Where is the Colorado River water going?

A mystery has been unfolding in the region encompassing the Colorado River Basin. For years, scientists have been scratching their heads, trying to understand why the water from Colorado’s snowpack isn’t making its way to the Colorado River as expected.

This conundrum has had significant implications for the seven U.S. states and Mexico that rely on the river for vital resources such as hydropower, irrigation, and drinking water.

Traditionally, the winter snowpack accumulated over the cold months melts in spring, feeding into the river system.

Water managers have used the snowpack’s size in early April to predict the annual water supply. However, since the early 2000s, these forecasts have been consistently overestimating the actual streamflow, leaving us with one burning question: Where is the water going?

A puzzle piece: Hotter spring seasons

A recent research endeavor from the University of Washington offers a crucial insight.

According to the findings, the discrepancy in predicted and actual water flow in the Colorado River could be largely attributed to recent warmer, drier springs. This climatic alteration is believed to account for almost 70% of the discrepancy.

The fundamental reason? Less rainfall in spring spells less water making its way into the streams.

Study lead author Daniel Hogan is a UW doctoral student in the civil and environmental engineering department.

“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,” noted Hogan.

This change in weather pattern has caused local vegetation to rely more on snowmelt for water, absorbing a significant portion that would have otherwise fed into the river system.

Decreased rainfall also translates into sunnier skies, promoting plant growth and increasing water evaporation from the soil.

Missing water in Colorado River

Hogan and senior author Professor Jessica Lundquist studied this phenomenon as part of a broader project to solve the big “whodunit” of where the water is going.

Initially, the research team hypothesized that the decreasing snowpack might be due to the snow directly transforming into water vapor, a process known as sublimation.

However, subsequent investigations revealed that sublimation was responsible for a mere 10% of the missing water, indicating another factor was predominantly responsible.

“There are only so many possible culprits, so I started to compare things that might be important. And we saw that springtime changes are a lot more exaggerated than they are in other seasons,” said Hogan.

“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.”

Water of the Colorado River Basin

Further analysis pointed towards a marked shift in springtime environmental changes. This period witnesses a rapid transition from thick snowpack to blooming wildflowers, all relying heavily on the melting snow for their hydration needs.

The research team conducted their study in 26 headwater basins at varying elevations in the Upper Colorado River Basin, incorporating diverse datasets, including streamflow and precipitation measurements dating back to 1964. The goal was to paint a comprehensive picture of each basin’s changes over time.

“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.”

Spring rainfall in the Colorado River

The outcomes of this research show that a lack of spring rainfall can lead to reduced streamflow in all basins. However, lower elevation basins witness a more significant deficit. This is because the snow in these basins melts earlier, providing plants with more growth time and hence, more snowmelt consumption.

“April is when everybody wants to know how much water is in the snowpack each year. But the problem with doing these calculations in April is that obviously spring hasn’t occurred yet,” explained Professor Lundquist.

“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.”

Efforts are now focused on refining our understanding of springtime changes and their impact on water availability. For instance, investigations are underway to determine whether residual snow patches can act as mini-reservoirs, providing a sustained water supply to surrounding plants.

The study’s findings also hint that the water calculations done each April will need to account for these changes, especially as the millennium drought continues. As the role of spring rain becomes more critical, its prediction will have to be factored into future forecasts for more accurate results.

The study is published in the journal Geophysical Research Letters.

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