Study: Small increases in Upper Colorado water use would cause big shortage risk

Increasing Upper Colorado River Basin water use by just 11.5 percent would double the risk that the Upper Basin fails to have enough water to meet its obligations under the Colorado River Compact, according to a new modeling study to be rolled out in a big meeting in Grand Junction, Colorado, next week.

The study hammers home an important point former Assistant Secretary of the Interior Anne Castle has been making: new uses on the Upper Colorado increase risk for existing users.

The analysis, done by John Carron of Hydros Consulting for Colorado’s four West Slope Basin Roundtables, suggests that even without increased use, the risk is high – 46 percent over the next 25 years based on hydrological assumptions chosen to take climate change into account. But Carron found that “an increase in annual Upper Basin Consumptive Use averaging 11.5% (approximately 500,000 acre-feet roughly doubles the risk.” The study’s results are being circulated in state of Colorado water management circles in preparation for a big meeting June 20 in Grand Junction.

(Disclosure: I worked with Carron on early phases of this work, as a consultant to the Colorado River Water Conservation District. I was not involved in the current work


The risk is basically this: taking more water for new uses leaves less of a margin, increasing the risk that the Upper Basin will not have enough water in storage during a spell of dry years to meet its obligations under the Colorado River Compact to deliver 82.5 million acre feet of water in each 10 consecutive years (7.5 million acre feet per year the Upper Basin owes the Lower Basin under the compact, plus an additional 750,000 to meet U.S. treaty obligations to Mexico). If that happens, 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.

To be clear, smart Upper Basin lawyers will argue that we don’t really owe all that water to the Lower Basin and Mexico, but the Lower Basin has smart lawyers too. I’d prefer not to go there.

There are tons of assumptions in the study that matter – Carron’s slides open with the famous George Box quote we use with our water resources modeling students: “All models are wrong, some are useful.” In particular, the key conclusions are based on the “stress test hydrology.” This is a technique developed by Carron and my book co-author Eric Kuhn that attempts to be more realistically pessimistic about the threat of climate change. I am not unbiased here – I think the stress test is a great “some models are useful” tool.

Water folks in Colorado will find the details of what Carron has done particularly useful, looking at the details of where within that state the vulnerabilities lie, based on a new analysis of the web of water rights seniority on the Colorado and its tributaries. This is a great model for what other states and the Upper Basin as a whole probably need to be looking at to better understand our water supply reliability risks. I’ve been trying to understand, for example, where similar vulnerabilities lie in my own state of New Mexico.

Details here.


  1. Not following the Colorado R. situation that closely; mostly focused on the Calif Delta and Central Valley, but in an article recently I read the assertion that the north basin has underutilized their allotment. Is this not right?

    “On stressed Colorado River, states test how many more diversions watershed can bear
    Arizona Public Media
    June 12, 2019
    “The 1922 Colorado River Compact, the river’s foundational governing document, gives Upper Basin states the legal cover to continue developing projects like the Gross Reservoir expansion. In the compact, each basin is allocated 7.5 million acre-feet of the river’s water. Over the decades the rapidly growing and intensely farmed Lower Basin has used much more than that. The less populated Upper Basin has never reached its full allotment. Those state have been using roughly 4.5 million acre-feet for the last 13 years, with the rest flowing downstream for the Lower Basin to use as it sees fit.”

  2. I’m kinda newbie to NM. In Silver City. Your last sentence: As I understand it, the NM CAP Entity is trying to get through a diversion plan on the Gila River. They could take up to 14,000 acre feet annually from the Gila and San Francisco. If Lake Mead drops below 1075 feet, under the DCP, the Gila River Indian Tribe has agreed to leave a certain amount of their apportionment in LM. So they would be getting less water from the Colorado.
    Meanwhile, the NM CAP Entity has to pay the going rate for CAP water to the GRIT for what they take. I keep thinking the GRIT would rather have the water. It’s a rather confusing mess, at least to me. It doesn’t seem like it is being addressed on any level? FD: I am against the NM CAP Entity. I’m just tossing this out. Been reading the blog a little, and appreciate your blogs. Thank you.

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