Simulating Land Subsidence
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)
Simulation of land surface elevation change in the Central Valley, 1961-2003
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.
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.
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.
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.
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
Using numerical models to simulate subsidence
MODFLOW is a three-dimensional (3D) finite-difference groundwater model that was first published in 1984. It has a modular structure that allows it to be easily modified to adapt the code for a particular application. Many new capabilities have been added to the original model. MODFLOW-2005 is the most current release of MODFLOW.
MODFLOW-2005 (Harbaugh, 2005) simulates steady and nonsteady flow in an irregularly shaped flow system in which aquifer layers can be confined, unconfined, or a combination of confined and unconfined. Flow from external stresses, such as flow to wells, areal recharge, evapotranspiration, flow to drains, and flow through river beds, can be simulated. Hydraulic conductivities or transmissivities for any layer may differ spatially and be anisotropic (restricted to having the principal directions aligned with the grid axes), and the storage coefficient may be heterogeneous. Specified head and specified flux boundaries can be simulated, as can a head-dependent flux across the model's outer boundary that allows water to be supplied to a boundary block in the modeled area at a rate proportional to the current head difference between a "source" of water outside the modeled area and the head in the boundary block.
Simulation of land subsidence by using a hydrologic model with the MODFLOW-SUB package.
Subsidence and Aquifer-System Compaction (SUB)
The Subsidence and Aquifer-System Compaction (SUB) package is used for simulating the drainage, changes in groundwater storage, and compaction of aquifers, interbeds and confining units that constitute an aquifer system. Delays in the release of groundwater from interbed storage, and thus the delays in aquifer-system compaction, can be simulated. Delayed drainage and compaction in confining units can also be simulated.
Subsidence and Aquifer-System Compaction Package for Water-table Aquifers (SUB-WT)
The Subsidence and Aquifer-System Compaction Package for Water-table Aquifers (SUB-WT) was developed to simulate vertical compaction in models of regional groundwater flow. The program simulates groundwater storage changes and compaction in discontinuous interbeds or in extensive confining units, accounting for stress-dependent changes in storage properties. The new program is a package for MODFLOW, the U.S. Geological Survey modular finite-difference ground-water flow model. Several features of the program make it useful for application in shallow, unconfined flow systems. Geostatic stress can be treated as a function of water-table elevation, and compaction is modeled as a function of computed changes in effective stress at the bottom of a model layer. Thickness of compressible sediments in a modeled unconfined layer can vary in proportion to saturated thickness.
Vertical section of a two-aquifer system with potential for compaction of fine-grained sediments. A, Hydrogeology of the system. B, Representation of the system by using three model layers (Leake and Galloway, 2007).
MODFLOW One Water Hydrologic Model (MF-OWHM)
The MODFLOW One-Water Hydrologic Flow Model (MF-OWHM) is an integrated hydrologic flow model used for the analysis of a broad range of conjunctive-use (the combined use of groundwater and surface water) issues (Hanson and others, 2014). MF-OWHM allows the simulation, analysis, and management of the components of human and natural water movement and use in a physically-based supply-and-demand framework. MF-OWHM includes a land-subsidence module with a vertically deforming mesh.
Diagram showing the relation between surface and subsurface processes with linkage to land subsidence (Schmid and others, 2014).
Groundwater Sustainability Indicators
Land Subsidence Modeling Publications
One-Water Hydrologic Flow Model (MODFLOW-OWHM)
USGS Techniques and Methods 6-A51
Economic Resilience through "One-Water" Management
USGS Open-File Report 2013-1175
MODFLOW-2005, The U.S. Geological Survey Modular Ground-Water Model-the Ground-Water Flow Process
USGS Techniques and Methods 6-A16
Planned updates and refinements to the Central Valley hydrologic model with an emphasis on improving the simulation of land subsidence in the San Joaquin Valley
Conference Paper, World Environmental and Water Resources Congress 2011
Integrated simulation of consumptive use and land subsidence in the Central Valley, California, for the past and for a future subject to urbanization and climate change
Conference Paper, Land subsidence, associated hazards and the role of natural resources development: EISOLS 2010