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Techniques and Methods 6–A37

Groundwater Resources Program

MODFLOW-NWT, A Newton Formulation for MODFLOW-2005

By Richard G. Niswonger, U.S. Geological Survey, Sorab Panday, AMEC Geomatrix Inc., and Motomu Ibaraki, Ohio State University

Thumbnail of and link to report PDF (10.4 MB)Abstract

This report documents a Newton formulation of MODFLOW-2005, called MODFLOW-NWT. MODFLOW-NWT is a standalone program that is intended for solving problems involving drying and rewetting nonlinearities of the unconfined groundwater-flow equation. MODFLOW-NWT must be used with the Upstream-Weighting (UPW) Package for calculating intercell conductances in a different manner than is done in the Block-Centered Flow (BCF), Layer Property Flow (LPF), or Hydrogeologic-Unit Flow (HUF; Anderman and Hill, 2000) Packages.

The UPW Package treats nonlinearities of cell drying and rewetting by use of a continuous function of groundwater head, rather than the discrete approach of drying and rewetting that is used by the BCF, LPF, and HUF Packages. This further enables application of the Newton formulation for unconfined groundwater-flow problems because conductance derivatives required by the Newton method are smooth over the full range of head for a model cell.

The NWT linearization approach generates an asymmetric matrix, which is different from the standard MODFLOW formulation that generates a symmetric matrix. Because all linear solvers presently available for use with MODFLOW-2005 solve only symmetric matrices, MODFLOW-NWT includes two previously developed asymmetric matrix-solver options. The matrix-solver options include a generalized-minimum-residual (GMRES) Solver and an Orthomin / stabilized conjugate-gradient (CGSTAB) Solver. The GMRES Solver is documented in a previously published report, such that only a brief description and input instructions are provided in this report. However, the CGSTAB Solver (called χMD) is documented in this report.

Flow-property input for the UPW Package is designed based on the LPF Package and material-property input is identical to that for the LPF Package except that the rewetting and vertical-conductance correction options of the LPF Package are not available with the UPW Package. Input files constructed for the LPF Package can be used with slight modification as input for the UPW Package. This report presents the theory and methods used by MODFLOW-NWT, including the UPW Package. Additionally, this report provides comparisons of the new methodology to analytical solutions of groundwater flow and to standard MODFLOW-2005 results by use of an unconfined aquifer MODFLOW example problem. The standard MODFLOW-2005 simulation uses the LPF Package with the wet/dry option active. A new example problem also is presented to demonstrate MODFLOW-NWT’s ability to provide a solution for a difficult unconfined groundwater-flow problem.

First posted May 11, 2011

For additional information contact:
Director, Nevada Water Science Center
U.S. Geological Survey
2730 North Deer Run Road
Carson City, NV 89701
http://nevada.usgs.gov/

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Suggested citation:

Niswonger, R.G., Panday, Sorab, and Ibaraki, Motomu, 2011, MODFLOW-NWT, A Newton formulation for MODFLOW-2005: U.S. Geological Survey Techniques and Methods 6–A37, 44 p.



Contents

Preface

Abstract

Introduction

Purpose and Scope

Description of MODFLOW-NWT

Additional MODFLOW Packages Modified for MODFLOW-NWT

Example Problems

Conclusions

Acknowledgments

References Cited

Appendix A. Input Instructions and List File for MODFLOW-NWT

Appendix B. Example for Modifying Nonlinear-Stress Packages for MODFLOW-NWT

Appendix C. χMD Users’ Guide Version 1.3: An Efficient Sparse Matrix Solver Library


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