The Colorado River is a lifeline for many cities and farms in the Southwest United States. It flows for about 1,448 miles before reaching the Gulf of California in Mexico and supplies water to numerous cities and farms along the way.
However, over the past 60 years, the amount of water in the Colorado River has been shrinking. In fact, in some years, the river's water has been used up completely before it reaches the gulf.
Landon Marston, assistant professor in civil and environmental engineering, teamed up with researchers from multiple universities and nongovernmental organizations to find out where the river's water goes and who uses it. This effort is informing state and federal decision-makers as they prepare to introduce new measures to bring the basin's water demands in balance with its dwindling supplies.
The research group, which includes two of Marston's graduate students, started by quantifying how much water people and businesses used from the Colorado River from 2000-19 from the records of the Bureau of Reclamation. These records serve as the basis for decision-making by local, state, and federal stakeholders concerned about the allocation of the Colorado River.
The group filled in gaps in federal records using a collection of models and data detailing crop-specific water consumption, water exported out of the basin by canals and pipelines, and evaporation from reservoirs and wetlands.
The study found that the agricultural demand for water is significantly higher than the water used by cities. The crops that need the most water are ones used for feeding cattle, such as alfalfa and hay, which are abundant in the area. The states that line the Colorado River raise roughly 14 million cattle per year.
"With water becoming scarcer and reservoir levels dropping, there is a growing need to find ways to use water more efficiently," said Marston. The reduction of water is crucial and will likely need to increase in the coming years due to competing demands and climate changes that reduce runoff into the Colorado River.
In 16 of the years analyzed, more water was taken from the river than what naturally flowed into it. To make up for the shortage, water was removed from reservoirs along the river, including Lake Mead and Lake Powell, which are among the largest reservoirs in the United States but have now become three-quarters empty.
The researchers found that the amount of water used actually varies a lot from year to year. In recent years, there has been less water used in the Lower Basin because of new rules that require more efficient use of water and cuts to the overall supply.
Another use of a significant amount of water is to support the natural environment along the river, including plants and wetlands.
"This is important because it wasn't accounted for in previous studies," said Marston. "This is not just about farms and cities. It is about protecting the river's ecosystem and ensuring a sustainable water supply for everyone in the southwestern United States."
Understanding where the water goes is crucial for making informed decisions about water use in the Colorado River Basin.
"The challenges are there," noted Marston. "But this data provides valuable information that can guide the seven states overlaying the Colorado River Basin and the federal government as they continue to negotiate how to share the dwindling water in the river."
