Northeast SJV (NESJV), Southeast SJV (SESJV), Western SJV (WSJV), and Southern SJV (SSJV). These regions were primarily defined from the California Department of Water Resources SJV groundwater subbasins (CDWR, 2003); minor modifications were made to the CDWR defined subbasin boundaries to optimize separation of groundwater derived from the Sierra Nevada to the east and from the Coast Ranges to the west under predevelopment conditions. Each region was divided into equal-area grid cells, ranging from 280 km2 in the SESJV to 984 km2 in the SSJV, for a total of 88 cells. These cell sizes were the minimum sizes in each region for which 60% of the cells in the region would contain at least 1 well with full major ion data in both time periods for which groundwater quality data were analyzed. These regions and grid cells were used for data analysis in the following publication: Hansen, J.A., Jurgens, B.C, Fram, M.S., Quantifying Anthropogenic Contributions to Century-Scale Groundwater Salinity Changes, San Joaquin Valley, California, USA: Science of the Total Environment, vol. XX, no. X, pp. XX-XX, 2018.