creek in central Utah and the Virgin River and Coal Creek in southwest Utah.
It can be tough to draw conclusions for the whole state from just a handful of examples, Gupta said, because each watershed is unique. But the signs don’t look promising.
“We are going from a stable state to an unstable state,” Gupta said. “The situation is not that bad yet, but we expect that it's going to get worse in the future.”
So what might the local impact of this trend look like in the southwest part of the state? Erich Mueller, a hydrologist with Southern Utah University, describes it as “flashy.”
Rivers rise and fall more rapidly. More severe storms and floods.
The exceptionally wet year Utah is experiencing now could be a symptom of this unpredictability, too. As climate change shifts temperature and precipitation trends over the long term, it could become more common to see extended periods of drought punctuated by short, intense wet seasons.
“It's not like we're not going to have wet years anymore,” Mueller said, “but maybe the pattern of wet and dry is changing.”
A study from the Utah Snow Survey showed that nearly all — north of 98% — of the water that sloshes down Utah’s streams comes from melting snow. Even in watersheds with lower elevations and warmer temperatures, such as the Virgin River in southwest Utah, the study estimates that snow contributes more than 90% of the streamflow.
Because snowpack elongates the time period in which that precipitation flows down, healthy snowmelt can provide dependable streamflow well into the summer. But warmer temperatures could drive more rapid snowmelt, both shifting the melting season earlier in the year and condensing it. So by the time the heat of summer arrives — when Utahns need water the most — there’s less streamflow around.
Streamflow petering out earlier in the summer, Mueller said, could even open the door for higher wildfire risks as soil and plants have more time to dry out.
And as rain becomes more prevalent in the colder months, another risk would be winter flash floods caused by showers falling on snowpack.
“It adds enough energy to both rapidly melt the snowpack [and] also we have that addition of the rain as well,” Mueller said. “So it's sort of like a double whammy.”
These types of floods haven’t been common, Mueller said, but have led to some of the worst flooding in southwest Utah’s history, such as the disasters that washed over the banks of the Santa Clara and Virgin rivers near St. George in January 2005 and December 2020, respectively.
Evaporation and population are two other factors that put extra pressure on Utah’s water supply.
As the climate gets warmer, more water is lost to evapotranspiration. In southwest Utah, Mueller said, up to 80-90% of all precipitation floats back into the atmosphere before it even ends up in streams and rivers. So towns like St. George are left with the remaining 10-20%.
If the region’s population today were the same as it was in 1950, Mueller said, that might not be much of an issue. But as one of the nation’s fastest-growing states, the demand for water is higher than ever.
“Just that alone,” Mueller said, “without any climate change at all, is going to put a stress on our water resource.”