In the 1960s, ecologists used shovels and pick axes to dig out the root systems of plants on the Great Plains to deduce how much water they retained.
Unearthing the length and depth of roots on a single plant was strenuous work that could take days or weeks. Black and white photos in old ecology textbooks allude to the labor involved.
Data from early researchers became part of a standard model to show a plant’s resilience or vulnerability in drought conditions, and to determine how much water an ecosystem retained.
The model, however, was erroneous.
“It was no more than a best guess,” said U of I researcher Meng Zhao.
Accurately measuring how much water plants can store in the soil remained a challenge until now, said Zhao, a professor in the Department of Earth and Spatial Sciences.
He and his team use NASA satellite data, not picks and shovels, to show that plant systems can hold far more moisture in their roots than previously believed. The retained water helps them overcome dry spells and maintain the viability of ecosystems.
“For many years we used the information from ecologists who dug into the ground to see how deep a tree’s roots go and estimated a parameter,” Zhao said. “Because they only measured a single tree, for example, there was a huge uncertainty brought on by different soils, geography or different growing conditions.”
Using satellite data, Zhao and his team have developed a method to more accurately quantify the water storage capacity of plant roots on a worldwide scale.
“Our findings significantly enhance scientific understanding of how vegetation interacts with water resources,” he said. “This is the first time water storage capacity of surface soils and plant roots has been mapped directly from satellite observations.”
The new findings have been integrated into a hydrological model developed by the U.S. Geological Survey. They are set to transform how scientists and policymakers understand and handle droughts.
His research, however, is multifaceted and shows that ecological models based solely on satellite data must be used with caution.
In addition to learning how much water an ecosystem holds, Zhao’s team measures the amount of water an ecosystem releases into the atmosphere through evaporation from surrounding waterways, and transpiration — the water loss from plants. The combined water loss from an entire ecosystem is referred to as evapotranspiration, or ET.
“Understanding how much water is used by ecosystems through ET is important for managing water resources and assessing drought impacts,” Zhao said.
Satellite systems collect climate, atmospheric and earth-monitoring data to help scientists build models to measure ET, but Zhao’s researchers have shown that those models are not as accurate as direct measurements from river flow meters, weather stations that collect rainfall, and sensors that show changes in total water storage.