CHD: Time-Variant Specified-Head Package Pane

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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.

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, only the last of the specified heads will be used in assigning the specified head.

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.

Time-series interpolation

In MODFLOW 6, time series files can specify the times and values to use for a boundary condition.

When MODFLOW 6 needs data from a time series or time-array series for a time interval representing a time step or subtime step, the series is queried to provide a time-averaged value or array of values for the requested time interval. For each series, the user specifies an interpolation method that determines how the value is assumed to behave between listed times. The interpolation method thus determines how the time averaging is performed. When a time-array series is used, interpolation is performed on an element-by-element basis to generate a 2-D array of interpolated values as needed.

The supported interpolation methods are STEPWISE, LINEAR, and LINEAREND. When the STEPWISE interpolation method is used, the value is assumed to remain constant at the value specified in one time-series record until the time listed in the subsequent record, when the value changes abruptly to the new value. In the LINEAR interpolation method, the value is assumed to change linearly between times listed in sequential records. LINEAREND is like LINEAR, except that instead of using the average value over a time step, the value at the end of a time step is used.

ModelMuse uses time-series interpolation when parameters are used. The parameter value becomes the SFAC (scale factor) in the time-series file.

ModelMuse sets the interpolation method to STEPWISE for all boundary conditions in which it is used except for the CHD package in which it is set to LINEAREND.

ModelMuse does not currently support time-series interpolation because it slows down MODFLOW to much as implemented in ModelMuse. Support for time-series interpolation may be supported in a future version of ModelMuse but in a different form.

Time series interpolation in MODFLOW 6 can only be used with parameters.