How does the atmosphere quench its thirst? Two NAU scientists look to isotopes for answers.

Pinyon juniper forest

June 26, 2019

In the desert, it’s good to know where your water is and where it’s going.

That information is actually far more complex than most of us realize, but a team of scientists led by geochemist Kimberly Samuels-Crow is taking on one piece of the puzzle—evapotranspiration, or the process of water moving from soil or plants into the atmosphere. Samuels-Crow, an assistant research professor in the School of Informatics, Computing, and Cyber Systems at Northern Arizona University, is the principal investigator (PI) on the grant from the National Science Foundation.

NAU will receive $496,555 from the NSF for the three-year project, with an additional $203,445 going to collaborators at the University of New Mexico.

The study looks at two types of water loss—evaporation, in which water moves directly from the soil to the atmosphere, and transpiration, when water is piped from the soil to the atmosphere by plants. Scientists have struggled to make the effects of these processes distinct, coining the catchall term “evapotranspiration” to describe water moving from Earth’s soil into the atmosphere.

Up to now, that’s worked. It doesn’t anymore.

“We’re at the point now where we have to pick it apart,” Samuels-Crow said. “Our goal in this project is to understand what climate drivers are behind these two really important processes that take water from the soil and stick it into the atmosphere.”

She’s joined in this research by co-PIs Kiona Ogle, also a SICCS professor; Marcy Litvak, a biology professor at the University of New Mexico; and John Bradford, a research scientist with the U.S. Geological Survey in Flagstaff. The team will collect data at six sites in New Mexico, from ecosystems that range from desert grasslands and shrublands to pinyon and juniper woodlands, Ponderosa pine forests and high-altitude mixed conifer forest. They picked these sites because they already have a decade of data about fluxes and climate conditions; Litvak has been measuring the flow of water, carbon dioxide and energy between the atmosphere and ecosystems in these sites since 2008.

The measuring process is a bit complicated, Samuels-Crow said. Using a machine that measures isotopes in water, which she and Ogle acquired through a Technology and Research Initiative Fund grant from the Arizona Board of Regents, the team can determine what isotopes are present in the soil water and in the water vapor in the atmosphere. They know that heavier isotopes of both hydrogen and oxygen (2H, 17O and 18O) evaporate more slowly than lighter isotopes (1H and 16O). Thus, if you spilled water on the sidewalk and let half of it evaporate, what would be left in the puddle would have a higher concentration of heavy isotopes, while the water vapor that evaporated from the puddle would have a higher concentration of lighter isotopes. The same holds true for water in the top layers of soil.

High-elevation mixed conifer site in New Mexico.
High-elevation mixed conifer site in New Mexico. The tower is an “eddy covariance” or “flux” tower. The instruments that measure the exchange of CO2, water and energy between the ecosystem and the atmosphere are installed on towers like this across the six sites of the NM Elevation Gradient network.

So that’s evaporation. In transpiration, the plant is getting water from its root source, which is likely much deeper in the ground—12 inches vs. three inches, for example. Using an instrument that measures water vapor coming directly off the plant in real time likely will show an isotope range that is more in line with the isotopes found in water that occurs much deeper in the soil.

On top of that, some of the sites have instruments that measure water flow (“sap flow”) in trees, which is a direct measurement of transpiration. This data is difficult to apply to an entire ecosystem, but it’s one more dataset they can collect.

Once they have all of this disparate data, Ogle will use them to create statistical models that attempt to break down the effects of evapotranspiration into evaporation and transpiration. It won’t answer all the questions, but it will introduce valuable new information into a critical conversation about the availability of water in an increasingly hot, dry climate.

“We expect to show that history matters for understanding water loss from these systems such that the amount of water lost from soils to the atmosphere via transpiration will depend on the preceding environmental conditions that occurred days, weeks or months ago,” Ogle said. “These ‘lagged effects’ are typically ignored by predictive models, yet they could be really important for predicting how water loss, and the water cycle in general, will change in the future.”

The USGS’s Bradford can use these new datasets and models to calibrate SOILWAT, a mechanistic model of soil water, to better separate evaporation and transpiration and to evaluate the potential importance of such lagged effects.

“Those types of models go into water resource planning in the Southwest,” Samuels-Crow said. “By doing this, what we’re hoping is to get a better sense of these different drivers of evaporation and transpiration, when they’re active, under what conditions they’re active. For instance, if you increase temperature, we can tell what changes about our water resources because we’ll know how much less is getting down into shallow groundwater because the physical processes or evaporation has moved that water elsewhere.

“It’s not going to radically change our understanding, but it is going to provide the SOILWAT with valuable information for its models, and water managers use that tool in planning.”

It’s a small but important piece of the water management puzzle in this region but also globally.

“Water is a critical resource in the Southwest, and understanding how the water cycle is impacted by plants, soils and climate is important to predicting changes in water availability,” Ogle said. “I think that people of the Southwest generally appreciate their surroundings and access to public lands and diverse ecosystems. Our ecosystems may be threatened by continued climate change, and this research will contribute data and results that are important to predicting how our ecosystems may change in the future and to helping identify those ecosystems or biomes that may be most vulnerable to such future changes.”

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Heidi Toth | NAU Communications
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