The Colorado River: Dwindling Under Climate Change

One of the disconnects between science as it’s practiced and science as it’s understood by the public is “the results of the latest study”. When we report on “the results of the latest study,” the public is likely to be left shouting: “But last week I though they told me coffee was good for me!”

The trick, dear reader, is to look for consistency. If one study says coffee is good for you and the next one says it’s bad for you, chances are good this is an area where the science is hard and/or immature, and it’s wise to wait. On the other hand, if all the studies seem to be pointing in the same direction, using different methodologies and approaches the question, then chances are you can begin to think about hanging your hat on the results. It’s not that any one of them is the answer, so much as their collective wisdom is likely to be pointing you in a fruitful direction.

Such should, I think, be the response to a new paper in GRL next week by Greg McCabe and David Wolock on the Colorado River and climate change. The Colorado is the primary water source for much of the Southwestern United States. It’s already stretched to the limit. And McCabe and Wolock say that climate change is likely to mean a lot less water in the river.

There’s significant uncertainty in the models about whether precipitation in the Upper Colorado River Basin goes up or down as a result of anthropogenic climate change. But increasing temperature means more evaporation, which means less of whatever water falls from the sky actually ends up in the river. The average flow under a 2 degrees C warming is 17 percent less, according to the analysis by McCabe and Wolock. That’s consistent with previous studies, which have shown decreases of anywhere between 8 percent and 45 percent. (See “is coffee good for me or not” above. This coffee is consistently “not”.)

The really interesting bit of the new study is the way the authors used the tree ring record and a river management model to estimate the percentage of the time the flow in the Colorado would be insufficient to meet the requirements of the Colorado River Compact.

During the 20th century, the river failed to deliver enough water to meet compact requirements 7 percent of the time. If you take the 20th century record, slap a 2 degree C warming on top, that rises to 37 percent of the time. But if you take the driest period in the tree ring record and slap global warming on top of it, that rises to a whopping 77 percent of the time!

More reading:

  • Warming may create substantial water supply shortages in the Colorado River basin, McCabe and Wolock, Geophys. Res. Lett., 34, L22708, doi:10.1029/2007GL031764. full paper for GRL subscribers, abstract (maybe, I was getting an error on the abstract link Saturday)
  • nice summary by Brad Udall of the various Colorado River climate change studies
  • Past peak water, Hoerling and Eischeid
  • Christensen and Lettenmaier, A multimodel ensemble approach to assessment of climate change impacts on the hydrology and water resources of the Colorado River


  1. Diversions from the tributaries of the Colorado River are the hidden dilemma…

    The US Supreme Court decided in 1964 that states can develop their Colorado River tributaries for use within their state and it does not count against a state’s Colorado River allocation.

    This unbelieveable decision has allowed hundreds of thousands of acre feet to be 100% diverted from the Colorado River basin without being subtracted from a State’s entitlement pursuant to the Compact.

    Rather like saying to someone they can make whatever money they wish and as long as they don’t deposit it in a particular bank account, they don’t have to pay any taxes on the money !

    Once the tributaries’ flow reaches the mainstem of the Colorado River, however, its legal status changes. In mingling with the waters of the Colorado River the tributary flow becomes interstate water controlled by the U.S., subject to the 1922 Colorado River Compact and subsequent acts and agreements dividing the river’s resources. In other words, the Law of the River rules rather than state law, with the water now available for diversion by the Lower Basin States and for meeting the water obligation to Mexico.

    Cute ! As long as the upper basin states divert their tributary waters and keep it out of the mainstem Colorado River, it doesn’t count towards their share of the Colorado River water.

    Not only have thousands of such diversions occurred since 1922, many huge diversions totally remove the water away from the Colorado River to population centers such as Denver, Fort Colliins, Colorado Springs and Greeley…which are on the east side of the continental divide. The Colorado River flows on the west side of the divide, so all water to Denver area is 100% lost to the downstream states.

    Basic water law states that a user is entitled to the conditions that existed at the time he made his appropriation…the conditions in 1922 when the downstrem states agreed to the Compact were that all those diversions since 1922 did not exist at the time and were not depleting the Colorado River!

    So much for honor among … water manipulators.

    As if things were not bad enough, Denver is planning another 350,000 acre foot per year pipeline diversion from Flaming Gorge on the tributary Green River to the Denver area which will be a 100% loss of 350,000 acre feet per year to the Colorado River.

    In addition to the drought, slowly but surely, the Colorado River will be depleted by diversions from its tributaries…”that don’t count” against Compact entitlements.

    Also, maybe slowly but surely, California & Nevada will come to appreciate a new fresh water Source that will yield ONE MILLION acre feet a year without damage to the water rights of others. The Source is not only legally & physically available, but economically feasible and environmentally balanced.

    Additional water attorneys encouraged to review disclosure of the Source to confirm its validity.

    Ray Walker (Retired Water Rights Analyst)

  2. Ray –

    Thanks for this informative comment. Can you point me to the Supreme Court decision itself, or any literature on it (law review articles, etc.)?

  3. There was a good article in the Rocky this past weekend about the Front Range starting to stir about doing something to take the water from the Yampa, reducing tributary flow to the Colorado.

    Also, speaking of tree-ring records and CO River flow, this brief, and Woodhouse C.A. and Lukas J.J. 2006. Multi-Century Tree-Ring Reconstructions of Colorado Streamflow for Water Resource Planning. Climatic Change 78:2-4 23pp are informative about past water-climate regimes. RP Sr also had something to say about societal adaptation, & I commented about that here (grumpy Dano).



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