Steps in Defining a Conceptual Model

Several steps are involved in setting up a MODFLOW conceptual model and converting the conceptual model to a numerical model. The basic steps are as follows:

Create a MODFLOW conceptual model
Decide how many coverages you will need and what attributes you want with each. It is typical to use three coverages, each with a different purpose. One coverage is used for sources and sinks, like wells, specified head boundaries etc. Another coverage is used for recharge, and another is used for hydraulic conductivity. However, it is not required to use coverages in this manner, and the attributes associated with a coverage are user defined, so you can define a coverage with any combination of attributes. Thus, if your recharge zones were the same as your hydraulic conductivity zones, you might use just one coverage for both.
Create the sources and sinks. The most effective way to do this is with the aid of a background image. A digital image representing a scanned map or an aerial photo of the site can be imported and displayed in the background using the image tools. Once the image is displayed, feature objects defining the model boundary, rivers, lakes, flow barriers, and specified head boundaries can be created on top of the background image.
Create the areal features, such as recharge zones and evapotranspiration zones.
Create the layer features, such as hydraulic conductivity zones.
Use the Grid Frame command to place an outline of the numerical grid on the conceptual model. The frame is placed so that it just surrounds the conceptual model. The frame can be rotated if necessary if the major axis of the model is at an angle.
Use the Map -> 3D Grid command to automatically generate a grid. The location of the grid is controlled by the Grid Frame and the density of the grid is automatically adjusted around user-specified points (typically wells).
Define the active region of the grid using the Activate Cells in Coverage command. This automatically activates all of the cells within the boundary of the conceptual model and inactivates all cells outside the boundary.
Initialize the MODFLOW data by selecting the New Simulation command in the MODFLOW menu. Select the type of mode (steady state vs. transient) and define stress periods if necessary. Define a set of starting heads. Go to the LPF/BCF/HUF Package dialog and define the layer type for each of the layers in the grid.
Select the Map -> MODFLOW command to automatically assign the MODFLOW boundary conditions, stresses, and material properties to the appropriate cells in the grid.
Use the specialized interpolation tools to define the layer elevations.

In many cases, it is useful to repeat some, but not all, of these steps. For example, suppose after running a simulation it is determined that one of the boundaries of the model corresponding to a groundwater divide is not properly located. The boundary can be moved by simply selecting and dragging the vertices and nodes of the arc(s) defining the boundary. Once the boundary is moved, step 6 should be repeated to redefine the active/inactive regions and then step 8 is repeated to reassign the model data to the cells.