Adaptive explicit‐implicit quasi three‐dimensional finite element model of flow and subsidence in multiaquifer systems

A quasi three‐dimensional finite element model is presented for the analysis of groundwater flow and land subsidence due to pumpage in multiaquifer systems. In the model, aquifers are simulated with the aid of two‐dimensional horizontal finite element grids. Each aquifer is connected to its neighbors above and below by one‐dimensional vertical finite element strings which allow leakage to take place across aquitards and aquicludes. Land subsidence is modeled by varying the void ratio of each vertical element according to a nonlinear version of Terzaghi's one‐dimensional consolidation theory. A major feature of the new method is its ability to solve the finite element equations explicitly in one part of the mesh and implicitly in another part, depending on the ratio between the stability limit of each node and any given time step size. Since the hydraulic diffusivity of aquitards is usually very small compared to that of aquifers, it is often possible to solve the finite element equations explicitly for most, and sometimes all, aquitard nodes. This leads to a virtual decoupling of the aquifer equations during a given time step, which results in a significant saving of computer time and storage.