In the San Joaquin Valley, water is pumped out of aquifers at roughly twice the rate of replenishment through precipitation. Groundwater overdraft is a common phenomenon all over California, where demand for water outweighs supply. This is especially true during drought years, and of paramount concern right now because of California’s multiyear drought. Underground water levels have declined as much as 200 feet in the San Joaquin Valley during the past two years alone.


This year, California water experts estimate that over-pumping from groundwater aquifers will make up for over 1.5 trillion gallons of water that will not be delivered through the state’s extensive surface water projects. Compensating with groundwater is a risky and costly enterprise. UC Davis predicts that the increased groundwater use will cost nearly $500 million, with the greatest share of resource and economic impacts occurring in the San Joaquin Valley and the Tulare Lake Basin.




Maximum land subsidence in the United States USGSThe signs on this pole in Mendota, CA show the approximate altitude of land surface in 1925, 1955, and 1977. The rate of subsidence is even faster now.



Like any natural resource, the surface of the earth is dynamic and ever-changing, and responsive to what happens around and underneath it. Land subsidence is the process by which land surfaces sink downward. Upper layers of the subsoil dry out and compact, reducing the pore sizes or eliminating the spaces between soil particles. This is a permanent, irreversible condition—watering the soil does not and will not cause the land to rebound in altitude. The land literally sinks under its own weight, filling voids where water has been extracted, thus decreasing the total storage capacity of the affected aquifer. As the water table drops, shallow wells can dry up, and the water levels in nearby lakes and streams also drop. Groundwater overdraft can cause seawater intrusion in coastal areas, further degrading the quality of remaining groundwater. The Mojave River Basin recently experienced desiccation cracks, sink holes, and fissures more than three feet wide and deep as a result of groundwater overdraft.

More than 80 percent of the identified subsidence in the United States (affecting over 17,000 sq. miles in 45 states) is a consequence of groundwater depletion by humans. In Merced County, for example, unsustainable groundwater pumping caused an alarming subsidence rate of nearly a foot per year during the past two years—much faster than previously documented.





In addition to the San Joaquin Valley, major subsidence has been measured in the Santa Clara Valley, south of San Francisco, Sacramento Valley, Coachella Valley, and Antelope Valley. In the Santa Clara Valley, downtown San Jose dropped 14 feet between 1910 and 1995 before the county halted the process by monitoring groundwater levels and recharging aquifers with imported surface water. Groundwater pumping in the Coachella Valley led to upwards of two feet of land subsidence in parts of Indian Wells, La Quinta, and Palm Desert between 1995 and 2010. The Coachella Valley Water District (CVWD) mitigated by recharging aquifers with imported water and by encouraging heavy water users, like golf courses, to transition to reclaimed water for irrigation. USGS found that these CVWD efforts had a positive effect, especially in La Quinta, where Colorado River water helps to recharge aquifers via a groundwater replenishment facility. Not surprisingly, desert water resources continue to be strained; the Desert Sun found that the average depth of 291 wells in the Coachella Valley has dropped more than 55 feet since the 1970s.


Sunken and Sinking Delta Islands

While Governor Brown’s Bay Delta Plan aims to address the ongoing disconnect between state water supply and demand, and powerful groups clamor for more even water diversions from streams and rivers, California already exports unsustainable amounts of water from the fragile Sacramento/San Joaquin River Delta system. In addition, extensive groundwater withdrawal from the San Joaquin Valley could result in operational and structural issues for aging infrastructure including the Delta Mendota Canal, which began pumping surface water from the Delta and conveying it throughout the Valley during the 1950s.



The islands in the Delta are heavily altered as a result of more than a century of human intervention and water exports, and highly susceptible to subsidence. In 2009, the already subsided Bradford Island nearly sank when it was hit by a straying freighter from the Port of Stockton. The subsidence, sea level rise, and seismicity of the Delta region has been studied for many years. The sand and other dredged material used for levee maintenance in the Sacramento-San Joaquin Delta may contribute to increases in future earthquakes in the Bay Area and Delta region. The same processes and conditions that cause subsidence can actually push mountains upward, triggering earthquakes along active faults.


Water Levels Drop, Mountains Move

The rapid subsidence of the San Joaquin Valley floor is caused by the substantial reduction in the mass of groundwater. Removing this weight from the Earth’s crust has caused it to flex upward in response, quite literally moving mountains in the process. This has significant but relatively unexplored impacts. Results of a recent study exploring the connections between groundwater use and seismicity suggest that the “flexural uplift” of the Coast Ranges brings the San Andreas fault closer to failure by reducing the stress on the fault. This long-term process—a result of decades and decades of groundwater overdraft—has the potential to affect long-term seismicity rates for fault systems adjacent to the valley. The San Andreas fault extends more than 800 miles through the western part of California, marking the boundary between the Pacific and North American plates, and has caused some of the most devastating earthquakes in recorded history. Scientists have also measured an uplift of the southern Sierra Nevada that was previously attributed to tectonic or mantle-derived forces, but the new report indicates that this uplift is at least partly a consequence of human-caused groundwater depletion during the past century.



Simplified fault map USGS


Humans are the Problem (and the Solution)

All of these impacts of groundwater overdraft provide urgent and sometimes dramatic reminders of the connections between natural processes, land and water resources, and human activities that can change ecological systems forever. Subsidence in the San Joaquin Valley has been referred to as the greatest human alteration of the Earth’s surface.



Meanwhile, California remains the only western state without a groundwater protection policy or statewide regulations to manage groundwater sustainably and for the public good.



The solution to subsidence in California (i.e. maintaining adequate levels of groundwater) is well known and has been implemented with success in several parts of the state. But this solution must be enacted on a wider scale and with a long-term view, as aquifer replenishment and management is by nature a long-term activity.


Similarly, land subsidence isn’t only a concern during drought years or in particularly susceptible locations; indeed, even in the precipitation record-setting years 2010-2011, water deliveries fell short of requests from irrigators in the Central Valley (part of the problem with demand is that senior water rights holders receive and use an incredible portion of California’s available water). Because of this ongoing and growing disconnect between supply and demand, declines in groundwater quantity and quality are likely to continue, and California’s land will continue to sink in response.