Building a MODFLOW Model

Two basic approaches are provided in GMS for constructing a MODFLOW model: the model can be completely defined using the tools in the 3D Grid module, or the model can be defined with the aid of the feature objects in the Map module. Also, scatter points and solids can be used to define layer elevation, and boreholes can be used with TPROGS to create material sets for the 3D grid.

Using the 3D Grid Module

For models with simple geometry and boundary conditions, the entire model can be constructed using the tools and commands in the 3D Grid module. With this approach, the editing of the MODFLOW data is performed directly on the grid on a cell-by-cell basis. The main steps are as follows:

Create a 3D cell-centered grid covering the domain to be modeled using the Create Grid command in the Grid menu.
Use the commands in the MODFLOW menu to initialize and define the data required by the MODFLOW packages. Sources and sinks such as wells are defined by selecting the cells and assigning the attributes directly to the cells.

Using the Map Module

In most cases, the best method for setting up a MODFLOW simulation is to use the feature objects in the Map module to define a MODFLOW conceptual model of the site being studied. The conceptual model is a high-level description of the site including sources/sinks, the boundary of the domain to be modeled, recharge and evapotranspiration zones, and material zones within each of the layers. The conceptual model is defined with feature objects, including points, arcs, and polygons, and is constructed independently of a numerical grid. Once the conceptual model is complete, a grid is automatically constructed to fit the conceptual model, and the MODFLOW data are converted from the conceptual model to the cells of the grid.

Once the conceptual model is constructed and converted to a grid, the package dialogs and interactive editing tools in the MODFLOW menu can be used to edit or review the data if desired.

Defining the Layer Data

An important part of a MODFLOW model is the definition of the layer data (hydraulic conductivity, layer elevations, leakance, etc.). While both the Grid and Map module approaches to constructing a MODFLOW model can be used to define the layer data, both approaches may lead to an overly simplistic definition of the stratigraphy. Layers with spatially varying thicknesses can be handled most effectively using a special set of layer data tools provided in GMS. Solids can also be used to define layer data with the Solids->MODFLOW and Solids->HUF commands.