Central Valley: Drought Indicators
The hydrology of the present-day Central Valley is driven by surface-water deliveries and associated groundwater pumpage, which in turn reflect the spatial and temporal variability in climate, water availability, and land use. Since the early 1990s, the availability of surface water has decreased because of operational changes of the federal Central Valley Project and the California State Water Project. Although irrigation has become more efficient, since 2000, land use in the Central Valley has trended toward the planting of permanent crops (vineyards and orchards), replacing non-permanent land uses. During the recent droughts of 2007-2010 and 2012-2017, groundwater pumping has increased from the combined effects of the drought and these changes. The increased pumping has re-initiated subsidence. In order to document historical subsidence and monitor continued changes, the USGS has gathered and interpreted data from a variety of sources.
Interferometric Synthetic Aperture Radar (InSAR) is a satellite-based remote sensing technique that can detect centimeter level land subsidence (and uplift) at high spatial detail. Synthetic Aperture Radar (SAR) imagery is produced by reflecting radar signals off a target area and measuring the two-way travel time back to the satellite. The InSAR technique uses two SAR scenes of the same area taken at different times which are then processed together to produce maps called interferograms that show relative land-surface-elevation change between the two SAR acquisition dates.
The Areas of Interest dataset is where InSAR data has been interpreted to produce a time-history of subsidence.
Continuous GPS (CGPS)
The CGPS data for this study were obtained from the UNAVCO Plate Boundary Observatory (PBO) network of continuously operating GPS stations. The PBO is the geodetic component of UNAVCO, a consortium of research institutions whose focus is measuring vertical and horizontal plate boundary deformation across the North American and Pacific Plates in the western United States using high-precision measurement techniques.
These 5 northern benchmarks are where the largest magnitude of subsidence was recently measured using repeat surveys. That doesn't mean these locations are where the largest magnitude of subsidence occurred, just where it's been measured. The southernmost benchmark in this set is where the largest magnitude was historically measured using repeat surveys.
Aquifer-system compaction has been monitored at selected locations for various periods with borehole extensometers by different agencies, including the USGS, DWR, SLDMWA, and CCID, and by Luhdorff and Scalmanini Consulting Engineers. A total of 35 extensometers have been monitored in the San Joaquin Valley, with most of the monitoring occurring in the 1950s and 1960s through the early 1980s. Four cable-type extensometers were refurbished: 12S/12E-16h3 (Oro Loma Deep, 305-m depth), 14S/13E-11D6 (Panoche, 414-m depth), 18S/16E-33A1 (DWR Yard, 314-m depth), and 20S/18E-6D1 (Rasta, 264-m depth). The refurbishment of the four selected extensometers was completed in early 2012. Since then, the sites have been maintained by USGS personnel who download the data, make manual dial-gauge and water-level measurements for quality control, and adjust equipment.
Groundwater levels and groundwater-level changes near continuous GPS sites, extensometers, and selected locations where InSAR data indicated larger magnitudes of subsidence, were evaluated by using water-level hydrographs from wells near these locations. On this webpage, a well is referred to as 'deep' if it was screened below the Corcoran Clay, 'shallow' if it was screened above the Corcoran Clay, or 'unknown' if the screened interval was not known. Where the Corcoran Clay does not exist, the term 'outside Corcoran extent' is used.
Central Valley Hydrological Model Regions
Surface Water Conveyance Features
Glaciated large drainage basin
Non-glaciated large drainage basin
Drought (2011 - 2017)
Wells with ≥ 5 yrs on record, latest measurement in 2015
|Decline ≥ 400|
|Decline 300 - < 400|
|Decline 200 - < 300|
|Decline 100 - < 200|
|Decline < 100||Change < 1|
Subsidence: Continuous GPS Data
Change in Land Elevation, 2011 - 2015
|Uplift > 1"|
|Uplift .5" - < 1"|
|Uplift .25 - < .5"|
|Uplift < .25"|
|Subsidence < 1"|
|Subsidence 1" - < 3"|
|Subsidence 3" - < 5"|
|Subsidence > 5"|
Compaction: Extensometer Data
Aquifer System Compation, 2011 - 2017
|Compaction < .25'|
|Compaction = .25' - < .5'|
|Compaction > .5'|
Subsidence Contours (2008-2010)25 mm (estimated)
Continuous GPS (Subsidence) Monitoring Sites
Extensometers (Compaction) Monitoring Sites
InSAR: Areas of Interest
InSAR: Delta-Mendota Canal North (Check Stations 1-16)
InSAR: Delta-Mendota Canal South (Check Stations 17-21)