Land Subsidence in the San Joaquin Valley

Science Center Objects

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.

Importance of Monitoring Subsidence

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.

San Joaquin Valley Land Subsidence Monitoring Network

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.

 

map of land subsidence in the San Joaquin Valley from 1926-1970, shaded by the amount of subsidence in meters

In the 1960s and 1970s, the USGS managed 31 extensometers operating at 21 sites and extensive spirit-leveling networks. (Public domain.)

Map of active San Joaquin Valley Land Subsidence Monitoring Network sites in the 1980s

By the 1980s, monitoring activities in the San Joaquin Valley were greatly reduced. While 3 new extensometers were constructed, 8 were discontinued, leaving 26 extensometers operating at 18 sites.  The decade also brought with it reduced spirit-leveling networks and a decrease in the frequency of measurements. (Public domain.)

Map of currently active extensometer and continous GPS sites in the San Joaquin Valley

California has experienced three droughts thus far in the 21st century (2001-2002, 2007-2009, and 2012-2016), bringing renewed subsidence to the San Joaquin Valley and the revitalization of the Monitoring Network. Four extensometers were refurbished in 2011-2012, which involved new reference tables and instrumentation, and the construction of new shelters.  These were added to the six extensometers that were operating at five sites.  Spirit-leveling and campaign GPS networks were generally maintained on major water-conveyance canals and highways only, and 13 Continuous GPS sites (maintained by various agencies/groups) are in operation on the Valley floor. (Public domain.)

Integrated Measurements

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.