Vanishing Water Mystery: Scientists Uncover Why the Colorado River Is Running Dry

Dry spring weather is causing plants to use more snowmelt, reducing Colorado River streamflow. This explains why runoff predictions have missed the mark since 2000, highlighting the need to improve forecasts by factoring in spring rainfall.

The Colorado River and its tributaries supply water for hydropower, irrigation, and drinking to seven U.S. states and Mexico. A significant portion of this water originates from snowpack that accumulates during the winter and melts in the spring. Each year, typically in early April, water managers assess the snowpack to forecast how much water will be available for the coming year.

However, since 2000, these forecasts have repeatedly overestimated the actual streamflow. Despite snowpack levels appearing sufficient, the resulting water flow has consistently fallen short of predictions. This persistent discrepancy has left researchers and water managers puzzled — where is the missing water?

According to new research from the University of Washington, the main cause of the discrepancy is a lack of spring rainfall. The study found that warmer, drier spring conditions in recent years account for nearly 70% of the gap between predicted and actual streamflow. With less rainfall, vegetation depends more heavily on snowmelt for moisture, reducing the amount of water that reaches rivers and streams. In addition, drier conditions mean clearer skies, which promote both increased plant growth and greater evaporation of water from the soil.

These findings were recently published in the journal Geophysical Research Letters.

A Long-Term Pattern Emerges

“The period of time when we were wondering, ‘Oh no, where’s our water going?’ started around the same time when we saw this drop in spring precipitation — the beginning of the ‘Millennium drought,’ which started in 2000 and has been ongoing to the current day,” said lead author Daniel Hogan, a UW doctoral student in the civil and environmental engineering department. “We wanted to focus on the cascading consequences of this. Less springtime rain means you likely have fewer clouds. And if it’s going to be sunny, the plants are going to say, ‘Oh, I’m so happy. The snow just melted and I have a ton of water, so I’m going to grow like gangbusters.’ This research really centers the importance of studying the whole snow season, not just when the snowpack is the deepest.”

Hogan and senior author Jessica Lundquist, a UW professor of civil and environmental engineering, studied this phenomenon as part of a bigger project to solve the big “whodunit” of where the water is going. At first, the researchers wondered if the snowpack was decreasing because the snow was simply turning into water vapor — a process called sublimation. But the team recently discovered that only 10% of the missing water was lost due to sublimation, meaning something else was the main culprit.

“There are only so many possible culprits, so I started to compare things that might be important,” Hogan said. “And we saw that springtime changes are a lot more exaggerated than they are in other seasons. It’s this really dramatic shift where you’re going from feet of snowpack to wildflowers blooming over a very short amount of time, relatively speaking. And without spring rains, the plants — from wildflowers to trees — are like giant straws, all drawing on the snowpack.”

Tracking the Water in Headwater Basins

The researchers looked at springtime changes in 26 headwater basins at various elevations in the Upper Colorado River Basin. To paint a picture of what was happening at each basin over time, the team used a variety of datasets, including streamflow and precipitation measurements dating back to 1964. The researchers then modeled how much water the plants at each basin would likely consume.

“We make an important assumption in the paper,” Hogan said. “We assume that the plants have an unlimited amount of water even with less-than-average precipitation, because they have access to snowmelt.”

All the basins the team studied showed reduced streamflow without springtime rain. But basins at lower elevations had an even more pronounced deficit in streamflow. This is because the snow at these basins is likely to melt earlier in the season, giving the plants more time to grow and consume the snowmelt, the researchers said.

Now that spring rain has been identified as the main culprit, the researchers are working to further refine their understanding of what’s happening during this season. For example, one project is investigating whether residual patches of snow are acting as mini-reservoirs that can provide a constant stream of water to nearby plants.

Regardless, the longer the Millennium drought continues, the more these results will affect the water calculations that happen each April.

“April is when everybody wants to know how much water is in the snowpack each year,” Lundquist said. “But the problem with doing these calculations in April is that obviously spring hasn’t occurred yet. Now that we know spring rain is actually more important than rain any other times of the year, we’re going to have to get better at predicting what’s going to happen rainwise to make these April predictions more accurate.”

Reference: “Recent Upper Colorado River Streamflow Declines Driven by Loss of Spring Precipitation” by Daniel Hogan and Jessica D. Lundquist, 16 August 2024, Geophysical Research Letters.
DOI: 10.1029/2024GL109826

This research was funded by the National Science Foundation, the Sublimation of Snow Project, and the Department of Energy Environmental System Science Division (the Seasonal Cycles Unravel Mysteries of Missing Mountain Water project).

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