Water managers must plan for a drier future
By Summit Voice
FRISCO — Just a few weeks after the U.S. Bureau of Reclamation released a major report on the future of the Colorado, a new study in the journal Nature Climate Change reinforced some of the federal government findings, predicting a 10 percent drop in Colorado River flows during the next few decades.
That may not sound like much, but it’s 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, according to the researchers from Columbia University’s Lamont-Doherty Earth Observatory.
“It may not sound like a phenomenally large amount except the water and the river is already over-allocated,” said Richard Seager, a climate scientist at Columbia University’s Lamont-Doherty Earth Observatory and lead author of the new study.
The study expands on previous findings suggesting that human-caused climate change is making the American Southwest more arid as temperatures rise and rainfall patterns shift.
“The projections are spot on,” said Bradley Udall, an expert on hydrology and policy of the American West, at the University of Colorado, Boulder. “Everyone wondered what the next generation of models would say. Now we have a study that suggests we better take seriously the drying projections ahead.”
The Columbia study focused on the Colorado River headwaters, the greater California-Nevada region and Texas, which gets nearly all of its water from within state borders. It incorporates data from the Intergovernmental Panel on Climate Change’s upcoming assessment to estimate seasonal changes in precipitation, evaporation, water runoff and soil moisture in the near future, 2021-2040.
“It’s a much finer grain picture than the one we had in 2007,” said Seager.
Drying is expected in all three regions. Warmer temperatures will trigger more evaporation, even in places that may see greater seasonal rain or snowfall, according to the study. The models project that temperatures between 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 percent, and as much as 25 percent during springtime, as warmer temperatures boost evaporation. California and Nevada will also see big changes in spring, with a projected 20 percent drop in spring runoff; Texas will overall become drier with a 10 percent 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 percent 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.”