Runoff still depends on temperature, dust on snow
By Heather Sackett, Aspen Journalism
The month of March in Colorado was wet, with several storms bumping snowpack and spring runoff forecasts to above average across nearly all of the state.
That is the main take-away from the April 2024 Water Supply Outlook Report from the National Resources Conservation Service and good news for those who depend on water from the drought-plagued Colorado River basin.
All major river basins around the state received above-average precipitation for March, boosting snowpack to above median. March precipitation ranged from 138% of median in the northwest corner of the state to 186% of median in the Arkansas River basin.
“The March 13 through 15 storm cycle brought an even greater increase in precipitation across the state, with some areas of the Front Range and southern mountains receiving three to five feet of snowfall,” the report reads.
Snow water equivalent — a measure of how much water is contained in the snowpack — ranged from 121% of median in the South Platte basin to 104% of median in the San Miguel/Dolores/Animas/San Juan basin in the southwest corner of the state as of April 1. The Colorado River headwaters stood at 108% of median and the Roaring Fork River basin was at about 112% of median.
Since most of the West’s water supply is snowpack-driven, a snowpack snapshot at the end of the season can be a predictor of runoff volume. But there are other factors that could affect how much water ultimately ends up in rivers and eventually in the nation’s second largest reservoir, Lake Powell.
Higher-than-normal temperatures and windstorms that drop dust on the snowpack can both cause runoff to happen earlier and faster, and hot temperatures can also reduce streamflow amounts in other ways.
“What really influences our water supply is yet to come and that’s temperature, it’s dust, it’s weather,” said Dave Kanzer, director of science and interstate matters with the Colorado River Water Conservation District. “It’s not only the weather for the next 8 to 14 days, but 30 to 60 days that can quickly turn a good thing bad or a bad thing good. We are right at the fulcrum of our water supply.”
Streamflow forecast volumes across the state are at 103% of median. For the Colorado River headwaters streamflow is forecast at 105% of median; the Yampa/White/Little Snake is at 120% of median and the Gunnison River basin is at 104% of median. The southwest corner of the state is lagging behind, with a forecast of just 82% of median streamflow. Locally, streamflows in the Roaring Fork basin are forecast to be 104% of median.
High temperatures and dust on snow
Higher-than-normal temperatures can rob rivers of their flow. Studies have shown that Colorado River flows have declined nearly 20% from the 20th century average and that about one-third of that can be attributed to higher temperatures driven by climate change. Higher temperatures mean both a thirstier atmosphere and thirstier plants, which can suck up snowmelt before it makes it to rivers.
According to temperature data from snow telemetry (SNOTEL) sites, the months of October through February were all above average in the Colorado River headwaters. March temperatures, which set record highs worldwide according to the National Oceanic and Atmospheric Administration, were 100% of median in the Colorado River headwaters.
“We’ve had some cold snaps, but overall this warm winter doesn’t do us any favors in preserving the snowpack,” Kanzer said.
Like Kanzer mentioned, dust on snow can also cause rapid melting. White snow reflects the sun’s rays, but when the snowpack is coated with a darker layer of dust, it absorbs solar radiation, causing earlier and faster-than-normal melt out.
According to Jeff Derry, executive director of the Silverton-based Center for Snow and Avalanche Studies, Colorado’s mountains have experienced four storms that dropped reddish-brown dust from the desert southwest onto the snowpack, including a severe wind event March 1-3.
“That was a walloper, that was a biggie. It was pretty dirty,” Derry said. “Once that dirt layer is at the surface, it’s going to really kick things into gear.”
Oil and gas development, grazing, off-road vehicles and anything else that disturbs soil makes that soil susceptible to being carried by prevailing winds to the Colorado mountains. Derry said Colorado averages about eight or nine dust-on-snow events a year, some of which are probably yet to come.
“We get the most dust events in March, April and May,” he said. “So we are maybe just halfway through the dust season.”
Even though conditions are above average in the Colorado River headwaters, Lake Mead and Lake Powell still hover just above crisis level, a result of more than two decades of drought, increasing temperatures and overuse. In its mid-March 24-Month Study, which is the most recent available, the U.S. Bureau of Reclamation put the most probable spring runoff inflow forecast for Lake Powell at 78% of average. Lake Powell is currently about 33% full, at elevation 3,558 feet.
“From a water supply planning perspective, things are good short term and locally,” Kanzer said. “The long term, regional picture rapidly declines when you get to Lake Powell and below Lake Powell. Those chronic or persistent water supply concerns remain.”
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