Simulating Land Subsidence

Science Center Objects

The California Water Science Center has been involved in multiple studies simulating land subsidence associated with groundwater withdrawal. The simulations can be used to estimate the magnitude, location, and timing of subsidence. They can also be used to evaluate management strategies to mitigate adverse effects from subsidence while also optimizing water availability.

Central Valley Hydrologic Model (CVHM)

California's Central Valley covers about 20,000 square miles and is one of the most productive agricultural regions in the world. More than 250 different crops are grown in the Central Valley with an estimated value of $17 billion per year. This irrigated agriculture relies heavily on surface-water diversions and groundwater pumpage. Approximately one-sixth of the Nation's irrigated land is in the Central Valley, and about one-fifth of the Nation's groundwater demand is supplied from its aquifers.

As part of a large groundwater availability study in the Central Valley, the USGS developed the Central Valley Hydrologic Model (CVHM), a computer simulation model of the entire Valley that helps to address water competition issues such as conjunctive water use (interdependent use of surface water and groundwater), declining water levels and land subsidence, the effects of land-use change on water resources, and the effects of climate change on water availability.

 

Cuyama

Continued withdrawals of groundwater, the Cuyama Valley's sole source of water supply, and associated water-resource management concerns prompted an evaluation of the hydrogeology and water availability for the Valley by the USGS, in cooperation with the Santa Barbara County Water Agency. The study included development of a digital three-dimensional geologic framework model and a computer simulation of the groundwater basin that included subsidence.

Change in groundwater storage with rapidly declining water levels in a sole-source aquifer were important factors in undertaking and completing this study. To better understand the system, the Cuyama Valley has been split into three groups of subregions: (1) the Main zone, (2) the Sierra Madre Foothills, and (3) the Ventucopa Uplands. Although partially connected hydraulically, the groundwater system in these subregions generally responds independently to different supply sources and demands. Data indicated small amounts of permanent subsidence of up to 0.2 ft since 2000 and reduced storage capacity in the aquifer sediments due to groundwater pumping. Simulations of historical conditions indicate nearly 1.6 ft of subsidence that is spatially centered near New Cuyama and coincident with the groundwater declines in the Main zone. An additional foot of permanent subsidence is projected in the Main zone if current demands continue.

Graph showing displacement data from 5 GPS stations in Cuyama Valley

Land-surface position, up coordinate, in millimeters, for the GPS stations Cuyama High School (CUHS), Ventucopa Station (VCST), McPherson_CS2008 (P521), Bitter Creek Wildlife Refuge (BCWR), and OZST_SCGN_CS2000 (OZST), Cuyama Valley, Santa Barbara County, California. The measured displacement at CUHS between December 5, 2002, and May 22, 2008, was -40 mm. It is likely that this downward trend, or subsidence, represents inelastic deformation and indicates compaction and reduced storage capacity of the aquifer sediments; a significant component ofthe seasonal fluctuations represented elastic deformation, as evidenced by various periods of partial recovery. (Public domain.)

Antelope Valley

Groundwater provides between 50 and 90 percent of the total water supply in Antelope Valley. Groundwater-level declines of more than 270 feet in some parts of the groundwater basin have resulted in an increase in pumping lifts, reduced well efficiency, and land subsidence of more than 6 feet in some areas. Natural recharge is an important component of total groundwater recharge in Antelope Valley; however, the exact quantity and distribution of natural recharge, primarily in the form of mountain-front recharge, is uncertain. To better understand this uncertainty, and to provide a tool to aid in groundwater management, a numerical model of groundwater flow and land subsidence in the Antelope Valley groundwater basin was developed using old and new geohydrologic information.

Map of Antelope Valley showing the location of geodetic benchmarks used to measure land subsidence in the area

Benchmarks used to measure land subsidence and to calibrate the transient-state groundwater-flow and land-subsidence model, Antelope Valley groundwater basin, California. (Public domain.)

 

Santa Clara

A revised numerical groundwater/surface-water flow model of the Santa Clara Valley was developed as part of a cooperative investigation with Santa Clara Valley Water District. The flow model was developed to better define the geohydrologic framework of the regional flow system and to better delineate the supply and demand components that affect the inflows to and outflows from the regional groundwater flow system.

Map of land subsidence contours in Santa Clara County, CA

Hand-contoured measured subsidence, 1939-80, and simulated ground compaction, 1983-99, for the Santa Clara Valley model, Santa Clara Valley, California. (Public domain.)