DENVER The changing climate could reduce the Colorado Rivers volume an average of 9 percent a year in the next half century, making it unlikely that Colorado will ever get to use all the water to which it is legally entitled, according to a new federal report.
Long droughts of five years or more will occur 40 percent of the time over the next 50 years, the report says.
Mondays report is the first of several in the Colorado River Basin Water Supply and Demand Study by the U.S. Bureau of Reclamation and the seven states that use the Colorado River.
Although several academic studies have been done on water and climate change, this is the first major effort by the federal government to see how warmer temperatures will affect the Southwestern United States.
The Colorado River basin includes all of Western Colorado. Thirty million people depend on the river, including residents of Los Angeles, Phoenix, Las Vegas, Denver and Albuquerque.
The river is already over-allocated, meaning people demand more water than nature usually supplies, according to the report.
Dan Grossman, regional director of the Environmental Defense Fund, greeted the findings.
Just as human health depends on healthy blood flow, the Colorado Rivers health depends upon healthy water flows that are being compromised by current management practices and policies, as well as a warming climate, Grossman said in a news release.
The report looked at four future scenarios, including one that applied global climate models to the Colorado basin. The climate scenario predicted future yearly flows averaging 13.6 million acre feet at Lees Ferry, the point just below Lake Powell that divides the Upper Basin states from California, Arizona and Nevada. The historic flow has been around 15 million acre-feet a year.
The last 11 years were the driest in a century of record-keeping on the river, according to the report.
Southwestern cities did not dry up because the many dams on the Colorado River system can hold about four times the rivers annual flow. But the two major reservoirs, Lakes Powell and Mead, saw large drops in their water levels.
The climate change part of the study predicted the San Juan Mountains will suffer some of the steepest declines in runoff from spring snowmelt in the whole seven-state area.
The lower predicted flow is bad news for Colorado, in part because of laws that govern which state gets water first. Under the 1922 Colorado River Compact, the first 15 million acre feet of water is divided in half between the Upper and Lower Basin states. But the more populous Lower Basin states get their half first.
Colorado politicians and water leaders have debated how much water Colorado has left to use. Legally, the state should expect more than 3 million acre feet from the Colorado River. But the study cautions against that optimistic assumption.
Absent the development of additional water supplies, the Upper Basin likely cannot realize full development of its Colorado River Compact apportionment with any level of certainty, the report says.
That bolsters the case of Western Colorado leaders, who say the state may not have enough water for more big pipelines to the Front Range.
The Colorado Water Conservation Board is working on its own study of how much water the state can expect from the river. Jennifer Gimbel, executive director of the CWCB, was happy that the seven states, which often spar over the river, set aside their differences to work on the study.
The Basin Study is a model of states working together on these issues in a more holistic and comprehensive manner, Gimbel said.
Reach Joe Hanel at joeh@cortezjournal.com