A new study of high-mountain snow and rain suggests researchers should pay more attention to spring conditions than scientists previously thought. The new data could help refine water supply forecasts for the streams that feed the Colorado River.

Researchers at the University of Washington initially conducted a study near Crested Butte, Colorado, to collect data on how snow behaves during the winter, but found that weather and climate factors during the warmer months had a noticeable impact on the amount of meltwater that ultimately flowed into streams and rivers.

“We thought it was because of the snow,” said Jessica Lundquist, a co-author of the study. “In fact, spring is the most dynamic season when we look at all the statistics for all seasons. That tells us we should focus on what’s happening in spring.”

Water forecasts for the Colorado River, which serves about 40 million people in seven western states, are based largely on snow in the mountains. Eighty-five percent of the Colorado River’s flow begins as snow in the high mountains of Colorado and Wyoming.

As climate change and constant demand shrink the river’s water reserves, scientists are working to collect increasingly accurate data on mountain snow and its melting behavior, allowing water managers – who sometimes live in cities hundreds of miles away – to more accurately estimate how much water will flow to them each year.

In this case, spring is defined as March, April and May. The factors that make these months so important are sunshine, plants and rain.

When it’s sunny in spring, plants are thirstier and take up more meltwater from the soil. When it’s cloudy and rainy, two things happen: plants are less thirsty and take up less water from the soil, and the water they get is freshly fallen rain rather than meltwater.

“When it gets sunny, the plants will say, ‘Oh, I’m so happy. The snow just melted and I have tons of water, so I’m going to grow like crazy,'” said Danny Hogan, the study’s other author. “This research really underscores the importance of studying the entire snow season, not just when the snowpack is at its thickest.”

Hogan and Lundquist said the research, published in the journal Geophysical Research Letters, is important because it helps explain why water flow has fluctuated so much in recent decades. Since 2000, water flow in the Colorado River basin has declined by 19 percent. The new data show that spring rainfall and crop water needs account for about two-thirds of that change.

Rising temperatures in the 21st century have made it harder to predict the amount of water entering the Colorado River each year. Historically, forecasts have focused primarily on the amount of snow in the mountains. But in recent years, a widening gap has emerged between the amount of snow that falls and the amount of meltwater that flows through streams, rivers and reservoirs.

Science is only just beginning to understand what causes this gap.

Hogan and Lundquist originally wanted to investigate another, less well-understood factor that influences the difference between snow amounts and water runoff. At their research site near Crested Butte, they first studied snow sublimation – the process by which snow evaporates before it can melt.

For the winter of 2023-2024, models predicted that 30 to 40 percent of snow would be lost to sublimation. The team found that about 10 percent of snow was actually lost to sublimation, less than the models predicted. These findings paved the way for the paper on spring conditions.

Other scientists are also studying the impact of climate change on runoff. Much of this work focuses on soil moisture. Early results suggest that warmer temperatures cause drier soils, which soak up meltwater before it can reach streams and rivers.

This story is part of ongoing coverage of the Colorado River produced by KUNC and supported by the Walton Family Foundation. KUNC is solely responsible for editorial coverage.