For all the talk about how much water exits Utah through the Colorado River, that’s not the primary way water leaves the state. Much of it simply floats off into the air.
Right now, Utah doesn’t have a lot of detailed data on the water lost to evapotranspiration. That’s a measurement of how much water evaporates from water bodies and soil and how much is released into the air from plants.
“That component has been very poorly understood in general,” said Kathryn Ladig, a project geologist with the groundwater and wetlands program of the Utah Geological Survey.
It’s why she and other scientists are putting together a new statewide set of sensors to track it.
“Water use is being looked at under a microscope really closely right now, and you have all of this legislation going on, so we are trying to provide the best data that we can to understand how much water, in theory, could also be conserved.”
The collection of monitoring stations is called the Utah Flux Network.
It began in 2021 with a site in the town of Wellington in Carbon County and another at the Matheson Wetlands near Moab. Ladig’s team has since grown it into a set of nine stations, from Bluff and Escalante in southern Utah to salt flats and wetlands in northern Utah. They plan to install one additional site in Green River next spring.
Each Utah Flux Network station is packed with scientific instruments that track evapotranspiration in real time by measuring several things simultaneously, including wind speed and direction, humidity, soil moisture and radiation from the sun.
The resulting data could have a big impact on Utah reaching its future water goals. Without clear information about how much water is leaving the Great Salt Lake through evapotranspiration, for example, it would be hard to know how much water the state needs to send into the lake to achieve its desired outcomes.
“In my mind, [evapotranspiration] is the single most important thing that we could measure on the lake,” said Jake Serago, an engineer with the Utah Division of Water Resources. “That's probably because it's the phenomenon that we perhaps know the least about.”
His team plans to use data from the Utah Flux Network to improve its modeling of how water moves through the lake system. The evapotranspiration estimates that Utah has historically leaned on come with a high degree of uncertainty, Serago said, especially for a vast, complex water body like the Great Salt Lake.
“Because the lake is so big, even if it's 10% uncertainty, that is an enormous amount of water,” he said.
One of the stations is set up among phragmites, an invasive plant, along the shore of the Great Salt Lake. That particular area is slated to have the plants cleared soon, Ladig said. So having evapotranspiration data from before and after will help answer questions about phragmites’ impact on water loss.
“Sure, the invasive plant is bad for all the ecological reasons, but also how much water is that invasive plant consuming?”
She hopes another station at Cedar Mesa in southern Utah could provide information about how pinyon-juniper forests encroaching on sagebrush landscapes might impact evapotranspiration in that natural habitat. In both cases, Ladig said, the new information could help lawmakers understand where the greatest urgency lies and how to prioritize funding projects.
That data’s largest impact, however, might be for farmers.
“Most agriculture users have a very good idea of how much water they are diverting from a stream or a well, for instance, but they have very little idea of how much water they're actually depleting,” said Marc Stilson, an engineer with the Colorado River Authority of Utah.
Some of the water that gets diverted for crop irrigation returns to the local water system, he said, such as the water that soaks into the ground toward an aquifer or runs off a field into a nearby stream. Water that evaporates, however, is essentially lost to that area — depleted — because it typically rains down on a different watershed.
Offering farmers and ranchers more detailed data about the water vaporizing in their fields could help them become more water-efficient, Stilson said. For instance, it could lead them to try different crops that might have less transpiration or test alternative irrigation practices that lose less water to evaporation.
Having more detailed statewide data could also help Utah’s position in the ongoing Colorado River talks before the current basinwide usage agreement expires in 2026.
“As we're involved in the negotiations in the larger Colorado River picture, we want to make sure that the data that we have is accurate and that it can be trusted,” Stilson said.
The data collected by the Utah Flux Network so far has mostly informed how to better set up the next monitoring site rather than directing policy, Ladig said. There are only so many broad insights you can extract from one or two seasons of information.
Her team plans to analyze the data they do have and provide it to decision-makers and the general public within the next year or two.
“Everybody has an interest in ensuring that the water is there,” she said. “As we see groundwater declines and surface water declines throughout the state, this is going to contribute so that the individual farmer can make decisions or legislators can make informed decisions about how water is being used.”
