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Modeling Study on Cattle Feed Crops & River Flow Depletion

a fish held up in someone's hand, with a rocky stream and blue sky in the background
A humpback chub – an endangered species found in the Colorado River Basin. Of the 60 species of fish that are threatened by lower flowing conditions, nearly 90 percent are primarily threatened by changes in streamflow from raising cattle specifically. Photo by US Fish and Wildlife Service, courtesy of Wikimedia Commons.

A new study uses a USDA Forest Service modeling tool – the Water Supply Stress Index, or WaSSI, ecosystem services model – to explore the relationship between water use, river flows, and fish populations across the conterminous U.S.

Brian Richter from the University of Virginia led the study. SRS researcher Peter Caldwell’s expertise with WaSSI was instrumental in relating water use to depletion of river flows.

Researchers found that in the western U.S., irrigating cattle feed crops is the greatest human-induced cause of declining river water levels. Declining water levels endanger fish and other aquatic species. In the South, river flow depletion is lower than in the West (on a percentage basis) but can exceed 20 percent in some river basins during dry years.

The study investigated the impact of temporary rotational fallowing in the Colorado River Basin. This practice of letting farmland go unused for some time – if financially incentivized for 20 percent of crops – could prevent river flow depletion in the river basin above Lake Powell without adverse economic effects on farmers or food security risks.

“Forests have and will continue to play a key role in providing a clean, stable water supply across the U.S.,” says Caldwell. “Our recent work has shown that in the South, for example, forested lands provide 44 percent of the total available water supply in the region, and ongoing work is showing that forests play an even greater role in the West.”

In addition to managing water demand, maintaining and managing forests on the landscape will be an essential component of water resource management in the future.

Read the article in Nature Sustainability. For more information, email Peter Caldwell at

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