How Irrigation Harms (or Helps) Streamflows in the West

Between mountain ranges that characterize the American West lay expanses of deserts and arid plains. Water is an invaluable resource here, so folks in the West often notice when the amount of water flowing through the landscape strays from the norm.

Over recent decades, changes in streamflows—decreases and increases alike—have often been chalked up to climate change. Largely across the board, science agrees. But one recent study suggests that a changing climate doesn’t account for the full spectrum of change. Enter irrigation.

Nearly 90 percent of human water consumption in the West goes toward irrigation that supports agriculture such as grain and livestock, according to the December study published in the journal Communications, Earth and Environment and led by hydrologist David Ketchum. Unlike other areas of the country, most of the water used to irrigate in the West comes from surface water. As such, the Upper Missouri, Colorado and Columbia river basins are “perhaps the most important natural resource in the region,” Ketchum, who works for the Montana Department of Natural Resources and Conservation, wrote in the paper. But irrigation comes at a price, and Ketchum’s research sought to quantify it.

The study compiled 35 years of irrigation data across 221 basins in 10 states to analyze the complex relationship between climate, irrigation and streamflow across the West.

“The patterns and the differences in the responses to climate change and to irrigation intensification reveal important information about those places,” Ketchum told Mountain Journal, “namely, that our irrigation impacts are different in different basins.”

Climate impacts, irrigation practices and management vary from place to place, likely accounting for many of the differences between basins. Idaho’s Snake River and the Upper Missouri River, two side-by-side watersheds with headwaters in Greater Yellowstone, have experienced similar climate-change impacts, yet the study revealed drastically different flow changes as a result of irrigation, demonstrating what the paper refers to as an “irrigation efficiency paradox.”

Efficient irrigation practices in the Snake basin result in much of the water diverted for irrigation being used by crops where excess water returns to the atmosphere through a process called evapotranspiration. Less efficient irrigation in southwest Montana leaves large quantities of water unused. Rather than being lost to the atmosphere, this unused water often makes its way back to the river and supplements flows at a later date, the study explains, which in a strange twist may ultimately be more efficient.

“This paper points to places where our irrigation infrastructure has worked for or against us,” Ketchum said. “[The study] shows that it’s possible to configure our irrigation system in a way that helps us … and have sufficient flows for our fisheries while also having sufficient water for irrigation.”

Through its in-depth analysis of the impacts of both climate and irrigation on western streamflows, the study aims to better inform future irrigation management decisions by providing detailed basin-specific insights. The U.S. Department of Agriculture uses survey results to inform irrigation estimates that Ketchum said exclude non-revenue-generating irrigators, while his study utilized satellite sensing for “more complete” estimates.

Ketchum believes that this study is just the beginning. Through his work with DNRC, Ketchum is developing processes that could predict how different irrigation management scenarios may impact river basins.

From agriculture production, outdoor recreation and wildlife habitats, water impacts nearly all facets of life in Greater Yellowstone and its neighboring western landscapes. This study reinforces that scientifically backed management decisions are key to protecting our region’s longevity, and streamflow is a vital component.