Land subsidence caused by underground fluid withdrawal is a problem in the United States and throughout the world. The term land subsidence has been traditionally used to denote only the vertical component of three-dimensional movement. However, land surface deformation actually results from the upward migration of both vertical and lateral deformation of the solid matrix within pumped aquifers at depth. T. Burbey developed the only current computer code that models transient three-dimensional deformation. His code, GDM requires a complex computation scheme that becomes impractical when applied to real world situations at the basin scale.

The key objective of this research is to develop a three-dimensional model that can be solved quickly for applications at the field scale. It is accordingly incorporated as a new module within an existing groundwater flow model. The widely respected USGS modular finite-difference groundwater flow model, popularly referred to as MODFLOW, is selected as the parent code. The new subsidence model, called DIS, is developed on the basis of D. C. Helm's generalized Darcy-Gersevanov equation. The new Darcy-Gesevanov law states that the velocity of the solid structure equals the difference between the total bulk flow and the flow of water relative to the aquifer system. What engineers call the "compressibility of the porous structure" is what hydrologists call "specific storage". At each modeled point, alternation between inelastic and elastic specific storage depends upon the relation of current hydraulic head to the previous maximum drawdown. This choice of compressibility is performed through an independent module feedback message to the MODFLOW program. As a result, the new model calculates three-dimensional movement directly from specified auxiliary conditions (such as boundary and initial conditions) within MODFLOW. The advantage of the new model is its speed in reaching a practical numerical solution in calculating transient land subsidence, and possible locations at depth for earth fissure genesis. The accuracy of its numeric results are the same as those of its parent groundwater flow model, MODFLOW. Seven contrasting cases are studied. Through these, the model is calibrated, verified and successfully applied to representative hydrogeological problems at the field scale.