Researchers at the University of California, Los Angeles (UCLA) claim in a new report that human-caused global warming has caused a substantial decrease in the amount of snowpack runoff leading to a decrease in flow of the Colorado River roughly equivalent to the size of Lake Mead.
The study published in Water Resources Research in July showed man-made carbon dioxide (CO2) emissions since 1880 have resulted in a roughly 10% reduction in water availability in the Colorado River under present-day conditions.
According to a press release by the authors, climate change during the megadrought of 2000-21 resulted in the loss of the equivalent of the size of Lake Mead in Nevada, or roughly 10 trillion gallons of water.
“While we knew warming was having an impact on the Colorado Basin’s water availability, we were surprised to find how sensitive the basin is to warming compared to other major basins across the western U.S., and how high this sensitivity is in the relatively small area of the basin’s crucial snowpack regions,” said Benjamin Bass, lead author of the study.
He continued, “The fact that warming removed as much water from the basin as the size of Lake Mead itself during the recent megadrought is a wakeup call to the climate change impacts we are living today.”
The study showed that from 1880 to 2021, the temperature in the Colorado River Basin rose 2.7 F, contributing to a 10.3% decrease in runoff. That amount would be closer to 13% were it not for the effects of plants in the basin. The study estimates that every 1.8 F increase in temperatures can lead to a drop of 6.8% in the Colorado River’s flow.
The most dramatic effect has occurred in snowpack regions, causing runoff to decline at double the rate compared to non-snowpack regions. Despite only making up roughly one-third of the basin’s drainage area, 86% of runoff decreases are driven by water loss in these regions. Snowpack regions are experiencing greater aridification than non-snowpack regions, resulting in a more significant reduction in water runoff efficiency.
According to Bass, “The rapid water loss in snowpack regions is a sign that the Rocky Mountain West is transitioning to a more arid climate rather than simply undergoing periodic droughts.”
The study concluded without reducing greenhouse gas emissions, the shortage of water and reduced runoff will likely continue as temperatures increase even with vegetation.
Conservation
The study comes after a second study showed runoff has declined while consumption remains constant.
Bass pointed out in the news release that when the 1922 Colorado River Compact was signed, the climate would remain stable, and 15 million acre-feet (an acre-foot is 325,851 gallons) would be provided from the river for the Upper and Lower Basin states.
“If they were outlining the compact now, they would need to adjust that value due to the impact of warming,” Bass said. “Going into the future, we may get some natural variability, wet or dry swings, but this study highlights that there’s been a decreasing trend in runoff.”
The study also confirms that runoff has declined throughout the 20th and 21st centuries. From 2000-23, the runoff at Lees Ferry, AZ, was 12.5 million acre-feet, a 13.5% decline from the average annual river flow between 1930-99 and 30% less than the annual flow between 1906-29.
The amount of water stored in Lake Mead and Lake Powell declined by 33.5 million acre-feet between January 2000 and April 2023. Both lakes are now at 30% capacity, according to the study published in June 2023. The study suggests a 13-20% decline in basin-wide use would allow for stabilization and some reservoir storage recovery.
In May, the Lower Basin states of Arizona, California and Nevada announced an agreement to conserve 3 million acre-feet through 2026. The federal government would compensate farmers and ranchers through the Inflation Reduction Act for an estimated $1.2 billion for water conservation.
Bureau of Reclamation Commissioner Camille Calimlim Touton called the agreement “an important step forward towards our shared goal of forging a sustainable path for the basin that millions of people call home.” — Charles Wallace, WLJ contributing editor
