Scientists have figured out a way to measure soil moisture by detecting the vibrations of traffic noise travelling through the ground.
The technique is called distributed acoustic sensing (DAS) and it is normally used to measure seismic waves during earthquakes.
Seismological instruments have never been used to measure soil moisture at such a large scale for such an extended and continuous timespan before.
Researchers in the US repurposed a DAS system to measure moisture content in a part of the Earth known as the vadose zone. This is the shallow region between the underground water table (where the soil is fully saturated) and the surface.
Plants obtain water through their roots from the vadose zone, so the ability to measure its moisture content in real time is crucial for managing water use and conservation efforts.
“Given the escalated regional drought risk under climate change, our findings highlight the promise of fibre-optic seismic sensing as a large-scale, long-term, and cost-effective observational tool to enhance our climate resilience in semi-arid regions,” the researchers write in a paper describing the work in the journal Nature Communications.
In DAS, laser pulses are sent along unused, underground fibre-optic cables. This light bends and refracts as vibrations, such as seismic waves, pass through the cable. Measuring this tells researchers important information about the passing wave.
DAS can also be used to pick up the vibrations of ambient noise caused by humans. The more moisture there is in the vadose zone, the slower these vibrations move through it.
The research team collected data from a DAS array in Ridgecrest, California, for 5 years to create models of how soil water content varies over time there.
“From the top 20 meters of soil in the Ridgecrest region, we can extrapolate to the entire Mojave Desert,” says Yan Yang, a California Institute of Technology (Caltech) graduate student in geophysics and 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.”
They found that moisture in the vadose zone decreased at a rate of 25cm per year, exceeding the average precipitation for the region.
The researchers say their next steps will be to trial the method in regions other than a desert.
“We know this method works really well for this particular site,” says hydrologist Xiaojing (Ruby) Fu, Caltech assistant professor of mechanical and civil engineering and co-author of the study.
“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.”
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