Frequently Asked Questions about Visual MODFLOW Flex

We are working hard to incorporate all Classic interface components in Flex. While most features that users require are now available in the Flex interface, not all features are supported yet. The best summary of which features are available only in the Classic interface is the features list on our Compare Editions page – those marked with an asterisk (*) are only available in the Classic interface.

You can leverage the power of 64-bit versions of the MODFLOW engines during running of your models to speed up your run time, if you are running 64-bit version of Windows. This support is available for the following Engines; MODFLOW-2000, 2005, MODFLOW-NWT, and SEAWAT in the Professional and Premium versions of Visual MODFLOW Flex.

The Visual MODFLOW classic interface supports MODFLOW-NWT package, a version of MODFLOW which provides enhanced capabilities for solving problems involving drying and rewetting non-linearities of the unconfined groundwater flow equation.

If your computer contains multiple processors, or dual core processors, you can leverage the power of parallel processing for your flow simulations using Visual MODFLOW Flow simulations using PCG, WHS, or SAMG solvers can be solved over multiple processors reducing the simulation run time.

The latest version of Visual MODFLOW supports SAMG v.2. This version of the SAMG solver is dramatically faster than its predecessor, and is ideally suited for multi-layered models with heterogeneous properties and is available with the Premium version only.

Similar to Recharge, the Visual MODFLOW classic interface provides an option to apply “Evapotranspiration” to different layers during the simulation

  • To the top layer
  • To a specified layer
  • To the top-most active layer

When defining a lake boundary condition, there is now an option to define horizontal leakance explicitly; this is important when there is both horizontal and vertical flow between the lake and aquifer, which is common in quarries or mine tailing ponds that have steep walls.

In the Output, there is a new Lake Budget option for Graphs that allow for showing time series plots of the lake parameters: stage, precipitation, evaporation, runoff, volume changes. You can chart results from a single model or load multiple model results on the same plot, for comparing different model scenarios.

This issue can happen if the entire installer did not get downloaded, this can happen if the internet connection time out during the download. Please check the installer size as it should be about 116MB. We would recommend downloading the software again, in the event of having a downloaded file size less than 116 MB. If the issue persists, please consider contacting technical support at along with the screen capture of the warning message.

MODFLOW-USG is a new version of the MODFLOW groundwater flow simulation code developed by the United States Geological Survey (USGS) which uses unstructured grids instead of rectilinear grids to discretize the model domain.

An unstructured grid is made up of triangular, rectangular or other polygon shaped geometries individually or in combination to discretize the groundwater model domain. This is in contrast to MODFLOW-2005 which uses structured, rectilinear grids. Unstructured grids require the use of the finite volume method whereas traditional structured grids are used with the finite difference method.

Structured finite difference MODFLOW-2005 grid (left) and an UnStructured grid with Voronoi Cells for MODFLOW-USG grid (right)

Structured finite difference MODFLOW-2005 grid (left) and an UnStructured grid with Voronoi Cells for MODFLOW-USG grid (right)

Although MODFLOW-USG can be used with any grid geometry – triangles, rectangles, polygons – Visual MODFLOW Flex supports polygonal unstructured grids made up of Voronoi Cells.

Voronoi Cells make up all unstructured grids in Visual MODFLOW Flex. Voronoi Cells are a special type of polygon that honors the Control Volume Finite Difference (CVFD) constraints required by MODFLOW-USG. Using this grid geometry with MODFLOW-USG results in an efficient and robust numerical grid that can handle complex hydrogeological geometries and boundaries with ease – similar to triangles, but with a higher degree of accuracy.

Learn more about Voronoi Cells.

Voronoi Cells

Voronoi Cells

Ghost Node Correction is a technique that is applied to some unstructured grid types, e.g., Nested or Quad-tree Grids, whose geometries do not inherently satisfy the requirements of MODFLOW-USG. Because Voronoi Grids by definition satisfy the constraints, ghost node correction is not required.

One of the geometric constraints for a Finite Volume grid is that the line formed by connecting the cell-center points of two adjacent Finite Volume cells must perpendicularly bisect the shared cell wall. While Voronoi Tesselations by definition fit this criteria, other grid types (such as Delaunary Triangulations and nested or Quadtree Grids) do not strictly conform to this constraint. In order to permit the mathematical solution of the Finite Volume Method for such grids, the USG package employs a technique called Ghost Node Correction (GNC) in which a temporary ‘ghost’ node is created within a cell such that it meets the geometric constraints and the equations may be solved. This solution at the ghost node is then interpolated (corrected) back to the actual cell center point, resulting in a Ghost Node Correction within each cell. (

Increased Simulation Accuracy

Unstructured grids permit local grid refinement along model features such as wells and boundary conditions, in addition to supporting fully discontinuous layers, both of which deliver high resolution detail and accuracy around areas that matter the most.

Reduced Simulation Runtimes

Traditional MODFLOW grid refinement often results in an excessive number of grid cells outside the areas of interest which leads to an inefficient numerical model with unnecessary long run times. With unstructured grids, the grid refinement is focused only to the areas of interest, resulting in much fewer grid cells, more stable grid structures, and therefore reduced run times.

Further Reading:
Case Study: MODFLOW-USG Yields 90% Reduction in Run Times for Regional Groundwater Model

Superior Modeling of Complex Geology

Unstructured grids are flexible and conform easily to the natural variability found in surface features and subsurface stratigraphy. Unstructured grid layers permit discontinuous layers that pinch-out to zero thickness, providing a more realistic representation of complex hydrogeological conditions, and improved model stability.

Increased Numerical Stability

Unstructured grids in Visual MODFLOW Flex 2014.1 are composed of a special type of polygon called Voronoi Cells are superior to other unstructured grid geometries (nested grids, quad-tree grids) as they completely honor the Control Volume Finite Difference (CVFD) constraints, and therefore do not require the Ghost Node Correction (GNC) package. In addition, well and boundary condition nodes coincide exactly with the grid nodes. This results in an efficient, robust and dependable numerical grid, with improved convergence and accuracy.

MODFLOW-USG can be used to simulate groundwater flow for virtually any application. However, It’s ability to handle complex geology makes it’s ideal for applications in mining where steep slope geology is often encountered. Flexible grid refinement capabilities make it suitable for large regional and watershed scale models.

The following processes and packages are supported:

  • CHD, FHB
  • SUB
  • New Packages: DISU, CLN, GNC, SMS

MODFLOW-USG is only available in the Professional and Premium editions of Visual MODFLOW Flex.

Yes. If you currently use Visual MODFLOW Flex, you will already have a 3D conceptual model that defines the geologic layers, properties and boundary conditions independent of the grid. To run your model with MODFLOW-USG, simply auto-generate the unstructured grid and then run the simulation. Visual MODFLOW Flex’s multi-model environment allows you to manage both your MODFLOW-2005 and MODFLOW-USG models within the same project, and both models can be easily compared side-by-side.

If your model currently exists in Visual MODFLOW Classic, you will need to import your model into Visual MODFLOW Flex before you can run the model using MODFLOW-USG.

Explore the powerful capabilities of MODFLOW-USG on the Waterloo Hydrogeologic MODFLOW-USG Learning Hub where you will find webinar recordings, videos, tutorials and related articles


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