California Water History in a Minute – Subsidence

The California Department of Water Resources (DWR) recently released a document on subsidence and how to stop it. When groundwater is pumped, particularly from wells deep underground, the ground above can sink, as the clays between the aquifer and the ground surface compress. This phenomenon of the ground surface sinking is called subsidence.

Subsidence has been a problem in California ever since we began using large capacity wells to do irrigation. The DWR document describes the history this way:

From 1926 to 1970, an area in the Central Valley southwest of Mendota had documented subsidence of more than 28 feet. Construction of the Central Valley Project began in the late 1930s to address water supply and distribution in California’s Central Valley. The introduction of Central Valley Project surface water imports via the Friant-Kern and Delta-Mendota Canals in the 1950s, and Central Valley Project and State Water Project surface water imports via the California Aqueduct in the 1970s, significantly reduced groundwater reliance, initiated groundwater level recoveries, and slowed—and even reversed—subsidence in some areas of the San Joaquin Valley [emphasis added].

However, expansions of agricultural acreage and drought periods between 2000 and 2023 coincided with reduced surface water availability [emphasis added], resulting in increased groundwater pumping, which resulted in accelerated subsidence rates of more than 1.0 ft/year in parts of the San Joaquin Valley and more than 0.5 ft/year in parts of the Sacramento Valley.”

DWR does not describe what caused reduced surface water availability during that period, but we know that major environmental regulations implemented in the mid-1990s diverted millions of acre feet of surface water away from farms and people and sent it to the ocean. Because groundwater was unregulated, much of that diverted surface water was replaced by groundwater. It really is not a surprise that subsidence again emerges as a major problem to be addressed.

The problem is real and, in this document, DWR outlines Best Management Practices for how to slow it down and ultimately eliminate it. Unfortunately, the only way to stop subsidence is to stop lowering groundwater levels and in fact increase those levels above what DWR describes as “critical head.”  Critical head is the groundwater level below which permanent compaction begins. See this video for more information on critical head.

As you can see from the map below, the dark areas have subsided between 4-7 feet in just the last nine years with the rate of subsidence showing no signs of slowing up. The subsidence issue is the most challenging problem facing a number of Groundwater Sustainability Agencies (GSAs). To simply stop pumping groundwater would cause massive economic, environmental (dust, weeds) and societal problems. What GSAs need to do is greatly increase their knowledge of where, and at what groundwater level, wells are producing in their areas. Without that knowledge, the pumping restrictions necessary to address subsidence will be broader and more restrictive than they need to be.

While we can observe the effects of subsidence from above ground, there is a lot we do not know about what is actually going on underground. There is an urgent need for much more data about what is happening underground. DWR has provided good guidance in this Best Management Practice document and has a role to play in helping to provide funding and expertise to assist the GSAs in getting the data necessary to develop effective but efficient subsidence management plans.

The government should also recognize that regulations that diverted surface water away from farms and people significantly contributed to this problem and addressing those regulations to get more surface water as well as facilitating the maximizing of local wet year water for recharge is also part of the fix. The Water Blueprint for the San Joaquin Valley is on the right track with its latest release today.