According to a study published in Nature Communications on August 5th, 2024, Caltech researchers have devised a novel method for measuring soil moisture in the shallow subterranean zone between the surface and underground aquifers.
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The vadose zone, the underground region crucial for plants and crops to access water through their roots, has traditionally been monitored using satellite imaging. However, this method only provides low-resolution, average data and cannot penetrate below the surface. Additionally, the moisture in the vadose zone can fluctuate rapidly, with a region that is saturated after a thunderstorm potentially drying out within days.
The new approach involves using seismic technology, which is usually employed to measure ground vibrations during earthquakes, to assess moisture levels in this zone. This method capitalizes on the fact that vibrations from human activities, such as traffic, also pass through the ground and are slowed by the presence of water—more moisture results in slower vibrations. By analyzing these seismic waves generated by everyday activity, researchers can gauge water content in the vadose zone.
This innovative technique is the result of a collaboration between hydrologist Xiaojing (Ruby) Fu and seismologist Zhongwen Zhan. Their findings leverage a method known as distributed acoustic sensing (DAS), originally developed in Zhan's lab. DAS involves sending laser light into unused underground fiber-optic cables. When seismic waves or vibrations pass through these cables, the laser light bends and refracts. By measuring these changes, researchers can use a single 10-kilometer cable as a network of thousands of conventional seismic sensors.
After the 2019 magnitude 7.2 earthquake in Ridgecrest, California, Zhan deployed a DAS array on a nearby cable to monitor aftershocks. Collaborating with Fu, the team realized that the array could also track how everyday vibrations vary with soil moisture. Over five years, they gathered data and developed models to illustrate moisture fluctuations in the vadose zone. Their research showed a significant decline in moisture during the historic California drought from 2019 to 2022, with a decrease of 0.25 meters per year, surpassing the average precipitation levels.
From the top 20 meters of soil in the Ridgecrest region, we can extrapolate to the entire Mojave desert. 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.
Yan Yang, Study Co-First Author and Graduate Student, California Institute of Technology
The ability to measure vadose zone moisture in real time is crucial when it comes to managing water use and conservation efforts. Next, the team plans on deploying the technology in regions other than the desert.
We know this method works really well for this particular site. 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.
Xiaojing (Ruby) Fu, Assistant Professor, California Institute of Technology
Seismological instruments have not previously been employed to measure soil moisture on such a large scale and over such an extended period. This groundbreaking project was made possible by funding and support from Caltech's Resnick Sustainability Institute (RSI).
This is exactly the type of interdisciplinary, creative science that the Resnick Institute was designed to support, bringing together colleagues that otherwise wouldn’t have worked together, and in that collaboration develop new tools that can help measure and manage water availability more sustainably.
Neil Fromer, Executive Director of Programs, Resnick Sustainability Institute
Journal Reference:
Shen, Z., et. al. (2024) Fiber-optic seismic sensing of vadose zone soil moisture dynamics. Nature Communications. doi:10.1038/s41467-024-50690-6