By watching from above, scientists can closely monitor aquifer levels deep below ground. A group led by Estelle Chaussard, a geologist at the University at Buffalo, has shown that it’s possible to use radar measurements taken from a satellite to monitor tiny changes in the rise and fall of Earth’s surface—differences that are strongly correlated with groundwater levels in aquifers below.
They found that satellite-mounted radar can accurately detect subsidence or deformation to within just a few millimeters, precise enough to help scientists accurately estimate groundwater levels. In doing so, the scientists may have developed a low-cost way to monitor aquifer health. Satellite radar could potentially supplement other groundwater monitoring systems, or serve as a primary tool in places where no monitoring is done today.
For their study, they tracked minute elevation changes associated with groundwater levels within the Santa Clara Valley Water District in California during the drought of 2012 to 2015. They chose this area in part because the local water district has a sophisticated monitoring network in place. This would give them readings against which they could check the results they received from space.
The water district’s robust monitoring system spans two basins that serve 2.1 million customers. Employees manually take monthly readings from 215 wells to measure groundwater levels, and also receive automated daily readings from 85 of those wells.
Those 85 wells, equipped with In-Situ pressure transducers and Campbell Scientific data loggers, send data back to headquarters over cellular networks at 20 solar-powered telemetry stations. The district has also anchored two extensometers, which are long pipes, 300 meters below ground to measure vertical motion that can occur when an aquifer expands or contracts.
Chaussard’s team used data from a type of radar known as InSAR, short for Interferometric Synthetic Aperture Radar. The radars are aboard a fleet of four satellites called the COSMO SkyMed system, which is operated by the Italian Space Agency. The orbits of these satellites were such that Chassard’s team could collect daily readings over the Santa Clara Valley.
InSAR emits waves that bounce off the Earth’s surface and reflect back to the satellite, where they are measured as a way to estimate the satellite’s distance to Earth. One advantage of InSAR is that, because it emits waves with 3.1-centimeter wavelengths at 9.6-gigahertz frequencies (which is in a region of the spectrum known as the X-band), its emissions can easily penetrate thick layers of clouds. The downside is that these relatively short wavelengths can be blocked or scattered by anything larger than 3.1 cm—even objects as small as a leaf.
Still, they were able to detect that the ground surface in the valley “breathed” about 3 cm over the course of a year, an amount considered normal for that region. During the drought period, they observed changes that were twice as great.
They also found that groundwater levels in the area began to rebound in 2014, a year before rains returned to the state. They attribute this turn to an aggressive water conservation effort by the Santa Clara Valley Water District, which appears to have successfully reversed the aquifer’s depletion before the drought was over. “The recovery was directly related to the efforts of the water district in conserving water,” Chaussard says.
Though they analyzed readings during a period of drought, Chaussard says the radar data is precise enough to be used to monitor daily changes in healthy aquifers. They recently published their work in the Journal of Geophysical Research. Earlier research by Rosemary Knight, a geophysicist at Stanford University, and others, has also found satellite radar to be an effective means of monitoring groundwater.
“I would see this as a very complementary tool,” says Vanessa De La Piedra, groundwater manager for Santa Clara Valley Water District. “With the satellite technology, they can measure millimeter-scale changes in the land surface, so it’s very impressive kind of precision with the data.”
Many water districts across the United States and in other countries do not have any kind of monitoring system in place, and could not afford to install one. What’s more, says Chaussard is that satellite radar could provide a more comprehensive view of an entire aquifer, rather than collecting data only from designated sampling sites.
“The data that comes from [terrestrial readings] is a lot more precise than what we can get with the satellites. but with the satellites, we have a lot better idea of the scale of the entire aquifer,” she says.
Editor’s note: This post was updated on 10/09 to correctly state the degree of elevation change during a drought compared to a normal period.