A New Object-Oriented MODFLOW Framework for Coupling Multiple Hydrologic Models

Abstract:

MODFLOW is a popular open-source groundwater flow model distributed by the U.S. Geological Survey. For 30 years, the MODFLOW program has been widely used by academic researchers, private consultants, and government scientists to accurately, reliably, and efficiently simulate groundwater flow. With time, growing interest in surface and groundwater interactions, local refinement with nested and unstructured grids, karst groundwater flow, solute transport, and saltwater intrusion, has led to the development of numerous MODFLOW versions. Although these MODFLOW versions are often based on the core version (presently MODFLOW-2005), there are often incompatibilities that restrict their use with one another. In many cases, development of these alternative versions has been challenging due to the underlying MODFLOW structure, which was designed for simulation with a single groundwater flow model using a rectilinear grid. A new object-oriented framework is being developed for MODFLOW to provide a platform for supporting multiple models and multiple types of models within the same simulation. In the new design, any number of numerical models can be tightly coupled at the matrix level by adding them to the same numerical solution, or they can be iteratively coupled until there is convergence between them. Transfer of information between models is isolated to exchange objects, which allow models to be developed and used independently. For existing MODFLOW users, this means that the program can function in the same way it always has for a single groundwater flow model. Within this new framework, a regional-scale groundwater model may be coupled with multiple local-scale groundwater models. Or, a surface water flow model can be coupled to multiple groundwater flow models. The framework naturally allows for the simulation of solute transport. Presently, unstructured control-volume finite-difference models have been implemented in the framework for three-dimensional groundwater flow and solute transport. The flexibility of this new framework is demonstrated through a collection of test problems involving multi-model flow, transport, and variable-density flow and transport.