Introduction to MODFLOW using Argus ONE

Back to MODFLOW Tutorial Table of Contents.

This guide introduces the use of Argus ONE for running the U.S. Geological Survey's Modular Finite-Difference Ground-Water Flow Model commonly known as MODFLOW (McDonald and harbaugh 1988; Harbaugh and McDonald, 1996), with Version 3 the MODFLOW GUI PIE. Version 1 of the MODFLOW GUI was developed by Dr. Shapiro and others (1997) of the USGS. This was subsequently modified by Hornberger and Konikow (1998) and then by Winston (1999) ( MODFLOW PIE).

This guide was originally provided by Argus Interware, Inc. It was subsequently donated to the USGS where it was further modified and placed in the public domain. By no means does this guide presents the "only" or "proper" way to work with MODFLOW, MODFLOW GUI PIE or Argus ONE. Since it describes the use of MODFLOW with Argus ONE, both of which are open and modular systems, you can, and probably will, find other ways of using MODFLOW with Argus ONE. Also, since the source code of the USGS MODFLOW GUI PIE and of MODFLOW are in the public domain, one can modify both to enhance and customize them to support additional technologies. We will welcome suggestions for improving this tutorial. Suggestions for enhancements of the MODFLOW GUI PIE should be sent to the USGS office of ground water, but you may carbon copy to Argus as well.

Many of the Argus ONE GUI PIEs for ground water models such as SUTRA are based on similar concepts, and thus we are certain that this guide might become handy for users of these other codes.

It is assumed that you have already installed

on your computer and you know how to start Argus ONE. The MODFLOW PIE and MODFLOW-96 can be downloaded for free from the USGS hydrologic software Web site ( http://water.usgs.gov/nrp/gwsoftware/modflow-gui/modflow-gui.html). It is also assumed that you are familiar with the general features of Windows 95® programs. It does not assume that you have any familiarity with either Argus ONE or MODFLOW or that you have read the manuals for either. However, It is certainly recommend that you do read

Unless you want to modify MODFLOW, you do not need to worry about the MODFLOW FORTRAN source code. When using Argus ONE, you also don't need to worry about the proper formatting of the MODFLOW input files as described in the MODFLOW documentation (McDonald and Harbaugh, 1988; Harbaugh and McDonald, 1996). Argus ONE and the MODFLOW PIE take care of that for you. In addition to the MODFLOW documentation, it is highly recommend that you read Anderson and Woessner's text on groundwater modeling (Anderson and Woessner, 1992). Another good resource is A Manual of Instructional Problems for the USGS MODFLOW Model (Andersen, 1993). It can be downloaded from http://www.epa.gov/ada/models.html. The problems presented here are largely based on those in Andersen (1993) but they have been reformulated for to present you with various capabilities of Argus ONE.

Argus ONE PIEs

Argus ONE PIEs (Plug-In Extensions) are one of the powerful features of Argus ONE. PIEs can provide new functionality for Argus ONE. This can be anything from a new function that could be used with any model to a complete specialized interface for a particular model. The MODFLOW PIE is an example of the latter.

Numerical Models, Conceptual Models and the Real World

MODFLOW is an example of a numerical model. In MODFLOW the region to be modeled is broken up into a series of grid cells. The modeler must assign hydraulic properties to each grid cell. Boundary conditions and such as recharge rates must also be assigned to grid cells. The grid cells are artificial and arbitrary; the real world is not broken up into grid cells.

In the real world, groundwater flow is bewilderingly complex. Among other things, it is affected by variations and temperature and salinity and by the complex porosity structure of the medium.

Between the immensely complicated real world and the artificial numerical model there is another level more suited to humans. This is the conceptual model. The conceptual model is a vast simplification of the real world. In the conceptual model, the groundwater flow medium is broken up into a series of aquifers and aquitards with homogeneous properties such as porosity and hydraulic conductivity. In the real world, these properties may be discontinuous at the level of individual pores. Thus it is only practical to apply them when the volume of aquifer being tested is sufficiently large.

Argus ONE acts as an interface between the conceptual model and the numerical model. For the numerical model to be a successful representation of the real world, there must also be an interface between the real world and the conceptual model. That interface is you. You must decide how much detail to include in the model and what details to exclude. For example, you might decide to ignore thermal and density effects and to ignore much of the small-scale variation in hydraulic conductivity. You must also decide what hydraulic properties and boundary conditions to include in the model. However, you don't need to assign these properties or boundary conditions to individual grid cells; Argus ONE takes care of that for you.

Modflow Capabilities

Let's briefly go through what MODFLOW is, what it can and can't do. If you are already familiar with MODFLOW, you can skip this section.

As it's name implies, MODFLOW is a modular program. It is broken up into a series of packages each of which is used to model a specific process affecting groundwater flow. One package, for example, deals with wells, and another deals with recharge. In the underlying mathematical formulation, wells and recharge are similar. Both add or remove a known quantity of water from a certain defined region. However, we normally have estimates of recharge rate in units of L/T (length/time) but well pumpage is in units of L3/T. Recharge also normally occurs over a wide area but each well pumps from only one location. Because of these differences, it is convenient to deal with wells and recharge using two separate packages even though the underlying mathematics are the same. You may have noticed that I said that both wells and recharge can add or remove water. Of course, normally recharge only adds water, not removes it but MODFLOW does not prevent you from specifying a negative recharge rate and this can be useful under some circumstances.

The USGS version of MODFLOW simulates groundwater flow in the saturated zone. It does not simulate the unsaturated zone or solute transport. (There are, however, commercial versions of MODFLOW that add both of those capabilities as well as at least two programs that uses the output of a MODFLOW simulation as input for a solute transport simulation.) MODFLOW does not simulate groundwater flow under conditions in which the density of water varies such as salt-water intrusion nor does it simulate flow involving more than one fluid. MODFLOW does not simulate fracture flow or dual porosity media but only normal Darcian flow. Clearly there are a number of groundwater flow problems for which MODFLOW is inappropriate. However, MODFLOW is appropriate for the most commonly occurring problems and its power and reliability have made it, by far, the most commonly used groundwater-flow modeling program. It can be downloaded for free from the World-Wide Web and the documentation can be purchased at a nominal price from the US Geological Survey.

MODFLOW does not include a graphical user interface. Instead, the user must prepare a series of text files that specify the problem to be modeled. These files can be voluminous and preparing them manually is a time-consuming and error-prone process. Argus ONE together with the MODFLOW PIE provide a graphical user interface for MODFLOW which vastly simplifies the process of creating MODFLOW input files. Argus ONE and the MODFLOW PIE support MODFLOW-96, the most recent version of MODFLOW.

When MODFLOW-96 was released all of the packages in the original version of MODFLOW were upgraded to the new MODFLOW-96 standard. However, there have been a number of other MODFLOW packages written both inside and outside the US Geological Survey (USGS) and those other packages have yet to be raised to the MODFLOW-96 standard. Most of those written at the USGS will work with MODFLOW-96 and they are included in the executable version of MODFLOW-96 from the USGS. Version 3 of the MODFLOW PIE does not support most of the packages that have not been upgraded to the MODFLOW-96 standard. The MODFLOW PIE supports the following packages.

Other USGS packages included in the MODFLOW-96 executable but not supported by the current MODFLOW PIE include the following packages.

You could use Argus ONE to prepare input files for these and other packages not supported by the MODFLOW PIE by creating an export template as described in the Argus ONE user guide.

USGS Packages not included in MODFLOW-96 include

(With the possible exception of the Preconditioned Conjugate Gradient 1 package, none of these packages is likely to be especially useful because they either have been superceded (River Package 2) or are incompatible with the usual version of other commonly used packages (Flow-Barrier and Canyon-Cutter packages).

The following public-domain packages were written by non-USGS personnel and are not included in the USGS version of MODFLOW-96.

There are a variety of commercial packages for MODFLOW including another package used to solve the finite-difference equations, packages to simulate solute transport and a package to simulate variable saturated flow conditions. These commercial packages are not supported by the MODFLOW PIE and are not covered in this document.