In the southern portion of the San Joaquin Valley, California, agricultural demands are so high that they account for roughly 10% of the pumped groundwater in the United States. Because of the high water demand of the crops that are grown, large volumes of groundwater are required to irrigate the crops. Groundwater pumping in this area has caused significant declines in groundwater levels over the past century, leading to subsidence as high as 10 m over this time period. Water managers are concerned that the subsidence represents a permanent loss of storage.

Using Interferometric Synthetic Aperture Radar (InSAR), we can measure subsidence with cm-scale accuracy over large areas. We have processed InSAR data over the study area from 2007-2010. Using groundwater level data, geologic models, and knowledge of the geomechanical properties of sediments in the area, in conjunction with InSAR, we estimated the permanent loss of groundwater storage.

Our results show that the vast majority (98%) of subsidence in our study area is related to the permanent loss of groundwater storage. By volume this is m3 of groundwater storage, or roughly 9% of the volume of groundwater used from 2007-2010.