How will global warming affect Colorado River flows?

Water woes ahead for the Southwest

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The Colorado River will be hit hard by climate change. @bberwyn photo.

Even if precipitation stays the same or increases slightly in the next few decades, Colorado River flows are likely to dwindle due to increasing temperatures in the West. The projected warming in the 21st century could reduce flows by half a million acre feet per year, according to a new study to be published in the AGU journal Water Resources Research.

Most previous studies on how global warming will affect Colorado River flows have lumped together the effects of precipitation and temperature changes. The latest research is the first to try and tease out the different effects of temperature and precipitation, according to authors Bradley Udall of Colorado State University and Jonathan Overpeck of the University of Arizona.

Another study last year linked declining flows with warmer spring temperatures, and other research shows how dust-on-snow events can shift runoff regimes, which also affects total flows. Declining Colorado River flows have already been the subject of congressional hearings, partly because of the concern over potential interstate water conflicts.

The team began its investigation because Udall learned that recent Colorado flows were lower than managers expected given the amount of precipitation.

Between 2000 and 2014, Colorado River flows were only about 81 percent of the 20th century average,  a reduction of about 2.9 million acre-feet of water per year. According to the research, between one-sixth to one-half of that reduction can be attributed to warmer temperatures. During that period, temperatures in the Upper Colorado River Basin ( Wyoming, Utah, Colorado and New Mexico) were 1.6 degrees Fahrenheit (0.9 C) higher than the average for the previous 105 years.

Projecting future changes is critical to the 40 million people who depend on the river in seven western states and Mexico.

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

Despite a few wet winters, The  Colorado River Basin has generally been experiencing drought conditions since 2000 and several studies suggest the risk of 20-year “megadroughts” is increasing with global warming.

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

The Colorado Water Institute, National Science Foundation, the National Oceanic and Atmospheric Administration and the U.S. Geological Survey funded the research. Read more on the study here.

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