Land Subsidence in California

Title: Land Subsidence: The Lowdown on the Draw-down

The link between groundwater use and sinking landscapes

• What is land subsidence, where does it happen, and why does it matter?
• How do weather and land use affect land subsidence in California?
• Why are we optimistic about the future of land subsidence in California?

National Geodetic Survey vertical control bench mark H1235 RESET in Merced County, California. H 1235 RESET is in the median of State Highway 15. This is one 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, this bench mark has some of the larger

National Geodetic Survey vertical control bench mark W990 CADWR in Merced County, California. W 990 CADWR is on the Mariposa Bypass Bridge on Washington Rd. This is one 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, this bench mark has some of

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-

Map showing estimated regions of subsidence derived from interferograms for 2008–2010 and selected surface-water conveyance infrastructure in the San Joaquin Valley area of the Central Valley, California.

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 dominant cause of land subsidence in the Sacramento-San Joaquin Delta is decomposition of organic carbon in the peat soils. Under natural waterlogged conditions, the soil was anaerobic (oxygen-poor), and organic carbon accumulated faster than it could decompose. Drainage of peat soils for agriculture led to aerobic (oxygen-rich) conditions. Under aerobic conditions,

Post-subsidence diagram showing the relation between surface and subsurface processes with linkage to land subsidence. The linkages to subsidence deformation are briefly summarized for surface-water, landscape and groundwater flow processes. The potential changes in movement of water before and after the linked effects of subsidence result in redirected or

Pre-subsidence diagram showing the relation between surface and subsurface processes with linkage to land subsidence. The linkages to subsidence deformation are briefly summarized for surface-water, landscape and groundwater flow processes. The potential changes in movement of water before and after the linked effects of subsidence result in redirected or reduced

Reduced surface-water availability (associated with droughts) during 1976-77, 1986-92, 2007-09, and 2012-2016 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,

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.