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UNIT 8 -- MODFLOW INPUT AND EXECUTION:

MODFLOW - A Modular Three-Dimensional Finite-Difference Ground-Water Flow Model by Michael McDonald and Arlen Harbaugh, is the world's most used groundwater modeling code. MODFLOW simulates: Single Phase, Saturated Flow of Constant Density in Porous Media (Darcy's Law applies) for 1, 2, or 3 Dimensional systems in 2D areal or cross-section mode, or Quasi-3D mode. MODFLOW simulates both Steady State and Transient Flow in Heterogeneous, Anisotropic, Layered Units. It uses a Block Centered Finite Difference Solution Technique applied to Variable grid spacing in the x, y, and z directions. Units can be Confined, Unconfined, or Convertible. Boundary conditions include: Dirichlet, Neumann, Cauchy, and Phreatic Surface options. Simulated stresses include: Wells, Recharge, Evapotranspiration, Rivers, Streams, and Drains.

If you haven't seen a numerical model before, look at this simple example (numerical model example).

View an example of a flow model (numerical model example).

View an example of a flow and transport model (numerical model example).

In this unit we study the basic features of MODFLOW from the perspective of inputting information to MODFLOW and executing the code. In order for MODFLOW to simulate groundwater flow, there are a number of ASCII text files that need to be written for MODFLOW to read. These files describe the grid, material properties, boundary conditions, initial conditions, time steps, stresses and solution and output options.

A graphical user interface (GUI) allows you to develop a nice image of model features on the computer screen and manipulate the model inputs graphically. When you are ready, the GUI creates the text files and executes MODFLOW. You never need to see the text files or know the commands that are necessary to run MODFLOW ... until something goes wrong!

If you do not have the ability to look in, and understand the content of these files or control these commands, you will not be a valuable modeler. This is because the GUIs are not infallible yet. If GUIs were as reliable and as self contained as your calculator, you would not need to know about these files and commands. That time is still fairly far off in the future.

Inevitably something does not work correctly in the GUI and you need to track down and resolve the problem. Or, you want to do something unique with your project that the GUI developer hadn't planned on. If you do not understand the file structure and commands, you cannot move forward. You are, so to speak, paralyzed, and your project is in jeopardy. I have seen this happen to many people. One year I tried to teach this class with only GUI access to MODFLOW and I was terribly disappointed at the students ability to resolve even simple problems that arose. Of course you can try to find someone who will help you and get the job done, but to be a valuable employee and a useful modeler, you want to be the person that people can go to with such questions.

In short, you are likely to dislike the tedium associated with the portion of the course where we work with text files, but please be patient and persevere because the effort will be worth it in the future. You will be a hero on modeling projects due to this knowledge and skill. AND we will move on to the GUI approach soon!

Here are a few words from individuals who found that knowing MODFLOW text files was useful on the job.

MODFLOW MAIN PAGE

• read Pages 1-44

In your copy of MODFLOW-2000, the US Geological Survey modular groundwater model -- Documentation of the Hydrogeologic-unit Flow (HUF) Package,

• read Pages 1-10

In your copy of A Modular Three-dimensional Finite-Difference Groundwater Flow Model,

• familiarize yourself with the data input sections of Chapters 4, 5, 12, and 14
• each time we start to use a new module, stop and read the data input section for that module before proceeding

If you have not yet done so, download the manual for MODFLOW96 from the Books section on the main class page. It is a pdf file.

In your copy of the MODFLOW96 manual,

• familiarize yourself with Pages 45-121 -- notice these are abbreviate instructions and where necessary new input instructions relative to the MODFLOW88 and the MODFLOW96 manuals -- if you have trouble understanding input instructions in later manuals you may be enlightened by looking at the instructions in older manuals

In your copy of MODFLOW-2000, the US Geological Survey modular groundwater model -- Documentation of the Hydrogeologic-unit Flow (HUF) Package,

• familiarize yourself with Pages 10-89