Drought-induced rapid subsidence in the Central Valley and its impact on the California Aqueduct

California’s Central Valley is the site of a complex heterogeneous aquifer system composed of alternating layers of coarse sediments and fine-grained confining material. Confined and semi-confined aquifer systems experience groundwater fluctuations coupled with elastic and inelastic land surface deformation. Data from the UAVSAR L-band synthetic aperture radar acquired between May 29, 2013 and November 27, 2018 were used to generate a high resolution deformation time series, and identify and track the development of a small subsidence feature that developed immediately adjacent to the California Aqueduct. By the end of the time series, the feature surface area that subsided 10 cm or more was 4452 hectares. The California Aqueduct supports Central Valley agriculture and large urban populations in Southern California, and a 10.5+ km segment of the aqueduct subsided 10 cm or more due to this one subsidence feature.  The Central Valley experienced a persistent drought starting in 2012, followed abruptly by a wet period from December 2016 to February 2018. The data were analyzed for the drought period in conjunction with hydraulic head level data from nearby wells to solve for aquifer storage parameters and volume storage loss.  We found the inelastic volume storage loss was 7.1x10⁶ m³, or an average rate of 7x10³ m³/day.

Compared to satellite SARs, UAVSAR has a higher spatial resolution (<2 m ground resolution) and signal-to-noise ratio. Because of these factors along with spatial averaging to reduce phase noise, accuracy is increased and temporal decorrelation is reduced, so a greater proportion of the scene produces useful measurements while maintaining a spatial resolution of 7mx7m. The resolution achieved with UAVSAR time series processing allows for modeling and monitoring localized subsidence features affecting the aqueduct that were not previously observed by satellite. The data, analysis, model, and results are described in this presentation.  It is notable that UAVSAR is a prototype for the L-band SAR to be launched on the NASA-ISRO SAR Mission (NISAR) in 2022.  In that context, we also discuss and compare the expected performance of the two instruments and highlight how these technologies can be used to study aquifer properties in areas where water level data are sparse in both space and time.