Diving Deep Into Conservation

A comprehensive environmental conservation effort needs to monitor many variables, including the amount of moisture that is available in the soil. This is the water that plants and crops take in through their roots to fuel their growth, so if it is in short supply, entire ecosystems can rapidly falter. The vadose zone, in particular, is an important area to monitor — this is the region between the surface of the ground and the water table below. The amount of water stored in this porous space can vary significantly over time as environmental conditions change.

Keeping an eye on these fluctuations is crucial for implementing intelligent, data-driven conservation efforts, as it allows for timely interventions and adjustments to water management strategies, ensuring the sustainability of ecosystems and agricultural productivity. However, this is easier said than done. Traditionally, these measurements are taken from satellite imagery. However, this method cannot peer beneath the surface of the ground and is only capable of providing researchers with low-resolution estimates.

A much more accurate means of measuring the moisture content of large regions of soil has recently been developed, which could provide conservationists with much better data in the future. Researchers at the California Institute of Technology found an indirect way to collect the measurements that relies on infrastructure that is already in place in many regions. Their work showed that unused, buried fiber-optic cables can be repurposed for the task.

It has been known for some time that if a laser is shined through a fiber-optic cable in the ground, it will be altered by seismic waves. Specifically, the vibrations will bend and refract the light, which can then be leveraged to, for example, measure the intensity of an earthquake. But what the researchers in this study realized is that those vibrations are influenced by other factors, like the amount of moisture that is present in the soil. So, by calculating how observed vibrations differ from expectations, one can infer how much water is present.

Fortunately, there is no need to wait for an earthquake to collect this data. The team demonstrated that even the vibrations produced by nearby automobile traffic can be utilized by their sensing system. Using their methods, they found that a significant loss of soil moisture — to the tune of 0.25 meters per year — has been taking place in Ridgecrest, California over the past 2.5 years.

You may be wondering why the researchers did not just use traditional seismic sensors to measure soil moisture in a similar way. In theory, that may be possible, however, a few miles of fiber-optic cable provides data equivalent to what would be captured by thousands of sensors, so covering large regions would quickly become impractical.

While these initial results are very encouraging, the method has only been tested in one particular area. Looking ahead, the researchers intend to validate their methods in a variety of sites to make sure that the results will hold up under differing conditions.