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Additional information on the San Joaquin Valley subsidence study


September 4, 2009
For Immediate Release

News release

How does groundwater pumping cause subsidence, or a drop in land elevation?
Land subsidence is a gradual settling or sudden sinking of the Earth’s surface owing to subsurface movement of earth materials. In alluvial aquifers that include silt and clay layers, such as in the San Joaquin Valley, long-term groundwater-level declines can result in a compaction of those layers, and that manifests itself as land subsidence.
Is this the first time subsidence has occurred in the San Joaquin Valley?
No. In past decades, extensive withdrawal of groundwater has caused widespread land subsidence in the San Joaquin Valley, in some places exceeding 28 feet. Land subsidence from groundwater pumping began in the 1920’s, and by 1970 significant land subsidence (more than one foot) had occurred in about half of the San Joaquin Valley. During the droughts of 1976–77 and 1986–92, diminished deliveries of imported water prompted pumping of groundwater to meet irrigation demands.  This increased pumping resulted in water-level declines and periods of renewed subsidence.  Following each of these droughts, recovery to pre-drought water levels was rapid and subsidence virtually ceased. Since about 2005, groundwater pumping has increased as a result of reduced surface-water deliveries. Groundwater levels are declining in response to the increase in pumping, and are approaching historic low levels, which could reinitiate subsidence.

What is InSAR and how will it be used?
InSAR – or Interferometric Synthetic Aperture Radar -- is a means for remotely mapping land-surface changes using satellite radar images. Paired, synthetic-aperture radar (SAR) images taken from earth-orbiting satellites are used to create an image. The image shows the change in the radar line-of-sight distance, or range, between land surface and the radar antenna. Under ideal conditions, it is possible to resolve changes in elevation on the order of 0.4 inches (10 millimeters) or less. The spatial resolution of InSAR is orders of magnitude better than any economical land-based methods.

However, results of previous studies indicate that conventional InSAR methods are not ideal for large areas of the San Joaquin Valley. That’s because agricultural land uses – growing crops, plowing and clearing of fields – can distort the results. In this study, USGS scientists will use a new type of InSAR known as “persistent scatterer,” which focuses on established landmarks such as roads, roofs and other fixed objects. Scientists will use the persistent scatterer technique to document historical land subsidence during 2003-2009, and will test the technique for providing “real-time” monitoring of land subsidence.

How will the research be conducted?
Selected SAR imagery for 2003-2009 will be processed using the persistent scatterer technique to create time series images of historical land subsidence at selected points in the study area. During 2010, SAR imagery will be processed on a quarterly basis to monitor land subsidence on a close to real-time basis. Researchers will compare the mapped land subsidence to groundwater data compiled by the California Department of Water Resources (DWR) during the same period (2003-2009) to assess the relationship of land-surface-elevation changes to groundwater levels. In addition, researchers will compare land-surface-elevations at two continuous GPS (Global Positioning System) stations in the northern part of the Westlands Water District area to water-level data in nearby wells.

When will the research be finished?
USGS researchers will provide interim results as needed to keep water managers informed of the status of the work and any findings. They plan to have publishable results within three years.

The U.S. Geological Survey's California Water Science Center operates project offices in Sacramento and San Diego and nine field offices where more than 200 scientists and technicians bring a broad range of disciplines to modern water-management issues.

Links and contacts within this release are valid at the time of publication.


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