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How Satellites Can Monitor California’s Underground Water



CALIFORNIA IS RUNNING on groundwater right now. As the state has cut down on surface water deliveries from rivers and reservoirs, farmers and municipal water suppliers have reacted by sucking more and more out of Madre Earth. The state’s land, in response, is sinking lower and lower, day by day, year by year.

In times of crisis, turning to groundwater is understandable (it may even be unavoidable). But—as it stares down its inevitably dessicated future—California is finally waking up to the need to monitor and protect these reserves. To do that, the state’s Department of Water Resources is turning to new techniques using satellite data which, by measuring changes in the ground above, can keep an eye on water levels below. Essentially, if the Golden State is going to weather this disaster, it will need some help from up high.

Earlier this week Tom Farr, a geologist at NASA’s Jet Propulsion Laboratory in Southern California, completed the first of many maps for the California Department of Water Resources with data collected by the European Sentinel-1 satellite. That map, of the state’s agriculture hub in the Central Valley, is part of a larger project to use NASA expertise to study—and try to help combat—California’s drought.

One of the ways California will use Farr’s maps is to identify groundwater trouble spots (the faster the land is sinking, the faster the water is being depleted). A new law signed last year by Governor Brown requires regional water agencies to devise groundwater sustainability plans. To do that, though, they’ll need good data. And, at the moment, good groundwater information is hard to find and expensive to gather.

The state can monitor groundwater directly by measuring water levels within wells—but digging new wells is expensive, and existing wells may be on private land. “The other problem,” Farr says, “is that you’re not sure what kind of aquifer they’re drilled into.” Aquifers can be confined, separated from the surface by an impermeable layer of dirt or rock, or unconfined, with water entering from the ground directly above. Well data can be hard to interpret as most wells penetrate more than one level in the aquifer system. So, not only is the water-level in the well hard to interpret in terms of groundwater volume, explains Farr, but they provide portals for water to move between different parts of the aquifer system—altering the very thing geologists seek to measure.

Traditional land surveying techniques can also track water—but that method is labor-intensive. After days of painstaking measurements taken with tripods and levels, a surveyor will be left with one small area of measurement. Surveyors can also use GPS data, Farr says, but there are very few GPS stations in the Central Valley.

A better way, Farr says, is to use interferometric synthetic aperture radar, or InSAR. This technique, first developed about a decade ago, monitors changes in ground deformation. In the early days it was used almost exclusively to study earthquakes and volcanoes—creating maps like these, of the April 25th earthquake in Nepal. “Groundwater was like the poor stepchild,” says Farr. But as California enters its fourth year of crippling drought, more researchers are working to refine InSAR’s water watching abilities.

InSAR works by beaming radar waves at the surface of the Earth. The wave bounces off the surface and returns to the satellite, traveling in an undulating, up-down, sinusoidal wave. The height of the ground determines where the wave is in its oscillation as it returns to the satellite. The same ground can be re-scanned again in, say, a month or two, to detect changes in the surface level and shape within an accuracy of a few centimeters. “It’s almost miraculous,” says Farr.

InSAR measurements have been made about once a month, around the entire globe, for decades. This gives gives researchers a four-dimensional picture of how the Earth has swelled and shrunk in the past 25 years.

But only recently has the technology been applied to characterizing groundwater in agricultural areas. Jessica Reeves and Rosemary Knight, geophysicists at the Stanford School of Earth, Energy & Environmental Sciences, were among the first people to apply this technique in this way, and Knight’s team continues to refine the calibrations linking ground level to groundwater levels.

The sinking that they’re tracking—as much as a foot a year in some places—threatens to become an enormous problem. That’s not just because the water will eventually run out, which (if pumping continues unabated) it will. It’s a more immediate threat to surface-level infrastructure: aqueducts, bridges, roads and train tracks. Damages due to sinking land in Santa Clara Valley is estimated at more than $756 million.

Groundwater is also crucial for supporting natural ecosystems, such as rivers, wetlands and lakes. Pumping is said to have sunk flows on the Mattole, Eel and Upper Klamath rivers, and groundwater extraction on the Central Coast is blamed on increased saltwater intrusion into previously fresh-water aquifers.

Despite the scope of California’s water problems, Knight is hopeful a revolution within Earth science is afoot—one that will help the state get through hard, dry times. She compares the potential of techniques like InSAR, electrical resistivity tomography and ground penetrating radar to the medical impact of CT scans and MRIs. “Doctors used to basically have to poke holes in their patient to see if something was wrong,” says Knight. Just as those medical imaging techniques made exploratory surgery a thing of the past, Knight thinks better data will help geologists and hydrologists address the state’s water woes.

California can’t solve its drought, just as doctors cannot cure most serious illnesses. Better imaging and better information however, will show the way to best treat the patient in the future.

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