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.
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