A Multidataset Assessment of Climatic Drivers and Uncertainties of Recent Trends in Evaporative Demand across the Continental United States

Study key findings & significance

• In arid Western states, the climate is becoming increasingly warmer and drier, leading to rises in the demand for water resources from humans and most ecosystems.
• Evaporative demand – known colloquially as atmospheric thirst – is a measure of the loss of water from the earth’s surface to the atmosphere, calculated through variables such as temperature, humidity, wind speed, and solar radiation.
• From 1980 to 2020, there were massive increases in evaporative demand in the Western states, particularly in the area centered around the Rio Grande and Lower Colorado rivers.
• Increases in temperature were responsible for 57 percent of the changes in evaporative demand observed in these regions, while humidity, wind speed, and solar radiation played important yet lesser roles (26 percent, 10 percent, and 8 percent, respectively).

Author quotes

“This means that atmospheric thirst conditions in parts of the country are now verging outside of the range that was experienced 20 to 40 years ago, especially in some regions of the Southwest. This is really important to understand, because we know that atmospheric thirst is a persistent force in pushing Western landscapes and water supplies toward drought.”

Christine Albano, lead author and assistant professor of Ecohydrology at the Desert Research Institute

“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."

Justin Huntington, study co-author and research professor of Hydrology at DRI

Abstract

Increased atmospheric evaporative demand has important implications for humans and ecosystems in water-scarce lands. While temperature plays a significant role in driving evaporative demand and its trend, other climate variables are also influential and their contributions to recent trends in evaporative demand are unknown. We address this gap with an assessment of recent (1980–2020) trends in annual reference evapotranspiration (ETo) and its drivers across the continental United States based on five gridded datasets. In doing so, we characterize the structural uncertainty of ETo trends and decompose the relative influences of temperature, wind speed, solar radiation, and humidity. Results highlight large and robust changes in ETo across much of the western United States, centered on the Rio Grande region where ETo increased 135–235 mm during 1980–2020. The largest uncertainties in ETo trends are in the central and eastern United States and surrounding the Upper Colorado River. Trend decomposition highlights the strong and widespread influence of temperature, which contributes to 57% of observed ETo trends, on average. ETo increases are mitigated by increases in specific humidity in non-water-limited regions, while small decreases in specific humidity and increases in wind speed and solar radiation magnify ETo increases across the West. Our results show increases in ETo across the West that are already emerging outside the range of variability observed 20–40 years ago. Our results suggest that twenty-first-century land and water managers need to plan for an already increasing influence of evaporative demand on water availability and wildfire risks.