U of I researcher coins ‘thirstwaves’ as new framework emphasizing prolonged, extreme water stressors

Researchers from University of Idaho (U of I) and University of Colorado Boulder (CU Boulder) have created a new metric to help farmers, researchers and practitioners better track and prepare for prolonged periods of extremely high evaporative demand.

Meetpal Kukal, an assistant professor of hydrologic science and water management with U of I, and Mike Hobbins, a senior research scientist with CU Boulder, have coined “thirstwaves” as a broad concept explaining when evaporative demand remains elevated for days at a time. Evaporative demand describes conditions governing the movement of water from the Earth’s surface into the atmosphere.

Their paper “Thirstwaves: Prolonged Periods of Agricultural Exposure to Extreme Atmospheric Evaporative Demand for Water” was published March 20 in the journal “Earth’s Future.”

In certain regions and seasons, windspeed, humidity or solar radiation from sunshine, rather than heat, drive evaporative demand. Evaporative demand drives evapotranspiration, as long as sufficient soil water is available.

“In this whole domain of environmental change and its impacts, we’ve been sort of obsessed about heat and heatwaves,” explained Kukal, who is the paper’s lead author. “As far as water consumption by vegetation is concerned, including agricultural crops, there are other variables besides temperature that are equally, if not more, important, and that includes humidity, wind speed and solar radiation.”

Their study evaluated the gridMET surface meteorological dataset covering the contiguous U.S. states from 1981 through 2021 growing seasons. They defined a thirstwave as having occurred any time when during at least three consecutive days daily evaporative demand was greater than its 90th percentile recorded historically.

Agricultural water managers traditionally consider mean values in their assessments. Thirstwaves, by contrast, focus on extremes persisting over multiple days.

“If one were to look only at means it appears the southwest desert is the most extreme from a thirst standpoint. However, when you start looking at extremes, places that do not necessarily have high evaporative demand stand out as outliers,” Kukal said. “The Midwest, for example, is not as consequential from a mean evaporative demand standpoint, but from an extreme standpoint it is really a hot spot.” Kukal published this evidence last year in the journal “Environmental Research Letters.”

During prolonged durations of extreme stress from high heat, low humidity, high wind or high solar radiation, irrigation deliveries and equipment may not be able to put out water fast enough to keep up with demand.

Kukal is currently developing a decision-support dashboard to help southern Idaho farmers manage irrigation, factoring in the possibility of such extreme conditions and design limitations of irrigation systems. He and Hobbins looked at three unique aspects of thirstwaves from historical records: intensity, the number of events and the duration. They concluded all three aspects have been worsening over time, with thirstwaves becoming more intense, greater in number and more frequent.

“These findings make us think about how our current water resources infrastructure, irrigation equipment and water management should mitigate and adapt,” Kukal said. “As these pressures grow, there’s less and less room for guesswork in irrigation, so if you are under limited water conditions, you’ve got to do a better job at really tracking your water.”

The researchers are optimistic that the concept of thirstwaves will add an important new framework to help agriculture bring the picture of evapotranspiration monitoring and forecasting for maintaining economic productivity into sharper focus. Kukal’s research program at U of I is building on Idaho’s capacity to monitor and predict evapotranspiration to aid in decision-making.

“This idea of thirstwaves I think is really going to catch on,” Hobbins said. “It’s a very powerful metric and it’s a crucial distinction from heatwaves because we have been hobbled for decades by this idea that temperature is really the only place where the information is.”