The CHD: Time-Variant Specified Head package pane is on the MODFLOW Features tab of the Object Properties dialog box.
The data that can be specified for the Time-Variant Specified Head package are the Starting time, Ending time, Starting head and Ending head. The Starting head is the specified head at the Starting time. The Ending head is the specified head at the Ending time. Specified heads for times between Starting time and Ending time will be calculated by MODFLOW using linear interpolation. In MODFLOW 6 models, Ending head can only be specified with parameters.
Starting time and Ending time are explained in the help for the MODFLOW Features tab.
Auxiliary Variables If the Groundwater Transport process in MODFLOW 6 is active, auxiliary variables will be included. They represent the concentration of flow into the groundwater from the boundary. Flow out of the groundwater will be at the concentration in the cell containing the boundary.
Density: Density is the density of fluid entering groundwater from the boundary. Density is only present if the Buoyancy package is active and Specify Density is selected.
Viscosity: Viscosity is the viscosity of fluid entering groundwater from the boundary. Viscosity is only present if the Viscosity package is active and Specify Viscosity is selected.
Density and Viscosity are only used with the General Head Boundary, River, Lake, Stream, and Multiaquifer Well packages. However, they may also appear in other packages where they will be treated as auxiliary variables but not used by MODFLOW.
If parameters are used, there are also multipliers for each parameter for both the Starting head and Ending head. MODFLOW will multiply the multiplier by the parameter value (specified in the MODFLOW Packages and Programs dialog box) to determine the starting and ending head to apply.
If more than one specified-head boundary is specified for the same cell, the final specified head for the cell will be the sum of the specified heads in MODFLOW-2005. This is true even if specified heads with parameters and without parameters are assigned by the same object. In MODFLOW 6, specifying the head at the same location more than once in the same stress period will cause MODFLOW to terminate with an error message.
Specified-head boundary conditions can not be deactivated in MODFLOW-2005 so even if a specified head is not defined in a cell in a stress period, it will still be a specified head cell if it has been defined as a specified head cell in any previous stress period. In MODFLOW 6, specified head boundaries are only active in the stress periods in which they are defined.
In MODFLOW 6, "Active Specified Head" can be used to turn on or off a CHD boundary. If the formula for Active Specified Head evaluates to True, a specified head boundary will be defined. If it evaluates to False, a specified head boundary will not be defined. This can be used to turn on or off specified head boundaries at different locations based on a criterion specified by the user. Such a capability can be useful for specifying tidal boundaries where some locations may be alternately flooded or exposed.
If Multipliers have been activated for the CHD: Time-Variant Specified-Head Package, the user may specify a Multiplier value. the specified heads will be multiplied by the value of the multiplier.
