MODFLOW-2000 ground-water model-user guide to the Subsidence and Aquifer-System Compaction (SUB) Package

Abstract

This report documents a computer program, the Subsidence and Aquifer-System Compaction (SUB) Package, to simulate aquifer-system compaction and land subsidence using the U.S. Geological Survey modular finite-difference groundwater flow model, MODFLOW-2000. The SUB Package simulates elastic (recoverable) compaction and expansion, and inelastic (permanent) compaction of compressible fine-grained beds (interbeds) within the aquifers. The deformation of the interbeds is caused by head or pore-pressure changes, and thus by changes in effective stress, within the interbeds. If the stress is less than the preconsolidation stress of the sediments, the deformation is elastic; if the stress is greater than the preconsolidation stress, the deformation is inelastic. The propagation of head changes within the interbeds is defined by a transient, one-dimensional (vertical) diffusion equation. This equation accounts for delayed release of water from storage or uptake of water into storage in the interbeds. Properties that control the timing of the storage changes are vertical hydraulic diffusivity and interbed thickness. The SUB Package supersedes the Interbed Storage Package (IBS1) for MODFLOW, which assumes that water is released from or taken into storage with changes in head in the aquifer within a single model time step and, therefore, can be reasonably used to simulate only thin interbeds. The SUB Package relaxes this assumption and can be used to simulate time-dependent drainage and compaction of thick interbeds and confining units. The time-dependent drainage can be turned off, in which case the SUB Package gives results identical to those from IBS1. Three sample problems illustrate the usefulness of the SUB Package. One sample problem verifies that the package works correctly. This sample problem simulates the drainage of a thick interbed in response to a step change in head in the adjacent aquifer and closely matches the analytical solution. A second sample problem illustrates the effects of seasonally varying discharge and recharge to an aquifer system with a thick interbed. A third sample problem simulates a multilayered regional groundwater basin. Model input files for the third sample problem are included in the appendix.

Inverse modeling of interbed storage parameters using land subsidence observations, Antelope Valley, California

“…[8] Delayed dissipation of overpressures from interbeds can be simulated with the subsidence package (SUB) [Hoffmann et al, 2003], developed by Leake [1990]. The package is based on the Terzaghi [1925] theory of onedimensional consolidation, which we refer to as compaction.…”

Examples of deformation-dependent flow simulations of conjunctive use with MF-OWHM

“…2). The effects of these deformationdependent flows were first analyzed with respect to a hypothetical problem that had typical properties and features (Schmid et al, 2014). This analysis demonstrated significant effects from the direct feedback of deformation onto physical properties of the land surface and subsurface.…”

“…In MF-OWHM (Hanson et al, 2014), new linkages were provided for the Subsidence Package (SUB) (Hoffmann et al, 2003) with other packages and processes in a fully integrated hydrologic model, which calculate flow rates that are affected by the deformation of the land surface or hydrostratigraphic boundaries (Fig. 1).…”

“…1) was used to explore and demonstrate the effects of the subsidence linkage within a fully coupled hydrologic model (Schmid et al, 2014). The coupling also affects the differential deformation of each layer (Fig.…”

Inverse modeling using PS-InSAR for improved calibration of hydraulic parameters and prediction of future subsidence for Las Vegas Valley, USA

“…The model simulates groundwater flow using MOD-FLOW_2005 (Harbaugh, 2005 and land subsidence using the SUB package (Hoffmann et al, 2003) for MODFLOW and considers the influence of basin-fill faults that are simulated using the HFB package (Hsieh and Freckleton, 1993) for the entire Las Vegas Valley using a 600 m × 600 m grid cell size. A four-layer model is used to represent the hydrogeologic conditions of the valley where the uppermost two layers represent the near-surface aquifer, layer three is the developed (principal) zone aquifer that extends 200-300 m below land surface and which is the primary focus of the calibration effort in this investigation because this is the layer from which all groundwater is pumped and virtually all subsidence occurs.…”