Evolution of Water Availability and Land Subsidence in California’s San Joaquin Valley
U.S. Geological Survey
The San Joaquin Valley covers about 26,000 km2 and is one of the most productive agricultural regions in California. Because the valley is semi-arid and the availability of surface water varies substantially from year to year and season to season, the agricultural industry developed a reliance on local groundwater for irrigation. Groundwater pumpage caused significant and extensive drawdowns, resulting in land subsidence at rates up to 0.3 meters per year. The completion of state and federal water distribution systems by the early 1970s eased the reliance on local groundwater as dependence shifted to diverted surface water. As a result, groundwater levels recovered and subsidence virtually ceased. In the last 20 years, however, land-use changes and an assortment of restrictions to surface-water availability—including droughts and environmental flows—have resulted in increased pumping and renewed land subsidence. The spatially variable subsidence has changed the land-surface slope in some places and caused operational, maintenance, and construction-design problems for water-delivery and flood-control canals as well as other infrastructure.The location, extent, and magnitude of land subsidence from the 1920s to 2015 were examined using Interferometric Synthetic Aperture Radar (InSAR), geodetic survey (spirit leveling and Global Positioning System surveys), extensometer, and continuous Global Positioning System (CGPS) data to estimate subsidence. Spatially and temporally dense data types are complementary and are needed to understand the mechanisms that underlie the spatial subsidence patterns and improve subsidence simulations. Since InSAR data became available in 1992, the comprehensive spatial coverage it provided has allowed the delineation of the spatial extent of subsidence: Geodetic survey, extensometer, and CGPS measurements show monthly, seasonal, and (or) inter-annual variations in subsidence rates at specific locations. Spirit-leveling surveys between the 1920s and 1970 indicated that more than half of the valley was affected by at least 0.3 m, and a local maximum exceeded 8 m of subsidence. Data from extensometers, combined with geodetic survey or CGPS data, indicated that compaction of sediments below the Corcoran Clay was the primary cause of subsidence. Data from extensometers, combined with other data sources, indicated that beginning around 1970, subsidence during the remainder of the 20th century occurred largely during drought periods. However, data from InSAR, geodetic surveys, extensometers, and CGPS during the 21st century showed subsidence patterns have changed, and in some circumstances, subsidence occurred irrespective of climatic conditions and was tied to land-use changes. Planning for the effects of subsidence in the San Joaquin Valley is important for water managers. As land use and surface-water availability continue to vary; long-term groundwater-level and subsidence monitoring, analysis, and modeling are critical to understanding the dynamics of historical and continued groundwater use resulting in water-level and groundwater-storage changes, and associated subsidence. Modeling tools, such as the USGS Central Valley Hydrologic Model, can be used in the evaluation of management strategies to mitigate adverse impacts due to subsidence while also optimizing water availability. This knowledge will be critical for successful implementation of recent legislation aimed toward sustainable groundwater use.