Researchers at Caltech have developed an innovative method to measure soil moisture in the vadose zone which is a crucial layer of soil between the surface and groundwater.
Traditionally, soil moisture measurements have relied on satellite imagery, which provides broad, low-resolution data and cannot penetrate below the surface. Moisture levels in the vadose zone can fluctuate quickly due to events like thunderstorms, making real-time monitoring challenging.
Caltech’s innovative approach involves seismic waves generated by everyday activities, such as traffic, which travel through the ground. Water in the soil slows down these vibrations; thus, by analyzing how seismic waves move, researchers can determine soil moisture levels. This technique is rooted in distributed acoustic sensing (DAS), a method developed in the lab of seismologist Zhongwen Zhan.
DAS uses lasers directed into unused fiber-optic cables, similar to those used for internet services. As seismic waves or vibrations pass through these cables, they cause changes in the laser light, which can be measured to provide detailed information about soil moisture. This setup transforms a 10-kilometer fiber-optic cable into a series of thousands of seismic sensors.
Following the 2019 Ridgecrest earthquake, Zhan’s team deployed a DAS array on a nearby fiber-optic cable to track aftershocks. Collaborating with hydrologist Xiaojing Fu, they discovered that the DAS array could also monitor changes in soil water content. Over five years, their research revealed a significant reduction in vadose zone moisture during California’s severe drought from 2019 to 2022, with a decrease of 0.25 meters per year.
“From the top 20 meters of soil in the Ridgecrest region, we can infer conditions across the entire Mojave desert,” said Yan Yang, co-first author of the study. They estimated that annually, the Mojave’s vadose zone loses water equivalent to the volume held by the Hoover Dam.
Caltech’s approach is part of a broader trend of technological advancements in soil moisture measurement. NASA’s recent missions include innovative tools for observing atmospheric gases and measuring freshwater. One such mission, the Signals of Opportunity P-Band Investigation (SNoOPI), uses a compact CubeSat equipped with a low-noise radio receiver to measure root-zone soil moisture through commercial satellite signals.
“Monitoring soil water content helps us understand crop growth and manage irrigation more effectively,” stated James Garrison, principal investigator for SNoOPI.
Accurate, real-time measurement of vadose zone moisture is crucial for efficient water management and conservation, especially in arid regions.