Subsidence

Subsidence is the sinking of the land surface due to changes in the soil or sediment beneath our feet. Subsidence occurs for a variety of reasons such as groundwater pumping, oil extraction, and geologic processes. Subsidence is a slow process and is usually measured in inches per year, but over time these small changes can add up to many feet and create significant problems for infrastructure and communities.  

Schematic showing the effects of land subsidence due to lowering of groundwater levels. Prior to groundwater withdrawals, clay and silt layers are loosely packed. After long term groundwater withdrawals, water level declines causing clay and silt layers to compact.
Subsidence due to reorganization of soil particles. Graphic courtesy of the Harris-Galveston Subsidence District, TX.

Subsidence due to groundwater pumping occurs when the water pressure in the subsurface sediment is reduced and the fine-grained sediment (such as silt and clay) compacts. As the water pressure is reduced by pumping, the fine-grained soil particles reorganize and compress into a flatter structure. This subsurface compaction results in the lowering of the land surface and, depending on the type of sediments, this change may not be reversable. Subsidence due to groundwater pumping is effectively minimized only when groundwater levels are allowed to rise as rapidly and as much as possible above historical levels where subsidence is known to occur.

Educational Resources

The resources below are provided to assist local Sustainable Groundwater Management Act (SGMA) implementation efforts. The resources are tailored for use in educating the public on various aspects of subsidence. While the resources may not be applicable to all GSAs, they are meant to engage audiences and communicate complex information. More resources and helpful links will be added to this section soon as DWR rolls out additional material on subsidence.

Subsidence 101 Video

 

Subsidence 101 Factsheet

Subsidence 101 Factsheet

The Role of Critical Head in Managing Land Subsidence

 

This video presents the science and mechanics of subsidence. It explains the principle of effective stress in soils, introduces the concept of critical head, and explains how subsidence can be managed using critical head. The second part of the video covers examples of how to calculate critical head based on examples in the San Joaquin Valley. 

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