Rapid drought-induced land subsidence and its impact on the California Aqueduct

The Central Valley in California is characterized by a semi-arid climate prone to droughts, a variable surface water supply, and immense agricultural areas dependent on groundwater irrigation. The complex aquifer system in the valley is composed of alternating layers of coarse sediments and fine-grained sediments acting as confining materials. Both elastic and inelastic land surface deformation are coupled with the groundwater fluctuations historically observed in the Central Valley. Surface deformation poses a hazard to the California Aqueduct, which supports Central Valley agriculture and large urban populations in Southern California. The risk of reduced aqueduct efficacy and expensive repairs compels the water resource managers to monitor land deformation. A persistent drought in the region began in 2012 and was abruptly followed by a wet period from Dec-2016 to Feb-2018. NASA’s UAVSAR L-band synthetic aperture radar acquired 31 high resolution radar images between May-2013 and Nov-2018. The interferometric phase difference between acquisitions is used to develop a time series of vertical displacement and identify and track a rapidly forming subsidence feature adjacent to the California Aqueduct. The surface area of the feature that subsided 10 cm or more by the end of the time series reaches 4452 hectares and a 10.5+ km segment of the aqueduct. This study also incorporates concurrent water elevation data, well construction reports, nearby extensometer measurements, aquifer material characterization, and environmental conditions to offer insight into subsurface properties and mechanics and estimate the permanent loss of aquifer storage volume.