We evaluate the benefits of space-derived ground deformation measurements for basin-wide characterization of aquifer-system properties and groundwater levels. We use Interferometric Synthetic Aperture Radar (InSAR) time series analysis of ERS, Envisat, and ALOS SAR data to resolve 1992-2011 ground deformation in the Santa Clara Valley, California. T-mode principal component analysis successfully isolates temporally variable deformation patterns embedded in the multidecadal time series. The data reveal uplift at 0.4 cm/yr between 1992 and 2000 and < 0.1 cm/yr during 2000-2011, illustrating the end of the aquifer-system's poroelastic rebound following recovery of hydraulic heads after the 1960s low stand. In addition, seasonal elastic deformation with amplitude of up to 3 cm, in phase with head fluctuations, is observed over the confined aquifer sharply partitioned by the Quaternary Silver Creek Fault (SCF). Integration of this deformation with hydraulic head data enables characterization of the aquifer-system storativity and elastic skeletal specific storage. Modeling of the deformation partitioning across the SCF constrains the fault's last tectonic activity, hydraulic conductivity, and material composition. The SCF likely cuts the shallow confining clays and was last active since~140 ka, it has a horizontal hydraulic conductivity several orders of magnitude lower than the surrounding aquifer-system, and is likely composed of clays, making it an effective barrier to across-fault fluid flow. Finally, we show that after a period of calibration, InSAR can be used to characterize basin-wide water level changes without well measurements with an accuracy of 70%, which demonstrates that it provides useful data for groundwater management.

Groundwater management typically relies on water‐level data and spatially limited deformation measurements. While interferometric synthetic aperture radar has been used to study hydrological deformation, its limited temporal sampling can lead to biases in rapidly changing systems. Here we use 2011–2017 COSMO‐SkyMed data with revisit intervals as short as 1 day to study the response of the Santa Clara Valley (SCV) aquifer in California to the unprecedented 2012–2015 drought. Cross‐correlation and independent component analyses of deformation time series enable tracking water through the aquifer system. The aquifer properties are derived prior to and during the drought to assess the success of water‐resource management practices. Subsidence due to groundwater withdrawal dominates during 2011–2017, limited to the confined aquifer and west of the Silver Creek Fault, similar to predrought summer periods. Minimum water levels and elevations were reached in mid‐2014, but thanks to intensive groundwater management efforts the basin started to rebound in late 2014, during the deepening drought. By 2017, water levels were back to their predrought levels, while elevations had not yet fully rebounded due to the delayed poroelastic response of aquitards and their large elastic compressibility. As water levels did not reach a new lowstand, the drought led to only elastic and recoverable changes in the SCV. The SCV lost 0.09 km3 during the drought while seasonal variations amount to 0.02 km3. Analysis of surface loads due to water mass changes in the aquifer system suggests that groundwater drawdowns could influence the stress on nearby faults.

Although still continuing, surface slip from the 2004 Parkfield earth-

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