Researchers at Caltech have developed an innovative technique to gauge soil moisture in the vadose zone, the shallow region between the surface and underground aquifers where plants and crops access water through their roots.
This method employs seismic technology, typically used to measure ground shaking during earthquakes, but it can also detect vibrations from human activities such as traffic.
As these vibrations travel through the ground, their speed is reduced by the presence of water; the more moisture, the slower the vibrations move. The new study leverages seismic vibrations from everyday traffic to measure the water content in the vadose zone.
Seismic waves indicate soil moisture
Traditionally, measuring fluctuations in underground moisture over time and across different regions has relied on satellite imaging, which provides only low-resolution averages and cannot penetrate beneath the surface.
Moreover, moisture levels in the vadose zone can change rapidly; a thunderstorm can saturate the area, which may then dry out within days.
The new method, based on a technique developed in seismologist Zhongwen Zhan’s lab called distributed acoustic sensing (DAS), offers a solution. This technique involves directing lasers into unused underground fiber-optic cables , similar to those used for internet service.
As seismic waves or any vibrations pass through the cable, the laser light bends and refracts. By measuring these changes in the laser light, researchers can gather detailed information about the passing waves, effectively turning a 10-kilometer cable into a line of thousands of conventional seismic sensors.
Studying the Mojave desert
Following the 2019 magnitude 7.2 earthquake in Ridgecrest, California, Zhan deployed a DAS array on a nearby cable to measure aftershocks. Collaborating with hydrologist Xiaojing Fu, the team discovered that the array could also track changes in underground vibrations related to soil water content.
Over five years, they collected data and developed models showing how moisture in the vadose zone varies over time. During California’s historic drought from 2019 to 2022, they observed a significant decrease in vadose zone moisture, at a rate of 0.25 meters per year, surpassing the average annual precipitation.
“From the top 20 meters of soil in the Ridgecrest region, we can extrapolate to the entire Mojave desert,” says Yan Yang, the co-first author of the study.
“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.”
Other soil moisture-mapping tech
This method is the latest in a line of innovations that endeavor to measure soil moisture.
Recently, NASA launched two pioneering missions into low-Earth orbit to demonstrate innovative technologies for observing atmospheric gases and measuring freshwater.
One of these missions, the Signals of Opportunity P-Band Investigation (SNoOPI), involves a low-noise radio receiver that tests a novel method for measuring root-zone soil moisture. It utilizes radio signals from commercial satellites, all packed into a compact 6U CubeSat the size of a shoebox.
“By monitoring the amount of water in the soil, we get a good understanding of crop growth. We can also more intelligently monitor irrigation,” said James Garrison, professor of aeronautics and astronautics at Purdue University and principal investigator for SNoOPI.
Beyond deserts
Real-time measurement of vadose zone moisture is essential for effective water management and conservation strategies.
The Zhan-Fu plans to extend the use of this technology to regions beyond desert areas.
“We know this method works really well for this particular site,” Fu says. “Many other interesting regions with the same climate could have different hydrological processes, like central California, where farming operations withdraw water, but the region also receives snowmelt from the Sierra Nevada mountains.”
The study was published in Nature Communications .
Most Popular
Comments / 0