Troubleshooting Well Exclusions in MODFLOW

If you’re running a MODFLOW model in Groundwater Modeling System (GMS) and notice that some of your wells aren’t showing up in your simulation, don’t worry—well exclusions are usually due to a disconnect between the well screens and the grid. To fix this, all you need to do is identify which cells are missing, understand why they’re being left out, and get them back into your simulation for accurate and reliable results.

Spotting the Issue

First, you need to figure out which wells are being excluded from the simulation. When you get the warning that the cells are being excluded from the MODFLOW model, click the Show Details… option on the dialog. This will give you a list of the wells that aren’t making it into the model. You can use the well names or IDs listed here to locate their corresponding coverage points.

Why Are the Wells Missing?

Once you’ve been able to identify the excluded wells, now you need to figure out why they’re not being included. Typically, this is the result of mismatches with the elevation or incorrect layer assignments.

• Compare Grid and Well Screen Elevations: Check the grid elevations near the excluded wells and compare them to the well screen elevations in your attribute table. If the well screen falls outside the grid’s vertical range, the MODFLOW simulation won’t include it.

• Check for Swapped Top and Bottom Values: Check that the top and bottom elevations of the well screen haven’t been flipped. To do this, just make sure that the top elevation is actually higher than the bottom elevation.

Fixing the Issue

Now that you know what’s causing the issue, here’s how you can fix it:

• Adjust the Well Screen Elevations: If the well screens are outside the grid’s vertical range, change them in the attribute table so they match the grid.

• Modify Grid Elevations: If adjusting the well screens isn’t a viable option, you might need to tweak the grid elevations instead to ensure they cover the well screens.

After making the necessary changes, make sure you remap the MODFLOW model coverage so that the updates you made are loaded. If you skip this step, the simulation will use the old, incorrect information and keep excluding your wells.

Missing wells in your MODFLOW simulation? Fix your well screen elevations in GMS today.