The vadose zone is crucial for plants and crops to obtain water through their roots, but measuring how this underground moisture fluctuates over time and between geographical regions has traditionally relied on satellite imaging that provides low-resolution averages and cannot penetrate below the surface. Additionally, moisture within the vadose zone changes rapidly, such as when a thunderstorm saturates a region that dries out a few days later.
The new method from Caltech (California Institute of Technology) is said to rely on seismic technology that can detect the vibrations of human activity such as traffic. As these vibrations pass through the ground, they are slowed down by the presence of water. The new study measures the water content in the vadose zone through seismic rumblings from everyday traffic.
The research is a collaboration between the laboratories of hydrologist Xiaojing (Ruby) Fu, assistant professor of mechanical and civil engineering; and seismologist Zhongwen Zhan, professor of geophysics. The work is detailed in Nature Communications.
The new method is based on distributed acoustic sensing (DAS). With this technique, lasers are pointed into unused underground fibre-optic cables. As a seismic wave, or any kind of vibration, passes through the cable, the laser light bends and refracts. Measuring the movements in this laser light gives researchers information about the passing wave, making the 10km cable equivalent to a line of thousands of conventional seismic sensors.
In the wake of the 2019 magnitude 7.2 earthquake in Ridgecrest, California, Zhan set up a DAS array on a nearby cable to measure aftershocks. In collaboration with Fu, the team realised that the array could also be used to measure how everyday underground vibrations change depending on soil water content.
Over five years, the team collected data and created models to illustrate how moisture in the vadose zone varies over time. They found that during the drought in California from 2019 to 2022, moisture in the vadose zone decreased significantly at a rate of 0.25 meters per year, exceeding the mean average precipitation.
"From the top 20m of soil in the Ridgecrest region, we can extrapolate to the entire Mojave desert," Yan Yang, a graduate student in geophysics and co-first author of the study said in a statement. "Our rough estimation is that every year, the Mojave vadose zone loses an amount of water equivalent to the Hoover Dam. Over the drought years of 2019 through 2022, the vadose zone has been drier and drier."
The ability to measure vadose zone moisture in real-time is crucial for managing water use and conservation efforts. Next, the team intends to deploy the technology in regions other than desert.
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