The General Head package is similar to the Drain and River packages in that flow in or out of a cell is proportional to a difference in head. General head cells are often used to simulate lakes. General head conditions are specified by assigning a head and a conductance to a selected set of cells. If the water table elevation rises above the specified head, water flows out of the aquifer. If the water table elevation falls below the specified head, water flows into the aquifer. In both cases, the flow rate is proportional to the head difference and the constant of proportionality is the conductance.
A set of selected cells can be specified as general head cells using the Point Sources/Sinks command in MODFLOW menu or general heads can be assigned using the conceptual model in the Map module. Once a set of cells have been specified, the General Head Package dialog can be used to view and edit the values assigned to the cells.
The stress period field shows the current stress period and allows you to cycle through the stress periods and view the different general head properties for those stress periods in the spreadsheet below.
Use this option on a stress period to use the general head values from the previous stress period for the current stress period. This option is unavailable for steady state models and the first stress period of transient models.
For cells where river type boundary conditions have been assigned, the stage, conductance, and bottom elevation assigned to each cell are displayed in the spreadsheet portion at the lower part of the dialog. The spreadsheet can be used to edit the row, column, layer, head, and conductance values. For a transient simulation, the values displayed in the spreadsheet are for an individual stress period. The values associated with other stress periods can be edited by entering the number of the desired stress period in the stress period edit box in the center of the dialog. If the Use previous option is selected for a given stress period, the values from the previous stress period are used and the spreadsheet is dimmed.
This button restores all values in the dialog to their default values
Related Links:
Packages Supported by GMS
Building a MODFLOW Model