‘The median of a bunch of model runs does not equal a prediction’ — Hannah Holm #coriver

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From the Grand Junction Free Press (Hannah Holm):

[Last] week, at the Colorado Water Congress annual convention in Denver, water providers and stakeholders from around the state had the opportunity to hear presentations and commentary on some of the study’s finer points and limitations. Here are a few of the key messages that came through in that discussion.

THE LOWER BASIN IS ALREADY IN A PICKLE
Lower basin water users, those that pull water from the Colorado River downstream from Lake Powell, are already using more water each year than they have a right to expect under the terms of the 1922 compact that divided the river between upstream and downstream users.

That’s because the upper basin states, including Colorado, have grown more slowly than California and Arizona, and have consequently allowed more water to flow downstream than is legally required. That’s why there’s still a debate in Colorado about how much more water can be taken out of the Colorado up here, despite the fact that basin-wide, uses are already exceeding supplies.

ENVIRONMENTAL AND RECREATIONAL FLOWS ARE VERY VULNERABLE
The study included modeling different supply and demand scenarios and management actions to see how they would affect the likelihood of hitting key indicators of shortages, both for human water users (levels in Lake Mead, for example), and the environment (low flows at key points). Projecting out toward 2060, the models indicate increasing numbers of years when fish are likely to be in trouble. Some of the management tools appear to have promise for reducing this vulnerability, but no actions would eliminate it. The study also showed that flows too low for enjoyable (and profitable) whitewater recreation were also likely to become more frequent.

THE MEDIAN OF A BUNCH OF MODEL RUNS DOES NOT EQUAL A PREDICTION
Several panelists made the point that climate change models, downscaled to fit the Colorado Basin, produce many different projections of water supply. The median of all the outputs shows water inflows to the basin reduced by 9% by 2060, but that doesn’t mean that this is what will actually happen.

More Colorado River Basin coverage here and here.

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