Groundwater is a critical resource in Southern California, where long-term drought and climate change place increasing pressure on local aquifers. Some regions, like the Hollywood Basin (a small region in and around the West Hollywood neighborhood), are increasing their reliance on these aquifers in order to reduce the amount of water imported from elsewhere. A new Caltech-led study provides the most detailed picture to date of how the Hollywood Basin responds to groundwater usage, revealing that current estimates of sustainable groundwater yield may be too high.
The study, led by former Caltech postdoctoral scholar Yujie Zheng, uses three decades of InSAR (Interferometric Synthetic Aperture Radar) satellite measurements to track subtle ground movements, on the order of millimeters, across the Los Angeles region. These movements reflect changes in groundwater storage, allowing researchers to infer how much water is being removed or replenished over time.
As rain percolates underground, it fills the space between soil particles (such as sand, gravel, and clay)-in the Hollywood Basin, this is located around 600 feet down. This aquifer is estimated to hold around 65 billion gallons of water, which can be pumped at wells to provide drinking water for the community. Too much removal of groundwater, however, has significant negative effects, including shrinking the available storage space itself and depressing the ground.
InSAR works by sending radar pulses from satellites to the Earth's surface and measuring how the returned signal changes over time. Over the last 30 years, multiple missions have used the technology to monitor ground movement in the Los Angeles area.
These long-term observations showed that the ground surface in the Hollywood Basin responds directly to groundwater pumping. During periods when pumping stopped, the ground rebounded. When pumping resumed, the ground subsided.
"We reconstructed a 30-year record of surface movements of the Hollywood Basin, and there's very clear correlation between the surface movement and the groundwater production," Zheng says.
InSAR was critical for the findings, because unlike larger groundwater basins in California, the Hollywood Basin lacks a dense network of monitoring wells and does not fall under certain state reporting requirements.
"This study allowed us to estimate sustainable yield without water-level data," Zheng says. "Groundwater management relies on water data from monitoring wells, which is often scarce in smaller basins and may not be comprehensive enough for accurate groundwater models."
By combining the long-term InSAR record with groundwater production history, the team assessed how much pumping the aquifer can withstand without causing permanent damage such as subsidence or loss of storage capacity.
The City of Beverly Hills, which utilizes the Hollywood Basin's groundwater resources, currently cites a safe yield of 3,000 acre-feet per year for local groundwater extraction. This figure comes from monitoring stations at wells, which are not comprehensive in the region. But the new satellite-based analysis suggests that the sustainable-yield figure should be significantly lower to prevent such permanent damage.
"We find that the sustainable yield we estimated from the InSAR is 1,200 to 1,400 acre-feet per year," Zheng says.
Most parts of the aquifer showed elastic behavior-meaning they recover after pumping stops-but smaller zones near production wells have in fact exhibited signs of permanent compaction, indicating potential long-term loss of storage.
The findings come at a time when climate variability is reshaping water planning in California. Los Angeles relies heavily on imported water, yet those supplies have become increasingly unreliable. Local agencies have begun shifting back toward groundwater use, including in the Hollywood Basin.
Climate extremes, including intense drought followed by unusually wet years such as the 2022-23 atmospheric river season, further complicate long-term planning.
With more open-access satellite data becoming available, including from the NISAR mission recently launched by NASA and the Indian State Research Organization (ISRO), researchers hope to provide water managers with powerful new tools for sustainable groundwater management. "Regional climate patterns are getting more and more unpredictable," Zheng says. "What we are hoping to show is that we can integrate long-term InSAR data for groundwater management."
While studying groundwater behavior, the team also observed another striking signal: a subsidence of about 30 centimeters over 30 years in the La Brea area.
The researchers suggest that this phenomenon is linked to oil and gas production-the Los Angeles Basin contains a major underground oil reserve and produces petroleum. "It's still subsiding very stably in this area," Zheng says, noting that even as oil production has declined, large volumes of water continue to be extracted along with remaining oil. Previous studies had noted subsidence in the area, but this research provides the first clear multi-decade record showing that the trend has continued for 30 years.
Zheng, now an Assistant Professor at UT Dallas, is currently leading a NASA-funded project analyzing groundwater recharge across the greater Los Angeles region during the exceptionally wet 2022-23 period. The team will combine InSAR, seismic data, and hydrological modeling to better understand how aquifers respond during rare high-rainfall periods.
A paper describing the research appears in the journal Water Resources Research on November 5 and is titled "Integrating Long‐Term InSAR Monitoring Into Local Groundwater Management: Insights From the Hollywood Basin, California." Zheng co-authored the study with Mark Simons , the John W. and Herberta M. Miles Professor of Geophysics and director of the Brinson Exploration Hub at Caltech . Funding was provided by NASA.
