Colorado River flows will keep shrinking as climate warms

Story by Mari N. Jensen at the University of Arizona

Warming in the 21st century reduced Colorado River flows by at least 0.5 million acre-feet — about the amount of water used by 2 million people in one year — according to new research from Colorado State University and the University of Arizona.

The research is the first to quantify the different effects of temperature and precipitation on recent Colorado River flow, said authors Bradley Udall of CSU and Jonathan Overpeck of UA.

“The future of the Colorado River is far less rosy than other recent assessments have portrayed,” said Udall, a senior water and climate scientist/scholar at the Colorado Water Institute, a unit within CSU’s Office of Engagement. “Our findings provide a sobering look at future Colorado River flows, and send a clear message to water managers that they need to plan for significantly lower river flows.”

The paper by Udall and Overpeck, “The 21st Century Colorado River Hot Drought and Implications for the Future,” went online Feb. 17 in the American Geophysical Union journal Water Resources Research. The Colorado Water Institute, National Science Foundation, the National Oceanic and Atmospheric Administration and the U.S. Geological Survey funded the research.

Significant water reduction 

From 2000 to 2014, the river’s flows declined to 81 percent of the 20th-century average, a total reduction of about 2.9 million acre-feet of water per year, with a warmer climate accounting for 0.5 million acre-feet per year and a reduction in precipitation levels making up the remainder. One acre-foot of water will serve a family of four for one year, according to the U.S. Bureau of Reclamation. Forty million people in seven U.S. Western states, plus the Mexican states of Sonora and Baja California, rely on the Colorado River for water.

From one-sixth to one-half of the 21st-century reduction in flow can be attributed to higher atmospheric temperatures since 2000, according to the researchers. Their analysis shows as temperatures continue to increase, Colorado River flows will continue to decline.

Current climate change models indicate temperatures will increase as long as humans continue to emit greenhouse gases into the atmosphere, but the projections of future precipitation are far less certain.

“This paper is the first to show the large role that warming temperatures are playing in reducing the flows of the Colorado River,” said Overpeck, Regents’ Professor of Geosciences and of Hydrology and Atmospheric Sciences at the University of Arizona and director of the UA Institute of the Environment.

The Colorado River Basin has been in a drought since 2000. Previous research has shown the region’s risk of a mega-drought – one lasting more than 20 years – rises as temperatures increase.

“We’re the first to make the case that warming alone could cause Colorado River flow declines of 30 percent by mid-century and over 50 percent by the end of the century if greenhouse gas emissions continue unabated,” Overpeck said.

The team began its investigation because Udall learned that recent Colorado flows were lower than managers expected given the amount of precipitation. The two researchers wanted to provide water managers with insight into how future projections of temperature and precipitation for the Colorado River Basin would affect the river’s flows.

Udall and Overpeck began by looking at the drought years of 2000-2014. About 85 percent of the river’s flow originates as precipitation in the Upper Basin – the part of the river that drains portions of Wyoming, Utah, Colorado and New Mexico. The team found during that time, temperatures in the river’s Upper Basin were 1.6 degrees F (0.9 C) higher than the average for the previous 105 years.

25 years of data

To see how increased temperatures might contribute to the reductions in the river’s flow that have been observed since 2000, Udall and Overpeck reviewed and synthesized 25 years of research about how temperature and precipitation affect the river’s flows. Water loss increases as temperatures rise because plants use more water, and higher temperatures increase evaporative loss from the soil and from the water surface and lengthen the growing season.

In previous research, Overpeck and other colleagues showed current climate models simulated 20th-century conditions well, but the models cannot simulate the 20- to 60-year mega-droughts known to have occurred in the past. Moreover, many of those models did not reproduce the current drought.

Those researchers and others suggest the risk of a multi-decadal drought in the Southwest in the 21st century is much higher than climate models indicate, and that as temperatures increase, the risk of such a drought increases.

“A mega-drought in this century will throw all our operating rules out the window,” Udall said.

Udall and Overpeck found all current climate models agree that temperatures in the Colorado River Basin will continue rising if the emission of greenhouse gases is not curbed. However, the models’ predictions of future precipitation in the Basin have much more uncertainty.

“Even if the precipitation does increase, our work indicates that there are likely to be drought periods as long as several decades when precipitation falls below normal,” Overpeck said.

The new study suggests Colorado River flows will continue to decline.

“I was surprised at the extent to which the uncertain precipitation aspects of the current projections hid the temperature-induced flow declines,” said Udall.

The U.S. Bureau of Reclamation lumps temperature and precipitation together in its projections of Colorado River flow, he said.

“Current planning understates the challenge that climate change poses to the water supplies in the American Southwest,” Udall said. “My goal is to help water managers incorporate this information into their long-term planning efforts.”

Read the journal article