The San Joaquin Valley is one of the most productive agricultural regions in the nation. Beginning around the 1920's, farmers relied upon groundwater for water supply. Over time, overpumping caused groundwater-level declines and associated aquifer-system compaction and land subsidence that resulted in permanent aquifer-system storage loss. By 1970, significant land subsidence (more than one foot) had occurred in about half of the San Joaquin Valley, or about 5,200 square miles (Poland and others, 1975), and locally, some areas had subsided by as much as 28 feet.
Reduced surface-water availability during 1976-77, 1986-92, 2007-09, and 2012-2015 caused groundwater-pumping increases in the San Joaquin Valley, declines in water-levels to near or beyond historic lows, and renewed aquifer compaction. The resulting land subsidence has reduced the freeboard and flow capacity of the Delta-Mendota Canal—as well as the California Aqueduct and other canals that transport floodwater and deliver irrigation water—requiring expensive repairs.
Continued groundwater-level and land-subsidence monitoring in the San Joaquin Valley is warranted because groundwater levels are poised to decline when surface-water deliveries do not meet demand, which may result in additional land subsidence. Even in precipitation record-setting years such as 2010-11, water deliveries fell short of requests in the Central Valley. Therefore, it is likely that groundwater levels will decline in the future. Integrating subsidence, deformation, and water-level measurements—particularly continuous measurements—permits analysis of aquifer-system response, which enables identification of the preconsolidation head and calculation of aquifer-system storage properties. This information could be used to improve numerical models of groundwater flow and aquifer-system compaction, to refine estimates of governing parameters, and to predict potential aquifer-system compaction which could be used to manage water resources while considering land subsidence.
A subsidence monitoring network in the San Joaquin Valley consisting of 31 extensometers was implemented in the 1960s to help quantify the extent and magnitude of the subsidence that was first discovered in the 1950s. By the 1980s, monitoring activities were greatly reduced. To identify existing and future subsidence, a new monitoring network is currently being developed. This includes refurbishing some of the extensometers and piezometers from the old network, and augmenting these ground-based measurements with remotely-sensed measurements from continuous Global Positioning System (CGPS) stations and Interferometric Synthetic Aperture Radar (InSAR). Preliminary results from the monitoring network indicate that subsidence is occurring in locations of known historical subsidence, as well as newly identified areas, such as Madera.
As the monitoring network developed fully, the integration of remotely-sensed measurements from InSAR and GPS with ground-based measurements from extensometers and spirit-leveling enabled construction of spatially and temporally dense timeseries of aquifer-system compaction and land subsidence. The high frequency measurements of compaction, land subsidence, and groundwater levels allow for inclusion of shorter-term elastic deformation in simulations that was not well-constrained in most of the San Joaquin Valley.
There are concerns that fluctuating land-surface elevations due to subsidence and uplift in the valley could cause serious operational-maintenance and design construction problems for the California Aqueduct surface-water delivery system.
Surface-water imports via the California Aqueduct in the late 1960's and early 1970's, and the associated decrease in groundwater pumping, resulted in a steady recovery of water levels and a reduced rate of compaction. During the droughts of 1976-77, 1987-92, and 2007-09, diminished deliveries of imported water prompted increased groundwater pumping to meet irrigation demands. This increased pumping resulted in water-level declines reaching near historic lows and periods of renewed compaction. Following each of these droughts, recovery to pre-drought water levels was rapid and compaction virtually ceased.
Land subsidence contours showing vertical changes in land surface in the central San Joaquin Valley area, California, during January 8, 2008-January 13, 2010. The top graph illustrates elevation changes computed from repeat geodetic surveys along Highway 152 for 1972-2004. The bottom graph depicts elevation changes computed from repeat geodetic surveys along the Delta-Mendota Canal for 1935-2001. Subsidence data along Highway 152 were computed from published National Geodetic Survey elevations. Subsidence graph for the Delta-Mendota Canal was obtained from the San Luis and Delta-Mendota Water Authority and the Central California Irrigation District
The U.S. Geological Survey, in cooperation with the U.S. Bureau of Reclamation and the San Luis and Delta-Mendota Water Authority, assessed land subsidence in the vicinity of the Delta-Mendota Canal as part of an effort to minimize future subsidence-related damages to the canal. The location, magnitude, and stress regime of land-surface deformation during 2003-10 were determined by using extensometer, Global Positioning System (GPS), Interferometric Synthetic Aperture Radar (InSAR), spirit leveling, and groundwater-level data. Comparison of continuous GPS, shallow extensometer, and groundwater-level data, combined with results from a one-dimensional model, indicated the vast majority of the compaction took place beneath the Corcoran Clay, the primary subsurface regional confining unit.