Experiencing non convergence error? Here are some tips

October 20, 2017

No one likes errors, am I right?

Errors such as “Solution Non-Convergence” , “Failed to meet solver criteria …”, “Abnormal termination. See file C:\……..\ProjectName.LST for details” when running MODFLOW are Non-Convergence messages. This typically occurs when MODFLOW cannot reach a successful simulation result.  Due to the complexity of ground water modelling, non-convergence errors are not unusual.  Please see the tips below for the most common convergence problems that our Visual MODFLOW clients face and suggestions for resolving them.

Here are some things to check when your model does not converge:

Problem #1: Zero thickness cells (this can only occur in older Visual MODFLOW models, the current version of Visual MODFLOW has built-in layer validation rules)

Solution #1: Visual MODFLOW contains an automatic check for zero thickness cells. When trying to translate a model that contains zero thickness cells, you will be prompted with a reminder to go back to the Input module and fix the thickness of the offending cells. Go back to the Input-Grid module, and then try to return to the Main Menu. Visual MODFLOW will not allow you to leave this module until the zero thickness cells have been corrected. You are provided with the option to fix the offending cells ‘globally’ or on a ‘cell-by-cell’ basis. The majority of users will select the global option. Once completed, you may return to the Run Module.

Problem #2: Active cells surrounded by dry cells: MODFLOW is trying to calculate head in those cells. You may receive an error referring to “… cells connected by a single conductance branch… ”

Solution #2: Activate the Cell Rewetting option (from the Run menu, click on MODFLOW Rewetting, and check the Activate Cell Wetting option) and first try “rewetting from below” only. Using the “from sides and below” option is an option that can help, but may be more prone to result in oscillation. For the “Head value in dry cells”, you can try using the “cell bottom” instead of -1e30 if you continue to experience difficulties (see your Visual MODFLOW User’s Manual for more details).

Problem #3: Large changes in Hydraulic Conductivity between adjacent cells.

Solution #3: Reduce the numerical difference in hydraulic conductivities between neighboring cells, or add additional conductivity zones, to produce a lateral ‘gradient’ of conductivity values.

Problem #4: Large differences in cell size between adjacent cells.

Solution #4: A rule of thumb to follow: Grid spacing should not increase by more than 50 percent between adjacent cells. As well, an aspect ratio must be respected: A grid cell should not be more than 10 times larger in
one direction than in another. Refine your grid in problematic sections, and if necessary, use the Grid-Smoothing feature. See your Visual MODFLOW User’s Manual for more details.

Problem #5: Lateral discontinuity between cells in the same layer.

Solution #5: Large changes in elevation (especially when your model has thin layers) can cause adjacent cells to become laterally “detached”, resulting in a lack of lateral continuity. This causes an inability to model flow between cells in the same layer. Avoid this by reducing large changes in elevation over short distances, and/or refining your grid, and/or changing layer thickness. As a rule of thumb, adjacent cells in the same layer should overlap 50% of their “depth” (Z axis). You can use the View Row and View Column tools in Visual MODFLOW to study areas in cross section.

Problem #6: Dry cells in your model domain, or pumping wells that are running dry.

Solution #6: The MODFLOW Flow Engine is designed to simulate saturated flow conditions only. If you have large areas of your model that are above the water table, or areas where the water table is dropping below a model layer, the dry cells can result in convergence problems. Additionally, if areas are cycling between saturated and unsaturated (wet and dry) conditions due to water table fluctuation, Recharge settings, or rewetting settings, this can also cause convergence problems due to oscillation.

1. Check to see that you have entered an appropriate Recharge value for your model, and that it is assigned to the “highest active layer” (from the Visual MODFLOW Run menu, click MODFLOW Recharge to select this option.) 2.  Consider re-assigning layer elevations, or combining the top layers in your model, to increasing the layer thickness, and reduce the probability of dry cells. Visual MODFLOW is a saturated flowmodeling program. If you have layers above your water table, they will become Dry during the model run, which can cause non-convergence. You may receive an error message referring you to the .LST file.

3. If you think that Rewetting is causing oscillation in your model, try reducing the Wetting interval to every 3-5 iterations.

4. If drawdown around pumping wells is causing wells to run dry, convergence problems can surface. You may need to revise your model inputs (e.g. Conductivity values, layer thickness, pumping rates, grid discretization, etc.) to keep the cell(s) in which the well is assigned from going dry If you are not able to resolve dry cell issues in your model, you may want to consider the MODFLOW-SURFACT Flow Engine. This add-on package for Visual MODFLOW is designed to simulate Unsaturated and Saturated conditions without difficulty, and is also capable of handling wells screened over multiple layers where one or more layers are being pumped dry, perched water tables, steep water table gradients, and other complex project scenarios. You can contact Waterloo Hydrogeologic Sales department at sales@waterloohydrogeologic.com, or   +1 519-746-1798, for more information on the MODFLOWSURFACT Flow Engine. If you have checked the previous common problems, and implemented the solutions, yet your model will still not converge, you may want to consider tweaking the parameters for your solver, or trying a different solver.

Hope this helps!