System and Boundary Conceptualization in Ground-Water Flow Simulation

The purpose of this U.S. Geological Survey report is to focus on the specific aspect of boundary conditions in the modeling process by describing the different ways of simulating, in a numerical model, the physical features that are boundaries of the groundwater system, and to discuss the ramifications, benefits, and limitations of each approach. The audience is both groundwater modelers and users of groundwater models. Groundwater models attempt to represent an actual groundwater system with a mathematical counterpart. The conceptualization of how and where water originates in the groundwater-flow system and how and where it leaves the system is critical to the development of an accurate model. The mathematical representation of these boundaries in the model is important because many hydrologic boundary conditions can be mathematically represented in more than one way. The determination of which mathematical representation of a boundary condition is best usually is dependent upon the objectives of the study. This report focuses on the specific aspect of describing different ways to simulate, in a numerical model, the physical features that act as hydrologic boundaries in an actual groundwater system. The ramifications, benefits, and limitations of each approach are enumerated, and descriptions of the representation of boundaries in models for Long Island, New York, and the Middle Rio Grande Basin, New Mexico, illustrate the application of some of the methods.