### GWF-CHDG

#### Structure of Blocks

_FOR EACH SIMULATION_

```
    BEGIN OPTIONS
      READARRAYGRID
      [AUXILIARY <auxiliary>]
      [AUXMULTNAME <auxmultname>]
      [PRINT_INPUT]
      [PRINT_FLOWS]
      [SAVE_FLOWS]
      [OBS6 FILEIN <obs6_filename>]
      [EXPORT_ARRAY_NETCDF]
    END OPTIONS
```
```
    BEGIN DIMENSIONS
      [MAXBOUND <maxbound>]
    END DIMENSIONS
```

_FOR ANY STRESS PERIOD_

```
    BEGIN PERIOD <iper>
      HEAD [LAYERED] [NETCDF]
            <head(nodes)> -- READARRAY
      [AUX [LAYERED] [NETCDF]
            <aux(nodes)> -- READARRAY]
    END PERIOD
```

#### Explanation of Variables

##### Block: OPTIONS

  * `READARRAYGRID` indicates that array-based grid input will be used for the constant head package.  This keyword must be specified to use array-based grid input.  When READARRAYGRID is specified, values must be provided for every cell within a model grid, even those cells that have an IDOMAIN value less than one.  Values assigned to cells with IDOMAIN values less than one are not used and have no effect on simulation results. No data cells should contain the value DNODATA (3.0E+30).

  * `auxiliary` defines an array of one or more auxiliary variable names.  There is no limit on the number of auxiliary variables that can be provided on this line; however, lists of information provided in subsequent blocks must have a column of data for each auxiliary variable name defined here.   The number of auxiliary variables detected on this line determines the value for naux.  Comments cannot be provided anywhere on this line as they will be interpreted as auxiliary variable names.  Auxiliary variables may not be used by the package, but they will be available for use by other parts of the program.  The program will terminate with an error if auxiliary variables are specified on more than one line in the options block.

  * `auxmultname` name of auxiliary variable to be used as multiplier of CHD head value.

  * `PRINT_INPUT` keyword to indicate that the list of constant-head information will be written to the listing file immediately after it is read.

  * `PRINT_FLOWS` keyword to indicate that the list of constant-head flow rates will be printed to the listing file for every stress period time step in which "BUDGET PRINT" is specified in Output Control.  If there is no Output Control option and "PRINT_FLOWS" is specified, then flow rates are printed for the last time step of each stress period.

  * `SAVE_FLOWS` keyword to indicate that constant-head flow terms will be written to the file specified with "BUDGET FILEOUT" in Output Control.

  * `OBS6` keyword to specify that record corresponds to an observations file.

  * `FILEIN` keyword to specify that an input filename is expected next.

  * `obs6_filename` name of input file to define observations for the constant-head package. See the "Observation utility" section for instructions for preparing observation input files. Tables ref{table:gwf-obstypetable} and ref{table:gwt-obstypetable} lists observation type(s) supported by the constant-head package.

  * <span style="color:red">`EXPORT_ARRAY_NETCDF` keyword that specifies input griddata arrays should be written to the model output NetCDF file.</span>

##### Block: DIMENSIONS

  * `maxbound` integer value specifying the maximum number of constant-head cells that will be specified for use during any stress period.

##### Block: PERIOD

  * `iper` integer value specifying the starting stress period number for which the data specified in the PERIOD block apply.  IPER must be less than or equal to NPER in the TDIS Package and greater than zero.  The IPER value assigned to a stress period block must be greater than the IPER value assigned for the previous PERIOD block.  The information specified in the PERIOD block will continue to apply for all subsequent stress periods, unless the program encounters another PERIOD block.

  * `head` is the head at the boundary.

  * `aux` is an array of values for auxiliary variable aux(iaux), where iaux is a value from 1 to naux, and aux(iaux) must be listed as part of the auxiliary variables.  A separate array can be specified for each auxiliary variable. If the value specified here for the auxiliary variable is the same as auxmultname, then the head array will be multiplied by this array.