#ColoradoRiver Compact obligations put #FrontRange transmountain diverters and Upper Basin in general at highest risk if there is a call #COriver #aridification

Here’s a report from Dennis Webb writing for The Grand Junction Daily Sentinel. Click through and read the whole article. Here’s an excerpt:

Upper Colorado River Basin water users are the most vulnerable on the Western Slope in the event of a call required by an interstate compact to curtail use, with much of that vulnerability resting with entities that divert water from that basin to the Front Range, new analysis shows.

John Carron, with Hydros Consulting, presented the latest work on potential risks facing the Western Slope in the case of continued long-term drought to a joint meeting of the four West Slope Basin Roundtables at the Ute Water Conservancy District in Grand Junction Thursday. This is the third phase of an analysis being conducted for West Slope water entities of risk arising from possible future water supply and demand levels.

Modeling used in the phase-three analysis found a 46 percent chance that over the next 25 years, the running 10-year average flow just below Lake Powell could fall below 82.5 million acre-feet. That threshold accounts for the average 7.5 million acre-feet a year that states in the Upper Colorado River Basin must allow to reach Lower Basin states over any 10-year period, plus another 750,000 acre-feet a year obligated to Mexico under a treaty.

Notably, the study finds that the risk doubles if annual Upper Basin water consumption increases an average of 11.5 percent a year…

New Mexico resident John Fleck…wrote a preview on Carron’s presentation last week on his blog, www.inkstain.net/fleck/. Fleck wrote that if average 10-year flows drop below 82.5 million acre-feet, “all sorts of hell could break loose, in court and/or as Colorado and other states scramble to cut back uses to meet downstream delivery obligations.”

[…]

A compact call is of most concern to those with water rights junior to 1922, when the interstate river compact was finalized. The Hydros study found that Colorado uses an average of about 2.5 million acre-feet of Colorado River water each year, with about 1.6 million of that involving pre-compact water rights, and some 932,000 acre-feet involving post-compact rights.

The upper Colorado River Basin accounts for about 626,000 acre-feet of post-compact water use, followed by southwest Colorado basins at 178,000 acre-feet, and the remainder spread between the Yampa, White and Gunnison basins. But about 532,000 acre-feet of the post-compact depletions within the upper Colorado basin are attributable to transmountain diversions, or about 57 percent of total post-compact depletions in the state, according to the report.

A curtailment of uses wouldn’t necessarily involve all 932,000 acre-feet of post-compact water in Colorado. The Hydros study dives into questions such as what are the oldest water rights (July 1957) that would be affected if 100,000 acre-feet were called, 300,000 acre-feet (September 1940), and 600,000 acre-feet (August 1935). It also breaks out the impact by acre-feet for each Colorado River sub-basin to reach each of those partial curtailment levels.

For purposes of discussion, and not because Hydros is advocating the idea, the report also explores the possibility that a curtailment in Colorado might be applied in some way other than simply on a statewide basis according to the age of water rights.

As its example, it calculates mandatory reductions in use based on each sub-basin’s post-compact water use as a percentage of statewide post-compact use.

Under such an approach, as an example, in the case of a 300,000 acre-foot statewide curtailment, the Gunnison Basin would be responsible for 18,436 acre-feet of that total, compared to 20,862 acre-feet if its percentage of overall use wasn’t considered. But southwest Colorado basins would be on the hook for about 57,000 acre-feet, about 17,000 more than when not considering sub-basin percentage of use.

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