Using Numerical Models to Simulate 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.

MODFLOW

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.

 

A depression in the ground, shaded by land-surface altitude

Simulation of land subsidence by using a hydrologic model with the MODFLOW-SUB package. (Public domain.)

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.

 

 

 

 

 

Cross-section of the subsurface showing the unsaturated zone, unconfined aquifer, confining layer, and confined aquifer.

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. (Public domain.)

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.

 

 

 

 

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.

before
after
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Diagram showing the relation between surface and subsurface processes before and after land subsidence. The linkages to subsidence deformation are briefly summarized for surface-water, landscape and groundwater flow processes. The potential changes in movement of water before and after the linked effects of subsidence result in redirected or reduced runoff, reduced streamflow and canal flows, reduced canal freeboard, additional connection of root zone to capillary fringe above water table, and reduced aquifer thickness and hydraulic properties (transmissivity and storativity).