California governor Jerry Brown declared a drought state of emergency in January 2014, following years of wintertime rainfall levels dipping below historic averages. A lack of rainfall throughout 2015—precipitation was 20% below average—sustained the drought. Surface water levels got so low that residents had to increasingly tap into groundwater resources in order to meet agricultural, urban, and industrial needs. This usage put immense pressure on groundwater resources and made it extremely difficult to manage water resources across the state.
These pressures can have long-term effects too. For example, rapid drawdown of groundwater resources can cause the land above it to sink. A new study examines groundwater levels and sinking land in California’s Santa Clara Valley in the context of the state’s widespread drought.
When an aquifer—a system of porous rocks that allow water to flow through them—reaches an all-time-low water level, large areas above it experience land subsidence. Subsidence is deeply damaging to infrastructure, such as buildings and roads, and can increase flooding in coastal areas.
The aquifer system in California’s Santa Clara Valley, home to the Silicon Valley’s robust tech industry, is made up of many alternating layers of clay and sand, deposited over time by the rising and falling sea level. It was here that in the early 1900s, scientists first observed land subsidence due to groundwater withdrawal within the United States.
For their study, Chaussard et al. compared physical changes in the Santa Clara Valley aquifer system during California’s recent drought to prior studies of the area to pinpoint changes that may be related to the drought. Past studies have used a type of satellite-based radar called interferometric synthetic aperture radar to measure changes in surface elevations. Although this technique provides many measurements over a large area, these data sets contain sizable gaps in time, with many days, and sometimes many weeks, passing between measurements.
Given the rapidly changing conditions of the aquifer system throughout the drought and drought recovery process, the team tried a different tack. Using a constellation of four Earth-observing satellites, Italy’s COSMO-SkyMed, the researchers were able to acquire data in much more frequent intervals (as often as once per day) from 2011 to 2017. Then, by analyzing these data, the researchers were able to trace small changes in ground elevation and relate them to the movement of water through the aquifer system.
The researchers found that water levels and elevations both hit an all-time low in 2014, shortly after Brown declared the drought state of emergency, but then started to rebound in late 2014, while the drought was still going strong, amid new groundwater management efforts. Water levels were back to normal by the start of 2017, whereas elevations took a bit longer to normalize.
Although no permanent water level changes were sustained, the researchers found, some land elevation was lost—more than 4 times the amount that is typically lost during seasonal variations. Also, the researchers found that changes in the aquifer’s water mass caused by tapping into groundwater stores could affect the stress placed on nearby faults and potentially influence the occurrence of earthquakes.
This study allows scientists to better understand rapidly changing drought conditions, as well as the long-lasting strain that these changes have on a region’s aquifer system. With a more intimate knowledge of these processes, researchers can better assess and help improve existing water resource management practices, which is especially important in a global climate that is increasingly prone to drought. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2017JB014676, 2017)
—Sarah Witman, Freelance Writer