The vadose zone—the underground region between Earth’s surface and the groundwater table—plays an important role in the water cycle. But so far, monitoring the vadose zone’s moisture content has been challenging. Now, seismologists have managed to trace moisture fluctuations in this zone using fiber-optic cables.

The new study, published in Nature Communications, focused on the city of Ridgecrest, Calif., which lies in a drought-prone region southwest of Death Valley and northwest of the Mojave Desert. The results suggested that soil moisture in the vadose zone is strongly affected by drought and may vary significantly over several kilometers.

“This study showcases a novel application of using telecommunication fiber-optic cables to monitor soil moisture with remarkable spatiotemporal resolution,” said Shujuan Mao, assistant professor of seismology at the University of Texas at Austin who was not part of the research. “[It] improves our ability to observe and understand water dynamics in the vadose zone and opens the possibility for broader applications.”

Studying the Shallow Subsurface, in Depth

Although the vadose zone contains only a fraction of Earth’s freshwater, it has a large impact on the hydrologic cycle: It supplies water to plants and crops, transports nutrients and pollutants, and controls the rate at which aquifers refill after droughts.

But determining how much water is in the vadose zone and how quickly it is lost or replenished isn’t easy. “Current [tools] for soil moisture monitoring, such as remote sensing satellites and physics-based land surface models, often suffer from low spatial resolution and struggle to measure below 200 centimeters [80 inches],” Mao said. These difficulties impede detailed studies of the vadose zone, which can extend more than 100 meters (330 feet) into the ground.

Instead of satellite data, the new study’s authors used distributed acoustic sensing (DAS) to monitor moisture. In DAS, a laser pulse is sent into an underground fiber-optic cable, such as those that supply Internet and telephone connections. Small defects in the cable scatter the light. When the cable bends or vibrates, the scattered light changes, too.

By analyzing wiggles and blips in the light, scientists can monitor vibrations along the entire length of the cable—producing data with meter-scale spatial resolution.

The 8-kilometer-long (5-mile) Ridgecrest DAS array was originally installed to record aftershocks of the magnitude 7.1 earthquake that rocked the city in 2019. But the researchers quickly realized that ambient vibrations—mainly from vehicular traffic—recorded in the DAS data could be used to monitor groundwater, said Zhichao Shen, a seismologist at Woods Hole Oceanographic Institution (WHOI) and the first author of the study.

To determine the soil’s water content from their data, the scientists looked to seismic wave speeds, which depend on the material through which the waves travel. If the ground is dry, seismic waves move through it quickly. If the soil is wet and soft, seismic waves travel more slowly.

By observing how seismic wave velocities beneath Ridgecrest changed across space and time, the researchers traced fluctuations in the vadose zone’s water content.

A Reservoir’s Worth of Water Lost from the Mojave

“The level of detail resolved by our fiber-optic seismic sensing principle is quite exciting and surprising,” Shen said. The results illustrate that water saturation in the vadose zone increases rapidly after rainfall, then decreases over timescales of days to months as the moisture is lost to evaporation or plant transpiration.

In times of drought, the loss of moisture can be drastic. Shen and his coauthors calculated that the vadose zone below Ridgecrest lost the equivalent of a 25-centimeter-deep (10-inch) layer of water each year from 2019 to 2022.

Ridgecrest receives only about 5 centimeters (2 inches) of annual rainfall. “[This result] implies that during drought years, groundwater is only minimally recharged by precipitation or surface water,” Shen said.

Extrapolating the water loss in Ridgecrest to the area of the Mojave Desert, the researchers estimated an annual loss of 30 cubic kilometers (7 cubic miles) of water. For comparison, Lake Mead, the reservoir created by the Hoover Dam, currently contains about 10.7 cubic kilometers (2.6 cubic miles) of water.

A Cost-Effective Water Monitoring Method

The high spatial resolution of DAS data could make this new tool useful for water monitoring, Mao said. The eastern end of the Ridgecrest cable recorded a higher moisture loss than the western end, suggesting that water dynamics in the vadose zone can vary considerably across several miles. Such information could help to identify areas that are especially sensitive to drought.

It’s a cost-effective way to complement current water monitoring methods, Shen said. Because it uses telecommunication cables that are already in place, it can be implemented quickly and cheaply.

“In water-stressed urban areas, this information is useful for supporting more effective drought mitigation strategies and water resource planning,” Mao concluded. “In agricultural regions, detailed soil moisture data can optimize practices such as irrigation scheduling and crop health monitoring.”

—Caroline Hasler (@carbonbasedcary), Science Writer

Citation: Hasler, C. (2024), Fiber-optic cables used to measure changing soil moisture, Eos, 105, https://doi.org/10.1029/2024EO240403. Published on 10 September 2024.

Text © 2024. The authors. CC BY-NC-ND 3.0
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