The first question posed about Colorado’s most precious resource, water, is always, “How much?”
The second water question is always, “Where?”
A study from the Colorado School of Mines poses a new third question that may prove just as important for Colorado’s drought-plagued future:
“When?”
The study from a hydrologist and an economist published April 30 in Nature Water combines theoretical calculations with real data from the Rio Grande headwaters in southern Colorado. Warmer temperatures from climate change are resulting in more streamflow coming from rain rather than snow, spreading out the peak runoff to a longer period, according to School of Mines economist Steven Smith and hydrologist and climate change researcher Adrienne Marshall.
The effect in Colorado’s major river basins could put senior and junior water rights holders even more at odds during seasonal extremes. Under Colorado water law, those who claimed and legalized water rights in earlier years get priority over junior (later) rights holders when there’s a shortage.
Spring and summer in the West after a poor snowfall season “tend to have more evenly distributed flows. This combination is likely to intensify drought impacts on junior water users, who receive water only after senior rights are fulfilled,” according to a CSoM writeup of the study. Water rights in Colorado are often allocated on the basis of daily flows, the researchers note, which may not account for variations over a full season.
But of course it’s not as simple as that, since water never is, Marshall and Smith add. Senior rights holders are not always relatively well-off farmers or cities. The most senior holders in the Rio Grande basin are sometimes small farming operations on communal ancient land-grant ditches, using relatively small amounts of water.
“Those senior rights are often tied to smaller allocations. As a result, changes in water availability do not map neatly onto broader measures of social or economic equity,” the Mines study says.
Over 70 years of Rio Grande basin data, the “spread” of water flows in some years can last 10 days longer than average and greatly impact the timing of water rights, Smith said.
“We were seeing a change where the junior irrigators were getting reduced by 20% of their water, whereas the seniors were getting 12% more water than normal,” Smith said, even as the total amount of stream water over the course of the year stayed the same. “That was a pretty large swing for just this change.”
The finding for less-snowy parts of the West hinted at an opposite effect: The pair of researchers say they have much more to study to map out the full implications of the timing trend. In the less-snowy (but possibly rainier) geography, “Warmer years are associated with more concentrated flows, potentially offering a relative advantage to junior water users,” the study concludes.
“Whether or not that’s more equitable or less equitable is not clear,” Smith said. “We want to dig more into that.”
They also want to find similar data from other major river basins, such as the much-fought-over Colorado River Basin. The upper portions of the Colorado in the state are heavily impacted by diversions to the Front Range and reservoirs. The School of Mines researchers would like to explore whether those heavily controlled sections of rivers will do a better job distributing water equitably over time.
“As water scarcity intensifies across many parts of the country, we hope our findings provide new insight for policymakers, water managers and communities working to adapt to a more variable and uncertain future,” Smith said.
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