The vadose zone, a crucial region for plant water absorption, has traditionally been challenging to measure accurately. Current methods, such as satellite imaging, provide only low-resolution averages and cannot penetrate below the surface. Additionally, the rapid fluctuations of moisture within this region make it difficult to monitor effectively.

However, Caltech researchers have introduced an innovative method to address this issue. By utilizing seismic technology originally designed for earthquake monitoring, the researchers have found a way to measure soil moisture in the vadose zone. This groundbreaking approach involves analyzing the vibrations caused by everyday activities, such as traffic, as they pass through the ground.

The presence of water in the soil affects the speed of these vibrations, allowing researchers to determine the water content in the vadose zone. This pioneering technique offers the potential for more precise and real-time monitoring of soil moisture, which is essential for optimizing agricultural practices and water resource management.

The groundbreaking research is a result of the collaboration between hydrologist Xiaojing (Ruby) Fu and seismologist Zhongwen Zhan. Their work introduces a new method based on distributed acoustic sensing (DAS).

This innovative technique involves using lasers to analyze unused underground fiber-optic cables, turning a 10-kilometer cable into the equivalent of thousands of conventional seismic sensors. This advancement has the potential to revolutionize seismic wave and vibration detection, offering new opportunities for scientific research and practical applications.

After the 2019 magnitude 7.2 earthquake in Ridgecrest, California, Zhan and Fu established a DAS array to monitor aftershocks. They soon realized that the array could also track changes in underground vibrations based on soil water content.

Over five years, they gathered data and developed models showing variations in moisture levels in the vadose zone over time. Their findings revealed a significant decrease in vadose zone moisture during California’s historic drought from 2019 to 2022, exceeding the average precipitation at a rate of 0.25 meters per year.

“From the top 20 meters of soil in the Ridgecrest region, we can extrapolate to the entire Mojave desert,” says Yan Yang, a 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.”

Measuring vadose zone moisture in real-time is essential for effectively managing water usage and conservation initiatives. The team plans to expand the deployment of this technology to areas beyond the desert.

“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.”

Seismic instruments have not previously been utilized to measure soil moisture on a large scale for such a prolonged and uninterrupted duration. The funding and support from Caltech’s Resnick Sustainability Institute (RSI) enabled the realization of this project.

“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,” says Neil Fromer, Executive Director of Programs with the Resnick Sustainability Institute.

Journal reference:

1. Zhichao Shen, Yan Yang, Xiaojing Fu, Kyra H. Adams, Ettore Biondi & Zhongwen Zhan. Fiber-optic seismic sensing of vadose zone soil moisture dynamics. Nature Communications2024; DOI: 10.1038/s41467-024-50690-6