Satellite radar measurements provided by interferometric synthetic aperture radar, or InSAR, can detect subtle, millimeter-scale changes in ground elevation as areas gain or lose groundwater. Researchers have recently utilized the technology to quantify the effects of California’s drought. And in an unexpected twist, they also detected shifts due to policy changes.
Moisture levels affect soil surface elevation. Fluctuating groundwater levels can inflate the tiny open spaces between sediment grains and affect the level of the earth’s surface. The loss of water in between grains of sediment lowers surface elevation by bringing these particles closer together.
From 2011 to 2017, a team of scientists led by the University of Buffalo’s Estelle Chaussard collected geophysical measurements across California’s Santa Clara Valley. This time period included measurements taken during the area’s extreme drought as well as after the state’s regulatory response. Data for the study was relayed from COSMO SkyMed, a constellation of four Italian satellites, as often as once a day.
“InSAR allows us to precisely measure small movements of the Earth’s surface over large areas,” Chaussard said. “Scientists have used it to measure surface deformation related to volcanoes and earthquakes; we expanded its use by applying it to tracking groundwater.”
In 2013, the satellites measured a 2-centimeter decrease in ground-level elevation, followed by another 3 centimeters in 2014. During that time, researchers estimate that about a tenth of a cubic kilometer of groundwater had disappeared.
But starting in September 2014, the ground surface began to rebound even before the winter rains had an opportunity to replenish groundwater sources. What could have produced this effect? The researchers determined that the recovery was due to the state’s newly instated water conservation efforts—policies that diverted surface water to refill aquifers.
During 2015 and 2016, the land surface rose nearly 2 centimeters. By the end of 2016, groundwater levels were back to where they started before the drought, and land elevations had almost completely rebounded.
Understanding fluctuations in groundwater levels and the rise and fall of ground surface elevation is of critical importance. A 2014 study found that groundwater depletion in California’s Central Valley affected stress patterns on faults and on the occurrence of earthquakes. Chaussard and her colleagues indicate that the water level changes they observed in the Santa Clara Valley could have a similar effect.
In all, this discovery seems encouraging—and proof of the regenerative effects provided by thoughtful conservation policy. What are your impressions?
