Documentation for the MODFLOW 6 Groundwater Flow Model

Techniques and Methods 6-A55
Prepared in cooperation with the U.S. Geological Survey Water Availability and Use Science Program
By: , and 

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Abstract

This report documents the Groundwater Flow (GWF) Model for a new version of MODFLOW called MODFLOW 6. The GWF Model for MODFLOW 6 is based on a generalized control-volume finite-difference approach in which a cell can be hydraulically connected to any number of surrounding cells. Users can define the model grid using one of three discretization packages, including (1) a structured discretization package for defining regular MODFLOW grids consisting of layers, rows, and columns, (2) a discretization by ver­tices package for defining layered unstructured grids consisting of layers and cells, and (3) a general unstruc­tured discretization package for defining flexible grids comprised of cells and their connection properties. For layered grids, a new capability is available for removing thin cells and vertically connecting cells overlying and underlying the thin cells. For complex problems involving water-table conditions, an optional Newton-Raphson formulation, based on the formulations in MODFLOW-NWT and MODFLOW-USG, can be acti­vated. Use of the Newton-Raphson formulation will often improve model convergence and allow solutions to be obtained for difficult problems that cannot be solved using the traditional wetting and drying approach. The GWF Model is divided into “packages,” as was done in previous MODFLOW versions. A package is the part of the model that deals with a single aspect of simulation. Packages included with the GWF Model include those related to internal calculations of groundwater flow (discretization, initial conditions, hydraulic conduc­tance, and storage), stress packages (constant heads, wells, recharge, rivers, general head boundaries, drains, and evapotranspiration), and advanced stress packages (streamflow routing, lakes, multi-aquifer wells, and unsaturated zone flow). An additional package is also available for moving water available in one package into the individual features of the advanced stress packages. The GWF Model also has packages for obtaining and controlling output from the model. This report includes detailed explanations of physical and mathematical concepts on which the GWF Model and its packages are based.

Like its predecessors, MODFLOW 6 is based on a highly modular structure; however, this structure has been extended into an object-oriented framework. The framework includes a robust and generalized numeri­cal solution object, which can be used to solve many different types of models. The numerical solution object has several different matrix preconditioning options as well as several methods for solving the linear system of equations. In this new framework, the GWF Model itself is an object as are each of the GWF Model packages. A benefit of the object-oriented structure is that multiple objects of the same type can be used in a single sim­ulation. Thus, a single forward run with MODFLOW 6 may contain multiple GWF Models. GWF Models can be hydraulically connected using GWF-GWF Exchange objects. Connecting GWF models in different ways permits the user to utilize a local grid refinement strategy consisting of parent and child models or to couple adjacent GWF Models. An advantage of the approach implemented in MODFLOW 6 is that multiple models and their exchanges can be incorporated into a single numerical solution object. With this design, models can be tightly coupled at the matrix level.

Suggested Citation

Langevin, C.D., Hughes, J.D., Banta, E.R., Niswonger, R.G., Panday, Sorab, and Provost, A.M., 2017, Documentation for the MODFLOW 6 Groundwater Flow Model: U.S. Geological Survey Techniques and Methods, book 6, chap. A55, 197 p., https://doi.org/10.3133/tm6A55.

ISSN: 2328-7055 (online)

Table of Contents

  • Abstract
  • Chapter 1. Introduction
  • Chapter 2. Formulation and Solution of the Control-Volume Finite-Difference Equation
  • Chapter 3. Spatial Discretization
  • Chapter 4. Internal Flow Packages
  • Chapter 5. Storage
  • Chapter 6. Conceptualization and Implementation of Stress Packages
  • Chapter 7. Conceptualization and Implementation of Advanced Stress Packages
  • Chapter 8. Groundwater Flow Model Exchange
  • References Cited
  • Appendix A. List of Symbols
  • Appendix B. Implementation of the Groundwater Flow Model in the MODFLOW 6 Framework

Additional publication details

Publication type Report
Publication Subtype USGS Numbered Series
Title Documentation for the MODFLOW 6 Groundwater Flow Model
Series title Techniques and Methods
Series number 6-A55
DOI 10.3133/tm6A55
Year Published 2017
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Office of Ground Water
Description Report: 197 p.; Application Site; Companion Files
Larger Work Type Report
Larger Work Subtype USGS Numbered Series
Larger Work Title Section A: Groundwater in Book 6 Modeling techniques
Public Comments This report is Chapter 55 of Section A: Groundwater in Book 6 Modeling techniques.
Online Only (Y/N) Y
Additional Online Files (Y/N) N