The water level of the Colorado River has been falling below forecast levels for decades, puzzling scientists.

According to a new article in the journal Geophysical Research Letters, researchers may have discovered that spring rains could be the cause.

Researchers estimate the amount of water in the Colorado River based on the snowpack that accumulates in the Rocky Mountains during the winter. However, since 2000, water levels have consistently been lower than predicted. This discrepancy may be due to lower precipitation in the spring months, the researchers suspect.

According to the study, 70 percent of this water shortage could be due to the fact that the springs have become drier over the past 20 years.

During the winter months, snow accumulates in the mountains, particularly in the Rocky Mountains. As temperatures rise in the spring, the snowpack begins to melt, flowing down mountain slopes as surface runoff and traveling through small streams and tributaries until it reaches larger rivers such as the Colorado.

“It is known that about 70 to 80 percent of the Colorado River’s water initially escapes as snow in the mountains. Since 2000, the Colorado River’s water volume has declined by 20 percent,” said study co-author Daniel Hogan, a researcher at the University of Washington in Seattle. Newsweek“It is difficult to determine exactly how much water comes from snow and other sources, but our research suggests that the contribution from snowpack may have decreased.”

When there is less rain in the spring, nearby plants rely on snowmelt rather than rain. This means less water overall makes it into streams and eventually rivers. Less spring rain also means sunnier weather is more likely, which increases water use by plants and promotes evaporation of water from the soil.

“We observed that the headwaters of the Colorado, where the rivers that feed the Colorado originate, have experienced a significant decline in spring precipitation since 2000 that was not seen in other seasons. This timing also coincided with the sharp decline in flow volumes seen in the Colorado River,” Hogan said. “We found that the decline in spring precipitation had two effects: less precipitation fell, which obviously reduces flow volumes, and plants and trees that normally rely on spring precipitation to grow later in the year can only rely on water from the winter snowpack to grow, resulting in less meltwater entering the rivers.”

The researchers describe that they initially thought that sublimation – the process by which snow or ice turns directly into water vapor – could explain the lack of water loss. However, they found that this process was only responsible for 10 percent of the water loss.

“There are only a limited number of possible culprits, so I started comparing things that might be important,” study co-author Jessica Lundquist, a professor of civil and environmental engineering at the University of Washington, said in a statement. “And we saw that the changes are much more pronounced in spring than in other seasons. It’s this really dramatic change where you go from a foot-thick blanket of snow to wildflowers blooming, and in a relatively short period of time. And without spring rain, the plants – from wildflowers to trees – are like giant straws, all feeding off the snowpack.”

The researchers modeled how much water the plants use along the Colorado’s 26 headwaters.

“We make an important assumption in our study,” Hogan said. “We assume that even when precipitation is below average, plants have an unlimited amount of water because they have access to melting snow.”

They found that models showed reduced runoff when there was less rainfall in the spring, with the largest deficit occurring in basins at lower elevations.

The decline in spring precipitation may be due to the megadrought that Colorado and other southwestern states have suffered since the turn of the millennium.

“The reasons for this change are not entirely clear,” Hogan said. “Most likely, a shift toward warmer and drier springs can be linked to annual variability. However, climate change may play an important role here, but unfortunately not enough research has been done to examine the long-term impacts of climate on spring precipitation in the region. If it can be shown that climate change is significantly affecting spring precipitation, then further declines in water volumes can be expected.”

This loss of water from the Colorado River could have future impacts on the water levels of Lake Mead and Lake Powell, both important reservoirs that provide water and hydroelectric power to millions of people.

“If dry springs continue, water levels are expected to continue to decline rapidly. The winter season plays a critical role in regulating water flow and deserves the attention it receives, but our work shows that spring is a critical bridge between the wet, cool months and the warm summer months. Therefore, understanding the impacts of spring weather conditions on water availability is important for Colorado River water storage,” Hogan said.

These findings may impact the way we calculate the amount of water available for the year, since this measurement is typically taken in April, after snowpack has peaked.

“In April, 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 has obviously not started yet. Now that we know that spring rain is actually more important than rain at any other time of year, we need to do a better job of predicting what’s going to happen in terms of rain to make these April predictions more accurate.”

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References

Hogan, D., & Lundquist, JD (2024). Recent water levels on the upper Colorado River
Declines are due to the decrease in spring precipitation. Geophysical Research Letters51, e2024GL109826. https://doi.org/10.1029/2024GL109826