### GWF-STO

#### Structure of Blocks

_FOR EACH SIMULATION_

```
    BEGIN OPTIONS
      [SAVE_FLOWS]
      [STORAGECOEFFICIENT]
      [SS_CONFINED_ONLY]
      [TVS6 FILEIN <tvs6_filename>]
      [EXPORT_ARRAY_ASCII]
      [EXPORT_ARRAY_NETCDF]
    END OPTIONS
```
```
    BEGIN GRIDDATA
      ICONVERT [LAYERED] [NETCDF]
            <iconvert(nodes)> -- READARRAY
      SS [LAYERED] [NETCDF]
            <ss(nodes)> -- READARRAY
      SY [LAYERED] [NETCDF]
            <sy(nodes)> -- READARRAY
    END GRIDDATA
```

_FOR ANY STRESS PERIOD_

```
    BEGIN PERIOD <iper>
      [STEADY-STATE]
      [TRANSIENT]
    END PERIOD
```

#### Explanation of Variables

##### Block: OPTIONS

  * `SAVE_FLOWS` keyword to indicate that cell-by-cell flow terms will be written to the file specified with "BUDGET SAVE FILE" in Output Control.

  * `STORAGECOEFFICIENT` keyword to indicate that the SS array is read as storage coefficient rather than specific storage.

  * `SS_CONFINED_ONLY` keyword to indicate that compressible storage is only calculated for a convertible cell (ICONVERT>0) when the cell is under confined conditions (head greater than or equal to the top of the cell). This option has no effect on cells that are marked as being always confined (ICONVERT=0).  This option is identical to the approach used to calculate storage changes under confined conditions in MODFLOW-2005.

  * `TVS6` keyword to specify that record corresponds to a time-varying storage (TVS) file.  The behavior of TVS and a description of the input file is provided separately.

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

  * `tvs6_filename` defines a time-varying storage (TVS) input file.  Records in the TVS file can be used to change specific storage and specific yield properties at specified times or stress periods.

  * `EXPORT_ARRAY_ASCII` keyword that specifies input grid arrays, which already support the layered keyword, should be written to layered ascii output files.

  * <span style="color:red">`EXPORT_ARRAY_NETCDF` keyword that specifies input gridded arrays should be written to the model output NetCDF file with attributes that support using the generated file as a MODFLOW 6 simulation input.  This option only has an effect when an output model NetCDF file is configured and the simulation is run in VALIDATE mode, otherwise it is ignored.</span>

##### Block: GRIDDATA

  * `iconvert` is a flag for each cell that specifies whether or not a cell is convertible for the storage calculation. 0 indicates confined storage is used. >0 indicates confined storage is used when head is above cell top and a mixed formulation of unconfined and confined storage is used when head is below cell top.

  * `ss` is specific storage (or the storage coefficient if STORAGECOEFFICIENT is specified as an option). Specific storage values must be greater than or equal to 0. If the CSUB Package is included in the GWF model, specific storage must be zero for every cell.

  * `sy` is specific yield. Specific yield values must be greater than or equal to 0. Specific yield does not have to be specified if there are no convertible cells (ICONVERT=0 in every cell).

##### 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.

  * `STEADY-STATE` keyword to indicate that stress period IPER is steady-state. Steady-state conditions will apply until the TRANSIENT keyword is specified in a subsequent BEGIN PERIOD block. If the CSUB Package is included in the GWF model, only the first and last stress period can be steady-state.

  * `TRANSIENT` keyword to indicate that stress period IPER is transient. Transient conditions will apply until the STEADY-STATE keyword is specified in a subsequent BEGIN PERIOD block.

#### Example Input File
```
    BEGIN OPTIONS
      SAVE_FLOWS
    END OPTIONS
    
    BEGIN GRIDDATA
      #cell storage conversion 0:confined, 1:convertible
      ICONVERT
        constant 1
      #specific storage (for all model cells)
      SS
        constant 1.e-5
      #specific yield (specified by layer because of LAYERED keyword)
      SY LAYERED
        constant 0.2
        constant 0.15
        constant 0.15
    END GRIDDATA
    
    BEGIN PERIOD 1
      STEADY-STATE
    END PERIOD
    
    BEGIN PERIOD 2
      TRANSIENT
    END PERIOD
    
    #stress period 3 will be transient because
    #a BEGIN PERIOD block is not provided.
    
    BEGIN PERIOD 4
      STEADY-STATE
    END PERIOD
```