The Colorado River has been overused for years, but no one knew exactly where all the water was going. Until now.

The Colorado River, vital for 40 million people, has suffered a 20% decrease in flows since the turn of the century. The two largest reservoirs in the U.S. — Lake Mead and Lake Powell — are just over a third full. Water negotiators across the American West are currently divided on how to manage the shrinking river and its reservoirs in the future.

For Brian Richter, a researcher who serves as the president of Sustainable Waters, an organization focused on water education, it’s never been more necessary to have a complete understanding of where the Colorado River’s water goes as it travels from its headwaters in the mountains of Colorado to its delta in Mexico.

His latest study, published today, is the most comprehensive look to date at how humans and nature use all of the water in the Colorado River Basin.

“Remarkably, the whole water budget of the Colorado River’s water has never been put together,” Richter told The Salt Lake Tribune. “This is a factual articulation of how all of the Colorado River’s water is being used, and we’re in a crisis situation. We have to reduce the overall consumption and then figure out how to shuffle it among all the different users.”

By “the whole water budget,” Richter referred to the study’s inclusion of the Gila River in Colorado River water calculations. The 1922 Colorado River Compact, the foundational document for managing the river, did not address the Gila River, which is located in Arizona and New Mexico. As a result, researchers previously did not fully account for consumption of the Gila River’s water when studying the Colorado River Basin.

“The most novel contribution of the paper,” according to Richter, is an account of how nature uses Colorado River water.

As water moves downstream, vegetation and wetlands rely on the river. Richter found that 19% of the Colorado River’s water is consumed by the natural environment.

The study, based on water consumption data between 2000 and 2019, also confirms a previously known fact: irrigated agriculture dominates the Colorado River. Richter and his team found that irrigated agriculture accounts for 52% of the overall consumption of the river’s water.

The majority of that water sustains water-intensive crops grown to feed cows, like alfalfa and hay. Of the Colorado River water directly used by humans, 46% goes to cattle-feed crops.

In the Upper Colorado River Basin — Colorado, New Mexico, Utah and Wyoming — these crops drink 90% of the Colorado River water used for irrigated agriculture. That’s three times the water consumed for municipal, commercial or industrial uses combined, according to the study.

“We’re going to have to see a substantial reduction in water use in irrigated agriculture if we’re going to bring this river back into balance,” Richter said.

Potential solutions include shifting to crops that require less water, which would likely require a financial incentive for farmers.

The new study breaks down water use to individual crops, a new level of specificity. For example, growing alfalfa consumed nearly 1.8 million acre-feet of water per year in the Upper Basin. An acre-foot of water is enough to sustain two households for a year.

In comparison, municipal, commercial and industrial uses in the Upper Basin consumed about 790,000 acre-feet of water per year — less than half of the water used for growing alfalfa.

“We recognize the inclination to point fingers at some of the bigger water users, and even some of the farmers that are growing particular crops,” Richter said. “But this is not an effort to generate blame. What we’re trying to do is create an honest, accurate base of knowledge to foster the most productive public dialogue possible.”

Richter and his team were focused on releasing the results of this study for water negotiators, who are in the midst of sensitive discussions about how to operate the Colorado River and its reservoirs in the future. Current operational guidelines expire in 2026.

Earlier this month, the Upper Basin states and the Lower Basin states (Arizona, California and Nevada) each submitted separate plans for doing so to the federal Bureau of Reclamation, an agency that owns and operates water infrastructure across the country.

Water negotiators haven’t come together to develop a united plan, divided on which states should have to cut their water use. Richter hopes that this new research can help.

“We wanted to make sure that the negotiators had the most accurate information possible for them to contemplate and debate,” he said. “We hope that this can reduce some of the noise.”