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/Visual MODFLOW Flex FAQ
Visual MODFLOW Flex FAQ2016-11-22T18:25:31-05:00

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, there are still a few features that are not yet available. The best summary of which features are available only in the Classic interface is the features list on our Interface Comparison page.

Yes, 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 all supported Engines in the Professional and Premium versions of Visual MODFLOW Flex.

Yes, the Visual MODFLOW Flex 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 Flex simulations using PCG, WHS, or SAMG solvers can be solved over multiple processors reducing the simulation run time. Note that the parallel/multi-core version of the SAMG solver is only available in the Premium Edition of Visual MODFLOW Flex.

The latest version of Visual MODFLOW Flex supports SAMG v4. This version of the SAMG solver is dramatically faster than its predecessors, and is ideally suited for multi-layered models with heterogeneous properties. The single core-version is available with Professional Edition of Visual MODFLOW Flex, while multi-core version is available with the Premium Edition of Visual MODFLOW Flex.

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

  • To the top 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 530 MB. We would recommend downloading the software again, in the event of having a downloaded file size less than 530 MB. If the issue persists, please consider contacting technical support at support@waterloohydrogeologic.com 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 previous versions of MODFLOW, including 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 Grid (middle) and Quadtree Grid (right) for MODFLOW-USG

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

Voronoi Cells are one of the types of unstructured grid supported 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: connection between Voronoi nodes (black) crosses the shared interface at right angles

Quadtree Grids are one of the types of unstructured grid supported in Visual MODFLOW Flex. Quadtree grids are based on locally refining rectilinear grids by recursively subdividing cells into four equal quadrants.  This allows you to locally refine the grid around non-linear features such as streams and lakes without having to carry those refinements laterally across the entire model domain as occurs when using fully structured rectilinear grids.  Using this grid geometry with MODFLOW-USG results in an efficient numerical grid that can handle complex hydrogeological geometries and boundaries with ease. However, the use of Quadtrees can introduce some errors as the geometry of Quadtree grids does not adhere to the assumption inherent in the Control Volume Finite Difference approach utilized in MODFLOW-USG that shared cell interfaces occur at the perpendicular bisector of the respective cell centroids (see discussion below on Ghost Node Correction [GNC]).

Ghost Node Correction is a technique that is applied to some unstructured grid types (e.g., Nested or Quadtree Grids), whose geometries do not inherently satisfy the requirements of MODFLOW-USG.   One of the geometric constraints for the Control Volume Finite Difference approach used in MODFLOW-USG is that the line formed by connecting the centroids of two adjacent Finite Volume cells must perpendicularly bisect the shared cell interface. While Voronoi Tessellation by definition fits this criteria, other grid types (such as Delaunay 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 certain 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. (http://pubs.usgs.gov/tm/06/a45/pdf/tm6-A45.pdf)

The ghost node correction package is generally not required when using Voronoi Grids because by definition they satisfy the geometric conditions required for the Control Volume Finite Difference approach used in MODFLOW-USG.

The use of Quadtrees may introduce errors in the simulation results as the geometry of Quadtree grids does not adhere to the assumption inherent in the Control Volume Finite Difference approach utilized in MODFLOW-USG: that shared cell interfaces occur at the perpendicular bisector of the respective cell centroids.  The interfaces between neighboring cells with different degrees of refinement will occur at an angle that departs from perpendicularity and the distance between the centroids will become increasingly asymmetric as the relative degree of refinement between the cells increases. In cases where the model is smooth (that is the model properties, cell elevations/thicknesses, aspect ratio between adjacent cells is small [2:1]) and/or the non-conforming cells are located far from the area of interest, the resulting error may fall within tolerances that are acceptable to the modeler.  For cases where this error may be unacceptable, Visual MODFLOW Flex supports the MODFLOW-USG Ghost Node Correction (GNC) package which corrects for this potential source of error. Note that the use of the GNC package is likely to increase model run times.


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 Voronoi grids in Visual MODFLOW Flex are composed of a special type of polygon called Voronoi Cells that are generally superior to other unstructured grid geometries (nested grids, Quadtree 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, cell centroids will coincide exactly with conceptual wells and boundary condition feature vertices. 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 ideal for applications in mining where steep slope geology is often encountered. Flexible grid refinement capabilities also 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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