Researchers use IPCC AR5 models to forecast 10% drop in Colorado River flow in next few decades

23 December 2012

In a new study in Nature Climate Change, climate modelers at Columbia University’s Lamont-Doherty Earth Observatory predict a 10% drop in the Colorado River’s flow in the next few decades, enough to disrupt longtime water-sharing agreements between farms and cities across the American Southwest, from Denver to Los Angeles to Tucson, and through California’s Imperial Valley. The Colorado River Basin encompasses much of the American West, covering parts of seven US states and two states in Mexico.

It may not sound like a phenomenally large amount except the water and the river is already over-allocated.

—Richard Seager, lead author

The study expands on findings published in 2007 in the journal Science that the American Southwest is becoming more arid as temperatures rise and rainfall patterns shift. It also comes on the heels of a major study of the Colorado River Basin by the US Department of Interior that projected longer and more severe droughts by 2060, and a 9% decline in the Colorado’s flows. (Earlier post.)

The present study narrows in on three key regions for water managers—the Colorado River headwaters; the greater California-Nevada region; and Texas, which gets nearly all of its water from within state borders. The study makes uses new simulations from the Coupled Model Intercomparison Project Five, to be assessed in Intergovernmental Panel on Climate Change Assessment Report Five (AR 5) due out next fall, to estimate seasonal changes in precipitation, evaporation, water runoff and soil moisture in the near future, 2021-2040.

Drying is expected in all three regions, as warmer temperatures trigger more evaporation, even in places that may see greater seasonal rain or snowfall, the study found. The models project that temperatures in 2021-2040 will be 1 to 2 degrees Celsius warmer than now. The Colorado headwaters are expected to see more precipitation on average, but annual stream flow is expected to decline by 10%, and as much as 25% during springtime, as warmer temperatures boost evaporation, the study found.

California and Nevada will also see big changes in spring, with a projected 20% drop in spring runoff; Texas will overall become drier with a 10% decline in annual runoff. For Texas the models predict that precipitation will decrease and evaporation rates will also go down in spring and summer, but only because “there is no moisture to evaporate,” said study co-author Mingfang Ting, a climate scientist at Lamont-Doherty.

Population growth in the Southwest is putting added pressure on regional water resources. To put the Colorado flow projections in context, a 10% decline is about five times the amount of water that Las Vegas uses in a year, said Udall. With alternate water sources tapped out, the West will likely have to meet the decline by cutting back on water use. “You can’t go build another water project,” he said. “That’s what makes this problem so difficult.”

Resources

• Richard Seager, Mingfang Ting, Cuihua Li, Naomi Naik, Ben Cook, Jennifer Nakamura & Haibo Liu (2012) Projections of declining surface-water availability for the southwestern United States. Nature Climate Change doi: 10.1038/nclimate1787