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Techniques and Methods 6–A44

Prepared in cooperation with the U.S. Department of Energy
A Product of the Groundwater Resources Program

MODFLOW-LGR—Documentation of Ghost Node Local Grid Refinement (LGR2) for Multiple Areas and the Boundary Flow and Head (BFH2) Package

By Steffen W. Mehl and Mary C. Hill

Thumbnail of and link to report PDF (2.07 MB)Abstract

This report documents the addition of ghost node Local Grid Refinement (LGR2) to MODFLOW-2005, the U.S. Geological Survey modular, transient, three-dimensional, finite-difference groundwater flow model. LGR2 provides the capability to simulate groundwater flow using multiple block-shaped higher-resolution local grids (a child model) within a coarser-grid parent model. LGR2 accomplishes this by iteratively coupling separate MODFLOW-2005 models such that heads and fluxes are balanced across the grid-refinement interface boundary. LGR2 can be used in two-and three-dimensional, steady-state and transient simulations and for simulations of confined and unconfined groundwater systems.

Traditional one-way coupled telescopic mesh refinement methods can have large, often undetected, inconsistencies in heads and fluxes across the interface between two model grids. The iteratively coupled ghost-node method of LGR2 provides a more rigorous coupling in which the solution accuracy is controlled by convergence criteria defined by the user. In realistic problems, this can result in substantially more accurate solutions and require an increase in computer processing time. The rigorous coupling enables sensitivity analysis, parameter estimation, and uncertainty analysis that reflects conditions in both model grids.

This report describes the method used by LGR2, evaluates accuracy and performance for two-and three-dimensional test cases, provides input instructions, and lists selected input and output files for an example problem. It also presents the Boundary Flow and Head (BFH2) Package, which allows the child and parent models to be simulated independently using the boundary conditions obtained through the iterative process of LGR2.

First posted September 19, 2013

For additional information contact:
Chief, Branch of Regional Research, Central Region
U.S. Geological Survey
Box 25049, Mail Stop 418
Denver, CO 80225
http://water.usgs.gov/nrp/

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Suggested citation:

Mehl, S.W., and Hill, M.C., 2013, MODFLOW–LGR—Documentation of ghost node local grid refinement (LGR2) for multiple areas and the boundary flow and head (BFH2) package: U.S. Geological Survey Techniques and Methods book 6, chap. A44, 43 p., http://pubs.usgs.gov/tm/6a44/.



Contents

Abstract

Introduction

Highlights and Compatibility

Description of Local Grid Refinement (LGR)

Examples

References Cited

Appendix 1. LGR2 Input Instructions and Selected Input and Output Files from Examples 1 and 3

Appendix 2. Independent Simulations Using the Boundary Flow and Head (BFH2) Package

Appendix 3. Error Propagation in LGR2

Appendix 4. LGR2 Input File for Multiple Refined Areas.

Appendix 5. Brief Program Description

Appendix 6. Relative Advantages of Ghost-Node versus Shared-Node Coupling


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