How to Fix MODFLOW General Head Boundary Errors Using GMS

When working in MODFLOW in the Groundwater Modeling System (GMS), have you ever come across a warning that the “general head elevation is below cell bottom elevation”? This error is not uncommon, and, fortunately, it can be easily resolved. This happens when the general head boundary condition is set to a value lower than the cell’s bottom elevation. Essentially, MODFLOW can’t assign a head below the cell bottom, marking it as dry and deactivating it. To fix this, all you need to do is adjust a few settings usng GMS so the cell stays active and the simulation runs smoothly.

Resolving the Issue

Here are some of the best ways to fix this issue in your model:

• Adjust the General Head Boundary Value: In the Sources/Sinks dialog—or in the General Head Package dialog—adjust the value of the general head boundary so the new value is above the cell’s bottom elevation.

• Modify the Coverage Attribute Table: Change the settings of the model and remap the coverage, making sure that the attribute table auto-assigns the general head boundary to the correct layers.

• Check Interpolated Values: If you are using an arc to assign conditions, ensure that the head values in the attribute table are first applied to the nodes, then interpolated across the arc. Sometimes, errors can happen if the interpolated values fall below the cell bottoms, so keep an eye out for that.

• Use the Model Checker: As a troubleshooting tool, the model checker under MODFLOW | Check Simulation… is extremely useful. By selecting a line in the checker, you can examine the cell's references, making it easier to change.

There are a few simple ways to resolve these issues when using the general head boundary condition for you MODFLOW project. By adjusting the attributes and double-checking the values, you can quickly dismiss any issues and get your model back on track. Take control of your MODFLOW models in GMS by making sure your general head boundary conditions are set correctly, so your simulations run smoothly and accurately! Download the latest version of GMS today!