The CSUB package allows the user to specify a stress offset that is added to the system in any stress period. In the example model that comes with MODFLOW 6, this stress offset is specified for every active cell in the top layer but the value of the stress offset is zero so it has no real effect. We will specify the stress offset here to demonstrate how it is done.
Use the Selection Cube to make layer 1 the selected layer. Then create a polygon or rectangle object covering all the active cells in the top view of the model.
Make sure that the Z-Coordinates enclose only the top layer of the model. Then, on the MODFLOW Features tab, select the CSUB package. On the Transient tab, set the Stress Offset for the first stress period to zero.
