Foreword: Understanding through Modeling

Abstract

Since 1998, five MODFLOW-themed conferences have been held, including ‘‘MODFLOW 98,’’ ‘‘MODFLOW 2001 and Other Modeling Odysseys,’’ ‘‘MODFLOW and More 2003: Understanding through Modeling,’’ ‘‘FiniteElement Models, MODFLOW and More 2004,’’ and ‘‘MODFLOW and More 2006: Managing Ground Water Systems.’’ This is the second theme issue of Ground Water to be derived from MODFLOW-themed conferences; the first was derived from the 2001 conference, as described by Hill et al. (2003). This theme issue is derived from the ‘‘MODFLOW and More 2003: Modeling through Understanding’’ conference held in September 2003 at the Colorado School of Mines in Golden, Colorado. Chosen contributions have evolved and expanded beyond their original scope and content as presented at the 2003 conference. In spite of using MODFLOW as a working theme, both the conferences and related journal issues involve many different modeling approaches and simulation programs. This theme issue begins with a Software Spotlight column by Barlow and Harbaugh. The column summarizes some of the current directions for MODFLOW development being pursued by the USGS. There include simulation of coupled ground water/surface water systems, variable-density flow and solute transport modeling, unsaturated-zone flow simulation, parameter estimation, and ground water management optimization modeling. The next two articles are issue papers by Hill and Gomez-Hernandez that debate the merits and pitfalls of simplicity vs. complexity in developing ground water models. Hill argues that all ground water models should start out simple and modelers need to add complexity carefully as warranted by the complexity (1) supported by the available data and (2) important to the predictions. On the other hand, Gomez-Hernandez argues that aquifers are inherently complex and their representation is uncertain. Thus, complexity in models, based on the stochastic, multiple-realization Monte-Carlo approach, is a must for proper evaluation and prediction. The exchange of the simplicity vs. complexity viewpoints is complemented by a following technical commentary by Haitjema. He advocates that training in basic hand calculations is important in ground water modeling curricula because analytic solutions to elementary ground water flow problems provide something that often is lost when using sophisticated computer models: insight. Nine research papers constitute the remainder of the theme issue. The first three deal with modeling techniques in flow and solute transport. Mehl et al. review several local grid refinement (LGR) methods and identify their advantages and drawbacks. They also demonstrate how a new LGR method can be successfully implemented to model surface water–ground water interactions. Romero and Silver take advantage of MODFLOW’s integrated finite-difference formulation to obtain more robust solutions for distorted grids. Herrera and Valocchi investigate a well-known but difficult problem in solute transport modeling: the presence of negative concentrations in solute transport solutions arising from advection and cross-dispersion terms. They propose corrective schemes aimed at preserving positive simulated concentrations. The next two papers address model calibration. Zyvoloski and Vesselinov demonstrate that grid properties and resolution, and the computational scheme, can have large effects on forward model predictions and on inverse parameter estimates. They quantify these effects using a series of oneand two-dimensional synthetic cases representing saturated and variably saturated flow problems. Edington and Poeter explore the connection between stratigraphic processes and the character of flow systems. They conclude that geologic processes controlling the ratio of accommodation space to sediment supply have a consistent effect on hydraulic properties. This suggests that joint inversion of geologic and flow models could be useful in characterizing the magnitude and distribution of hydraulic properties important to ground water flow and transport. 1Department of Geological Sciences, University of Alabama, Tuscaloosa, AL 35487; czheng@ua.edu 2International Ground Water Modeling Center, Colorado School of Mines, Golden, CO 80401; epoeter@mines.edu 3U.S. Geological Survey, Boulder, CO 80303; mchill@usgs.gov 4Watermark Numerical Computing, Corinda 4075, Australia; johndoherty@ozemail.com.au Copyright a 2006 The Author(s). Journal compilationa2006National GroundWaterAssociation. doi: 10.1111/j.1745-6584.2006.00270.x