The pane for this package is on the MODFLOW Packages and Programs dialog box under Specified Head.
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.
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). In MODFLOW 6, the CHD package is officially called the Constant Head package even though the heads are not necessarily constant in time.
In MODFLOW-2005 and related versions of MODFLOW, there is no way to convert a specified-head cell to an ordinary active cell; once a cell becomes a specified-head cell, it remains a specified-head for the remainder of the model.
In MODFLOW 6, a constant head will be converted to an active cell if the specified head is not specified again each time the specified heads are changed.
In MODFLOW 6 models, "multipliers" can be used with specified head boundaries. Multipliers will be used if the "Use Multiplier (MODFLOW 6)" checkbox is checked.. Ordinarily, multipliers are not needed. If multipliers are used, the specified head is multiplied by the value of the multiplier. One use for multipliers might be when the specified heads are defined using a time-series and it is desired to multiply the specified head values in the time series by a different amount at different locations.
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.
