What is MODFLOW? It’s one of the most commonly asked questions, and one of the hardest to find a succinct, easy-to-understand answer for. We hope to remedy that here.
MODFLOW is a finite-difference groundwater flow modeling program, written by the United States Geological Survey (USGS), which allows you to develop a numerical representation (i.e. a groundwater model) of the hydrogeologic environment at a field site that you are investigating. It uses the finite-difference method to divide the groundwater flow model domain into a series of rows, columns and layers, which defines a unique set of grid blocks (i.e. model cells) to represent the distribution of hydrogeologic properties and hydrologic boundaries within the model domain (See Figure 1). When you develop a groundwater model, you assign properties and boundaries to these model cells, and MODFLOW uses the cell dimensions, property values and boundary values to write a set of finite-difference equations that it solves to calculate the hydraulic head at the center of each model cell.
Figure 1: The Layer View of the MODFLOW Model Finite Difference Grid Cells
Developing a MODFLOW Model in 3 Steps
The first step in developing a MODFLOW model is to combine all of the field data from the project area (e.g. ground surface elevations, well locations, borehole log data, pumping test and slug test results, geophysical surveys, groundwater levels, surface water elevations, etc …) into a hydrogeologic conceptual model (See Figure 2) of the site using a Geographic Information System (GIS).
Figure 2: 3-Dimensional View of the GIS Conceptual Hydrogeologic Model
The second step is to construct the MODFLOW numerical model (See Figure 3) so that it represents the GIS conceptual model by choosing appropriate MODFLOW properties, boundaries and observations for each of the conceptual model objects (i.e. layer-by-layer distributions of hydrogeologic properties, hydrologic boundaries and field observations).
Figure 3: 3-Dimensional View of the MODFLOW Finite-Difference Grid and Property Zones
The last step in model development is to run the model to simulate groundwater heads and flows and to calibrate model output (i.e. MODFLOW calculated heads and flows) to observations of heads and flows from the field sampling program.
Unfortunately, the input files that USGS MODFLOW requires to run a groundwater flow simulation are a series of text files, which describe all of the attributes of the model domain including cell-by-cell dimensions, properties and boundaries. MODFLOW reads these text files and uses them to calculate the hydraulic head in every active finite-difference cell. The fact that these input files are text files makes it difficult and time consuming to use the USGS MODFLOW directly to create a groundwater model. This has led to the development of Graphical User Interfaces (GUIs), including Visual MODFLOW, that allow you to develop groundwater models graphically on-screen, with the help of imported DXF, TXT and SHP files in an extremely easy manner. Using Visual MODFLOW, you assign the cell dimensions, property values and boundary conditions graphically in row, column and layer view and then translate this Visual MODFLOW information into the set of MODFLOW input text files that are run by USGS MODFLOW to generate a groundwater flow solution.
The real value of using MODFLOW as a modeling tool to help with hydrogeologic investigations is that once a groundwater model has been developed and calibrated, you can use it to ask “what if” questions about how the hydrogeologic system responds to design changes in the future. For example, you can add a groundwater design (i.e. the addition of pumping wells and injection wells, infiltration galleries, grout cut-off walls, seepage drains, etc …) to the model to simulate their impact on the hydraulic heads and flow before they are ever constructed in the field. The results are presented graphically, as 2D and 3D images in Visual MODFLOW, which makes it easy to visualize the impact of these designs on the hydrogeologic system and, therefore, makes modeling an indispensible tool in the design process.