Simple Model: Specified-Head Boundary

Four boundary condition packages are used in this model, the CHD, DRN, RCH, and WEL packages. To activate them, select Model|MODFLOW Packages and Programs. Then expand Boundary conditions and each of the items beneath it. Locate the check boxes for each of the packages and check them. Then click OK to close the dialog box.

There is a specified-head boundary with a head of 0.0 in the first column of the model. It is present in both the upper and middle aquifers but not the lower aquifer. (It also is not present in the confining bed because the confining bed is not simulated.) Select Object|Create|Polyline and then create a polyline on the top view of the model through column 1 for the full length of the model (row 1 to row 15). Double click to end creating the polyline. It doesn't matter if the object goes past the end of the model or doesn't go completely through the first or last cell. Change the formula for the Higher Z coordinate and Lower Z coordinate to "Model_Top" and "Middle_Aquifer_Bottom" respectively. These will cause the object to extend vertically from the top of layer 1 to the bottom of layer 3. Go to the MODFLOW Features tab and check the check box for the CHD package. Enter a Starting time of 0, and Ending time of 86400, and a Starting head and an Ending head of 0. (Note that when entering times, you can select times for the start and end of the stress periods from the drop-down list in the cells for the times.) If the starting and ending heads were different, the specified head at the end of each time step between the starting time and the ending time would be determined by linear interpolation between the starting head at the starting time and the ending head at the ending time. Click OK to close the Object Properties dialog box.

To check that the specified heads were assigned correctly, select Data|Data Visualization|Color Grid... and set the Data set or boundary condition to "CHD Starting Head." (It is found under "Boundary Conditions, Observations, and Other Features|MODFLOW CHD|CHD Starting Head." Click the Apply button. Then change the selected layer and verify that the specified heads are assigned only in layers 1 and 3. (They are not assigned to layer 2 because layer 2 is not simulated.)