The GRACE satellites are, unfortunately, about to die. The pair of formation-flying gravity sensors has provided revolutionary data over the past 15 years, tracking ice loss and groundwater depletion through the subtle change in gravity triggered by that lost mass. Replacements will hopefully be launched soon.
But GRACE isn’t the only way to track groundwater in places like California. InSAR is a satellite radar measurement technology that can detect extremely small changes in ground elevation—including before-and-after mapping of the shifts around an earthquake. But they also work well for tracking elevation changes that occur as areas gain or lose groundwater. A new study takes advantage of that to track the golden state’s response to its recent drought, and it even detected changes due to the policy response.
Rise and fall
Parts of California have lost impressive amounts of elevation over the years as groundwater levels were lowered. The loss of water in between grains of sediment allows the sediment to compact more densely. And if you compact the sediments, the surface elevation drops.
That is largely a permanent change to the aquifer, but there can also be shorter-term, reversible surface movements tied to fluctuating groundwater levels. In aquifers that are essentially sealed, losing or gaining water changes water pressure, slightly inflating or deflating the tiny open spaces in between the sediment grains. So when groundwater levels rise, the ground swells a bit; when levels sink, the surface sinks, too.
InSAR satellites could measure that imperceptible movement, but you need enough instruments in orbit to make the frequent passes required to track seasonal changes. Taking advantage of an Italian quartet of satellites, a group of researchers led by the University of Buffalo’s Estelle Chaussard collected measurements across California’s Santa Clara (Silicon) Valley almost daily from 2011 to 2017. That time period includes California’s recent extreme drought and the accompanying changes in water use—which the satellites were able to observe from space.
In the first half of the 20th century, the Santa Clara Valley lost up to four meters of elevation as people pumped groundwater much faster than rainfall could replace it. After 1965, major water use changes put an end to the sinking, and the area has actually been slowly rising a few millimeters each year.
But in 2013, as the drought kicked in, the increased reliance on groundwater reversed that trend. The satellites measured the ground sinking as much as two centimeters that year and another three centimeters in 2014. In total, the researchers estimate that about a tenth of a cubic kilometer of groundwater had disappeared—as if five summers had been strung together without a single winter to replenish the lost water.
But starting in September 2014, things started to rebound—a month before the winter rains even started and about a year before the drought ended. Why then? That was when strict, mandated water conservation efforts kicked in, and surface water was brought in to help refill the aquifers. Over 2015, the land surface responded accordingly, popping back up almost two centimeters. By the end of 2016, groundwater levels were back to where they started before the drought, and land elevations had rebounded most (but not all) of the way.
When groundwater levels drop to new lows, the resulting compaction can actually permanently reduce the aquifer’s capacity. Fortunately, the researchers found that this didn’t happen. Water levels stayed above historic lows in the Santa Clara Valley, and the researchers found that the fall and rise of the land surface seems entirely tied to that reversible “deflation” of sealed aquifers.
A 2014 study found that groundwater depletion in California’s (much, much larger) Central Valley may actually be altering stress patterns on faults in the region, influencing the timing of earthquakes. The researchers mention that it’s possible the water level changes they observed in the Santa Clara Valley could have a similar, albeit smaller, effect. But that’s a question for its own study.
The researchers argue that this work shows how useful high-resolution InSAR monitoring can be. After all, a few satellites whizzing overhead beats a few thousand trips to measure well levels across the valley each day.
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