USGS directions in MODFLOW development.

Introduction The development and application of ground-water models has been an important component of USGS hydrologic investigations since the early 1970s. During the past 35 years, the USGS has developed a wide range of computer models to simulate saturated and unsaturated subsurface flow, solute transport, and chemical reactions. The most widely used of these programs is the MODFLOW model, which simulates threedimensional (3D) ground water flow (GWF) using the finite-difference method (Harbaugh 2005). Although originally conceived solely as a ground water flow model, MODFLOW’s modular structure has provided a robust framework for integration of additional simulation capabilities that build on and enhance its original scope. The family of MODFLOW-related models now includes capabilities to simulate coupled ground water/surface water systems, solute transport, unsaturated zone flow, parameter estimation, and ground water management (GWM) (optimization modeling). This column is intended to summarize some of the current directions for MODFLOW development within the USGS. This is an opportune time to provide such a summary because of the several MODFLOW-related programs that have been published during the past 2 years.