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New study offers first complete accounting of Colorado River water use

The Colorado River through Grand Canyon National Park.
B. Healy
NPS Photo
The Colorado River through Grand Canyon National Park.

A paper published Thursday gives, for the first time, a full accounting of where the Colorado River’s water goes.

KNAU’s Melissa Sevigny spoke with lead author Brian Richter about how the information can help guide the ongoing negotiations between seven U.S. states and Mexico on how to manage the river’s diminished water supplies in a future of drought and climate change.

Let’s talk about this breakdown in numbers…. Half of all the water that you found in your accounting is going to irrigated agriculture and a good part of that, if I remember, is going specifically to crops that feed cattle.

Exactly. So one of the things that about our numbers that is catching a lot of attention is the volume of the water or the percentage of the water that goes to producing two cattle feed crops, alfalfa and grass hay; very essential to dairy and beef production, of course. Our estimate is that about a third of all of the river’s water is going to those two cattle feed crops.

And it was surprising to me that 11 percent of the water in the Colorado River Basin is lost through evaporation from reservoirs. That’s a pretty big number.

Yes, it is. And unfortunately there’s not a whole lot we can do about evaporated water…. As the reservoirs shrink in area, which has been happening pretty precipitously over the last couple of decades, the total evaporation does decrease a little bit, but for the most part it’s about 11 percent of all the water in the Colorado River system. Lake Mead and Lake Powell each account for about 3 percent each.

And the remainder of the water—we’ll just run through the rest of the categories quick—about a fifth is going to the environment itself, to riparian areas, and another fifth roughly going to cities and industrial uses.

Right. One of the new findings in our paper was our ability to account for all the water that is used by natural ecosystems as the water flows through that extensive river system… We do make the point in the paper that even though that seems like a lot of water, it is of course essential to the culture and the sense of place in the Colorado River Basin.

And these numbers we’ve been talking about are starting in the year 2000, which happens to be when we really started plunging into drought here in the Southwest. How much are we overusing the river? After doing all this accounting, how much do we really need to cut to be in balance with our water budget?

It’s a great question. Our estimates are that we need to cut about 20 percent. We have reduced our water use in the lower half of the basin to a notable degree so far, but to bring the river system back into a balance we need to reduce our current levels of consumption by about 20 percent overall.

How would you do it, if you were in charge?

The folks that are negotiating the future of the Colorado River need to make two basic decisions. One is that they need to agree on the overall volume of water that’s available on a reliable and sustainable basis. And the second is they then need to decide how to share that available water among the seven states and Mexico. When I say reliable and sustainable, it’s really important to emphasize a portion of the water needs to be left in the river system to support the ecological health of the system. We also need to address Native American rights, which haven’t been properly allocated and enabled up until this point…. I do think we’re going to be able to resolve this water crisis, but it has to begin with an affirmation of how much water is truly available…. That affirmation of how much water is actually available is a necessary precursor to enabling everybody to start planning for a future that’s going to look different.

Brian, thank you so much for speaking with me today.

You’re very welcome, Melissa, and thank you for your interest in our story.

Read the paper in Communications Earth & Environment, New water accounting reveals why the Colorado River no longer reaches the sea."

Melissa joined KNAU's team in 2015 to report on science, health, and the environment. Her work has appeared nationally on NPR and been featured on Science Friday. She grew up in Tucson, Arizona, where she fell in love with the ecology and geology of the Sonoran desert.
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