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.