A mystery is emerging in the region around the Colorado River basin. For years, scientists have been puzzling over why water from Colorado’s snowpack does not find its way to the Colorado River as expected.

This puzzle has significant implications for the seven U.S. states and Mexico, whose vital resources such as hydroelectric power, irrigation and drinking water come from the river.

Traditionally, the snow cover accumulated during the cold months melts in the spring and flows into the river system.

Water managers have used the size of the snowpack in early April to predict annual water volumes. But since the early 2000s, these forecasts have consistently overestimated the actual amount of water, leaving us with a burning question: Where is the water going?

A piece of the puzzle: Hotter spring seasons

A current research project at the University of Washington offers crucial insights.

The results indicate that the discrepancy between the forecast and actual flow of the Colorado River is largely due to recent warmer and drier springs. This climatic change is believed to account for nearly 70% of the discrepancy.

The main reason for this is that there is less rain in spring and therefore less water flows into the rivers.

Lead study author Daniel Hogan is a doctoral student in the Department of Civil and Environmental Engineering at the University of Washington.

“The period of wondering, ‘Oh no, where is our water going?’ began around the same time that we experienced this decline in spring rainfall – the beginning of the ‘millennium drought’ that began in 2000 and continues to this day,” Hogan noted.

This change in weather patterns has caused local vegetation to become more dependent on meltwater for its water supply, absorbing a significant amount of it that would otherwise have been fed into the river system.

Less rainfall also leads to sunnier skies, which promotes plant growth and increases water evaporation from the soil.

Lack of water in the Colorado River

Hogan and lead author Professor Jessica Lundquist studied this phenomenon as part of a broader project to solve the major crime story surrounding the whereabouts of the water.

The research team initially suspected that the decreasing snow cover could be due to the direct conversion of snow into water vapor, a process called sublimation.

However, later studies showed that only 10% of the missing water was due to sublimation, suggesting that another factor was primarily responsible.

“There are only a limited number of possible culprits, so I started comparing things that might be important. And we saw that the changes are much more pronounced in the spring than in other seasons,” Hogan said.

“It’s a really dramatic change where you go from feet of snow to wildflowers blooming, and you do it 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.”

Water of the Colorado River Basin

Further analysis indicated a significant shift in environmental changes in spring, when there is a rapid transition from thick snow cover to blooming wildflowers that rely heavily on melting snow for their water needs.

The research team conducted its study in 26 headwaters at different elevations in the upper Colorado River basin, using a variety of data sets, including flow and precipitation measurements dating back to 1964. The goal was to create a comprehensive picture of how each basin has changed over time.

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

Spring precipitation in the Colorado River

The results of this study show that a lack of spring precipitation can lead to reduced water runoff in all basins, but the deficit is even greater in basins at lower elevations. This is because snow melts earlier in these basins, giving plants more time to grow and thus absorb more meltwater.

“In April, everyone wants to know how much water is in the snowpack each year. The problem with doing these calculations in April, however, is that spring has obviously not started yet,” explained Professor Lundquist.

“Now that we know that spring rains are actually more important than rains at any other time of year, we need to be better at predicting rainfall amounts to make our April forecasts more accurate.”

Current efforts are focused on refining our understanding of spring changes and their impact on water availability. For example, studies are underway to determine whether residual snow patches can act as mini-reservoirs and provide a sustainable supply of water to surrounding plants.

The study’s findings also suggest that these changes need to be taken into account in the water calculations carried out each April, especially as the millennium drought continues. As the role of spring rains becomes increasingly important, its prediction needs to be included in future forecasts to produce more accurate results.

The study was published in the journal Geophysical Research Letters.

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