Why isn’t Colorado snow flowing into the Colorado River? Research suggests it could be due to lack of spring precipitation

The Colorado River and its tributaries provide water for hydroelectricity, irrigation, and drinking water in seven U.S. states and Mexico. Much of this water comes from snowpack that accumulates during the winter and melts each spring. Each year in early April, water managers use snowpack to predict how much water will be available for the coming year.

But since 2000, these forecasts have proven wrong, with actual flows consistently falling short of forecasts. Water managers and researchers are left scratching their heads: Where is the water going?

The problem lies in the lack of precipitation in spring, according to a new study from the University of Washington. The researchers found that recent warmer, drier springs were responsible for nearly 70 percent of the gap.

With less rain, plants in the area rely more on snowmelt for hydration, leaving less water to flow into nearby streams. Less rain also means sunny skies, which help plants grow and water evaporate from the soil.

The researchers published these results on August 16 in Geophysical Research Letters.

“The period of wondering, ‘Oh, where is our water going?’ started around the same time we saw this decline in spring precipitation — the start of the ‘millennium drought,’ which began in 2000 and is still going on today,” said lead author Daniel Hogan, a UW doctoral student in the Department of Civil and Environmental Engineering.

“We wanted to focus on the cascading consequences of this. Less rain in the spring probably means fewer clouds. And if it’s sunny, the plants will say, ‘Oh, I’m so happy. The snow just melted and I have a ton of water, so I’m going to grow like a prey.’ This research really emphasizes the importance of studying the entire snow season, not just when the snowpack is deepest.”

Hogan and lead author Jessica Lundquist, a UW professor of civil and environmental engineering, studied this phenomenon as part of a larger project aimed at solving the great “mystery” of where water goes.

At first, researchers wondered whether the snowpack decrease was due to the snow turning into water vapor, a process called sublimation. But the team recently discovered that only 10 percent of the missing water was lost due to sublimation, meaning another factor was causing the problem.

“There are only a limited number of possible culprits, so I started comparing things that might be important,” Hogan said.

“We found that the changes in spring are much more pronounced than in other seasons. It’s a dramatic shift from several meters of snow to wildflowers blooming in a relatively short period of time. And without the spring rains, plants, from wildflowers to trees, are like giant straws, all drawing on the snow.”

The researchers studied spring changes in 26 watersheds at different elevations in the Upper Colorado River Basin. To build a picture of what was happening in each watershed over time, the team used a variety of data sets, including streamflow and precipitation measurements dating back to 1964. The researchers then modeled how much water plants in each watershed were likely to consume.

“We make an important assumption in our study,” Hogan said. “We assume that plants have unlimited water available, even with below-average precipitation, because they have access to snowmelt.”

All of the basins the team studied showed a decrease in streamflow in the absence of spring rains. But basins at lower elevations had an even more pronounced flow deficit. That’s because snow in these basins is likely to melt earlier in the season, giving plants more time to grow and consume the melted snow, the researchers explained.

Now that spring rains have been identified as the main culprit, researchers are working to refine their understanding of what happens during this season. For example, one project is investigating whether residual snow patches act as mini-reservoirs that can provide a steady flow of water to nearby plants.

Either way, the longer the millennium drought continues, the more these results will affect the water calculations that take place every April.

“April is when everyone wants to know how much water is in the snowpack each year,” Lundquist said.

“But the problem with doing these April calculations is that spring hasn’t arrived yet. Now that we know that spring rains are greater than rains at any other time of year, we’re going to have to do a better job of predicting what’s going to happen in terms of rainfall to make these April forecasts more accurate.”