Three Methods for Assigning CHD

Do you have a Ground-water Modeling System (GMS) project where you are needing to edit values in your constant head cells? If it is a simple grid approach, academic-style project, editing each cell is likely fine. But if you have a large, more complex, conceptual model, editing the constant head cells may prove problematic. Here are some ways that you can accomplish this with a minimum of effort.

A constant head (CHD) cell is an area where the water levels will remain fairly steady no matter what the inflow or outflow may be. Lakes, coastal areas, etc. are good candidates of constant head boundaries. Assigning cells to be constant heads provides a steady boundary where the water levels will stay the same for the model duration.

The constant head cells are typically assigned or edited in one of three ways. One method is to directly edit the IBOUND array. When setting up a MODFLOW simulation or when editing packages, the MODFLOW | Global Options menu will let you choose the IBOUND package to edit the options there including the array. Copying and pasting data from an external spreadsheet into the array could save you time here.

Another method is to select a set of cells and use the 3D Grid Cell Properties dialog. You can right-click on a cell to bring up the dialog. Make sure you are on the MODFLOW tab and edit the “IBOUND” and “Starting head” rows. If multiple cells are selected, this will edit all selected cells.

The simplest method is to define the constant head zones using feature objects as part of a conceptual model in the Map module. When you set up a coverage using the feature objects, the Coverage Setup dialog will allow you to assign CHD to cells in the grid. You can change what the default assignment will be if you also select “Layer Range” and make changes using the Attribute Table.

Whichever method you choose to assign CHD, when you are ready to run your ground-water model, the Model Checker can be used to see if there are errors in the assigned constant head cells. Then you can correct those cells mentioned specifically by using the IBOUND array or the Cell Properties dialog as mentioned above.

Head on over to GMS and see which method of defining your constant head cells works best for you!