The European Space Agency's (ESA) ENVISAT satellite was used to map and measure displacement. The satellite is side-looking, orbits the Earth at an altitude of approximately 500 miles (800 kilometers), and has 35-day repeat cycles. Illustration by the European Space Agency.
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.
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.
CGPS Station P303 in Los Banos, CA.
Figure 1: Recording-extensometer installations. A, the cable, and B, free-standing-pipe extensometers.
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-16H2 (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.
Benchmarks (also called geodetic monuments or control points) generally consist of flat metal disks or other sturdy materials that are anchored in the ground or to a structure and can be surveyed for elevation repeatedly to calculate elevation change. Historical benchmark elevation data generally can be obtained by contacting the National Geodetic Survey. The Bureau of Reclamation has been surveying a network of over 70 benchmarks across the San Joaquin Valley in July and December of each year, beginning in 2011, to monitor ongoing subsidence. Reclamation's data is available online.
National Geodetic Survey vertical control mark, Santa Rita Bridge, California State Highway 152.
Example of a hydrograph derived from groundwater level data found in the USGS National Water Information System (NWIS).
Approximate location of maximum subsidence in the U.S., identified by research efforts of Dr. Joseph F. Poland (pictured). Signs on pole show approximate altitude of land surface in 1925, 1955, and 1977. The compaction of aquifer systems that can accompany excessive groundwater pumping is by far the single largest cause of subsidence in California. In some systems, when large amounts of water are pumped, the sediments can compact, thus reducing in size and number the open pore spaces that previously held water. The overdraft of aquifer systems, such as what has occurred in the San Joaquin Valley, has resulted in permanent subsidence, reduced aquifer-system storage capacity, and infrastructure damage.
Bench mark H1235 RESET along State Highway 152 south of El Nido, CA
Bench mark W990 CADWR on the Mariposa Bypass Bridge on Washington Rd south of El Nido, CA
National Geodetic Survey vertical control bench marks, H1235 RESET and W990 CADWR in Merced County, California. H 1235 RESET is in the median of State Highway 152 and W 990 CADWR is on the Mariposa Bypass Bridge on Washington Rd. These are two of several bench mark locations used to help measure the largest recent subsidence in the area using repeat surveys. The exact maximum subsidence location is unknown; however, these two bench marks have some of the larger magnitudes measured during the last 5 years.
The USGS used satellite-based interferometric synthetic aperture radar (InSAR) images to further characterize this large subsidence bowl first seen by California Department of Water Resources surveys. The InSAR images processed by USGS indicate that this bowl is much larger than originally believed, encompassing roughly 1,200 square miles. Centered near the town of El Nido, it is bounded roughly by the towns of Merced on the north, Mendota on the south, Los Banos on the west and Madera on the east. It also includes part of the San Joaquin River, the Delta-Mendota Canal, most of the Eastside bypass—the primary flood control channel east of the river—and other water conveyance structures. At the center of the subsidence bowl near El Nido, the rate of subsidence in this area—nearly 1 foot a year—is among the highest ever measured in the San Joaquin Valley. Continued subsidence could cause infrastructure damage in local communities as well as adversely affect the already subsided canal area.
H1235 RESET: 37.06195, -120.54307
W990 CADWR: 37.11306, -120.58861
Reduced surface-water availability (associated with droughts) during 1976-77, 1986-92, 2007-09, and 2012-current caused groundwater-pumping increases, water-level declines to near or beyond historic lows, and renewed aquifer compaction. The resulting land subsidence has canal buckling and reduced flow capacity of the Delta-Mendota Canal (pictured), the California Aqueduct, the Eastside Bypass, and other structures that transport floodwater or deliver irrigation water.
CHECK STATION 18 LOCATION:
Graph showing surface water deliveries and cumulative storage changes simulated by the Central Valley Hydrologic Model (CVHM). One cubic kilometer is about 811,000 acre-feet. Since the majority of the surface water delivery system has been in place, the CVHM simulates that on average about 40% of the water supply of the Central Valley has come from groundwater (ranging from about 30% during wet years to 70% during dry years). Over time, the extra pumping has stressed the aquifer, which for decades has had an overall loss in storage. The Central Valley has been depleted by about 1.85 km3 per year on average since 1960 (Faunt et al. 2009), and has been depleted about twice this rate during the current drought.
Groundwater levels declined during the 2007-10 drought period in response to increased pumping, approaching historical low levels, which reinitiated compaction, as shown at continuous Global Positioning System (CGPS) station P304 and well 13S/15E-31J6 near Mendota. Discrete measurements of water levels in well 13S/15E-31J6 were screened below the Corcoran Clay Member, and vertical displacement measurements were taken at continuous Global Positioning System (GPS) station P304 near Mendota, California, 2004-10. 13S/15E-31J6 data before May 1, 2010, were obtained from Glenn Browning, Luhdorff and Scalmanini Consulting Engineers, and 13S/15E-31J6 data after May 1, 2010, were obtained from the U.S. Geological Survey. P304 daily data were obtained from the University NAVSTAR Consortium.
P304 STATION LOCATION:
The Delta-Mendota Canal check station (a structure built to control the water-surface level and flow in a canal) had to be extended vertically (highlighted in red) to keep the structure operational. These are expensive retrofitting procedures. At this location, embankments and bridges were raised on this canal and the nearby Outside Canal in 1971.
CHECK STATION 18 LOCATION:
The Russell Avenue Bridge at the Outside Canal, south of Dos Palos, CA. The bridge and canal run parallel to, and are adjacent to, the Delta-Mendota Canal. In 1995, the land has subsided such that side walls had to be constructed to keep water from the canal flowing onto the road surface of the bridge. The photograph shows that there is no freeboard, or space between the water surface and the bridge. According to a 2006 Central California Irrigation District Report, this canal has lost 50% of its conveyance capacity as a result of subsidence. Because the subsidence has degraded its structural integrity, plans are underway to replace the bridge.
CHECK STATION 18 LOCATION:
Protruding well casings, such as shown in this photograph in California's San Joaquin Valley, are evidence of subsidence. The ground has subsided enough, relative to the well casing (which is anchored in a deeper part of the aquifer), to suspend the broken concrete foundation of the well head above land surface. When originally constructed, much of the well foundation was below land surface.
This view looking into a typical collapsed well screen shows the damage caused by compaction. This photograph was made by lowering a light into the well, followed by a camera; the crumpled vertical ribbing of the steel well screen produced this radiating effect.
In many areas of the arid Southwest, earth fissures, like the one pictured above that crosses an aircraft runway at the Fort Irwin National Training Center in California, are associated with land subsidence. Although earth fissures can be caused by a variety of processes, the earth fissure pictured here was caused by horizontal movement of sediments that occurs when groundwater is pumped. Earth fissures can be more than 100 feet deep and several hundred feet in length. These features start out as narrow cracks, an inch or less in width. They intercept surface drainage and can erode to widths of tens of feet at the surface.
Extensometers are used in land subsidence studies to measure the compaction and expansion of the aquifer system to some depth. This pipe borehole dual-stage extensometer was built in 2008 in San Lorenzo, CA and measures compaction from 10 to about 300 meters. The illustration highlights the features that can be seen in the photograph.
More than 2 dozen extensometers in the San Joaquin Valley were constructed between the 1950s and 1970s by the U.S. Geological Survey in cooperation with the California Department of Water Resources, and represent the first extensometers ever built in the United States. Four of those original extensometers have recently been refurbished with state-of-the-science techniques and equipment to measure hourly aquifer-system compaction.
Central Valley subsidence study launched
Sep 04, 2009
Hydrologist, Land Subsidence Specialist
Geophysicist, Remote Sensing
Hydrologist, Hydrologic Modeling
Hydrologist, Hydrologic Modeling
Research Hydrologist, Sacramento-San Joaquin Delta
Research Soil Scientist, Sacramento-San Joaquin Delta