CHD: Time-Variant Specified-Head Package Pane

The Time-Variant Specified-Head (CHD) package allows the user to specify the heads in cells. The specified heads can vary with time and parameters can be used to specify the specified-head. However, once a cell becomes a specified-head cell, it remains a specified-head for the remainder of the model. There is no way to convert a specified-head cell to an ordinary active cell. The pane for this package is on the MODFLOW Packages and Programs dialog box under Specified Head.

The Time-Variant Specified-Head package is sometimes referred to as the Constant Head package. The abbreviation (CHD) for the Time-Variant Specified-Head package comes from this alternate name. "Constant Head" however does not accurately describe how the boundaries in the Time-Variant Specified-Head package actually work because the specified heads in the package can vary with time (Arlen Harbaugh, personal communication, 2007).

On the CHD: Time-Variant Specified-Head package pane, the user can define parameters for the specified head boundaries. These parameters will be multiplied by user-specified factors at the beginning and end of each stress period to determine the specified heads defined by parameters. The user can also set up specified head boundaries that do not involve parameters. The specified heads at any particular time step will be determined by linear interpolation between the specified heads at the beginning and end of the stress period. Objects must be used to define the locations of the specified head boundaries.

Parameters can also be edited in the Manage Parameters dialog box.