Hotter atmosphere = thirstier crops

Study finds crops in Rio Grande Valley need 8% to 15% more water, and they’ll need more water yet in the future. Same methodology shows 8% increase in Colorado’s Arkansas Valley

by Allen Best

A warming atmosphere is a thirstier atmosphere, and that has significant implications for agriculture.

A new study found substantial increases in atmospheric thirst across many Western states during the past 40 years, with the largest and most robust increases in areas centered around the Rio Grande and the lower Colorado River.

Atmospheric thirst increased 8% to 15% between 1980 and 2020 in the Rio Grande Valley of Colorado, New Mexico, and Texas. If all else is equal, that means 8% to 15% more water is now required to maintain the same irrigation of crops.

The change in the Colorado portion of the Rio Grande was 8%.

The methodology also produced the same 8% or so figure for the Arkansas River Basin in Colorado.

In other areas—notably the upper Colorado River Basin—there is less certainty about this phenomenon of evapotranspiration. That’s because the temperature increases in the datasets were not as strong as some other areas.

The study conducted by a team from three Western institutions found that conditions in parts of the United States are now verging on being outside the range of variability of 20 to 40 years ago.

“This is really important to understand because we know that atmospheric thirst is a persistent force in pushing Western landscape and water supplies toward drought,” said Christine Albano, an ecohydrologist at the Desert Research Institute in Reno.

The largest changes in atmospheric thirst have been centered over the Rio Grande region of Colorado, New Mexico and Texas. Imagine/Desert Research Institute

Unlike prior studies, the researchers teased out the relative influences in determining atmospheric thirst. They found that temperature increases caused 57% of the changes observed in all regions of the continental United States. Humidity had a role of 26%, wind speed 10%, and solar radiation 8%.

“The study shows the dominant role that warming has played on the increasing evaporative demand and foreshadows the increased water stresses that the West faces with continued warming,” said John Abatzoglou, a professor who explores the how and whys of weather and climate at the University of California, Merced.

For farmers and other water users, greater atmospheric thirst mean that more water will be needed in the future.

“Our analysis suggests that crops now require more water than they did in the past and can be expected to require more water in the future,” said Justin Huntington, a research hydrologist at the Desert Research Institute.

In some areas, warming temperatures and lower-than-average precipitation have already stressed water supplies. Climate models have previously identified the Rio Grande drainage of Colorado, New Mexico, and Texas as an area likely to get hammered by climate change. This study further corroborates that it’s happening.

“Our results indicate that, decade by decade, for every drop of precipitation that falls, less and less water is likely to drain into streams, wetlands, aquifers, or other water bodies,” said study co-author Michael Dettinger, of Scripps Institution of Oceanography and the Desert Research Institute. “Resource managers, policymakers, and the public need to be aware of these changes and plan for these impacts now and into the future.”

Uncertainty remains. The researchers cited the upper Colorado River Basin—including much of Western Colorado—as a place where changes in evaporation and transportation are less clear. In an e-mail to Big Pivots, Dr. Albano said that one dataset for the upper Colorado River Basin shows lesser temperature increases, while the rest of the datasets are generally on par with other places in the United States.

“The capacity of the Earth’s atmosphere to hold water vapor is increasing nonlinearly with rising global temperatures,” says the paper published in the Journal of Hydrometeorology. In other words, the capacity of the atmosphere to hold water vapor—itself an important, heat-trapping greenhouse gas—has been rising more rapidly than temperatures.

Western states – including Colorado – have had obvious signs of a shifting climate, including outsized temperature increases, drought (or what some prefer to call aridification), tree mortality, and longer fire seasons with forest fires that have been also becoming much larger.

Previous studies had focused on one set of forcing conditions, i.e. temperature or wind or solar radiation, or on a limited number of ground observations. This more comprehensive study attempted to address the gaps and reduce uncertainty.

But uncertainty remains—including in the upper Colorado River Basin.

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