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 groundwater is stored in a complex aquifer system composed of alternating layers of coarse sediments and fine-grained sediments acting as confining materials. Groundwater fluctuations are coupled with both the elastic and inelastic land surface deformation 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 water resource managers to carefully monitor land deformation in the Valley. A persistent drought in the region began in 2012, intensified in 2014, and was abruptly alleviated 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 ha and a 10.5+ km segment of the aqueduct. This study also incorporates ex-tensometer measurements, precise leveling surveys, Sentinel-1A displacement, concurrent water elevation data, well construction reports, nearby extensometer measurements, aquifer material characterization, and environmental conditions. Spatiotemporal data availability limits the appropriateness of calculations and models able to be performed for different sites along the aqueduct. We aim to offer insight into heterogeneous subsurface properties and mechanics, estimate the permanent loss of aquifer storage volume, and identify additional data that would aid water management.