### PRT-PRP

#### Structure of Blocks

_FOR EACH SIMULATION_

```
    BEGIN OPTIONS
      [BOUNDNAMES]
      [PRINT_INPUT]
      [EXIT_SOLVE_TOLERANCE <exit_solve_tolerance>]
      [LOCAL_Z]
      [EXTEND_TRACKING]
      [TRACK FILEOUT <trackfile>]
      [TRACKCSV FILEOUT <trackcsvfile>]
      [STOPTIME <stoptime>]
      [STOPTRAVELTIME <stoptraveltime>]
      [STOP_AT_WEAK_SINK]
      [ISTOPZONE <istopzone>]
      [DRAPE]
      [DRY_TRACKING_METHOD <dry_tracking_method>]
      [RELEASE_TIME_TOLERANCE <release_time_tolerance>]
      [RELEASE_TIME_FREQUENCY <release_time_frequency>]
    END OPTIONS
```
```
    BEGIN DIMENSIONS
      NRELEASEPTS <nreleasepts>
      NRELEASETIMES <nreleasetimes>
    END DIMENSIONS
```
```
    BEGIN PACKAGEDATA
      <irptno> <cellid> <xrpt> <yrpt> <zrpt> [<boundname>]
      <irptno> <cellid> <xrpt> <yrpt> <zrpt> [<boundname>]
      ...
    END PACKAGEDATA
```
```
    BEGIN RELEASETIMES
      <time>
      <time>
      ...
    END RELEASETIMES
```

_FOR ANY STRESS PERIOD_

```
    BEGIN PERIOD <iper>
      <releasesetting>
      <releasesetting>
      ...
    END PERIOD
```

#### Explanation of Variables

##### Block: OPTIONS

  * `BOUNDNAMES` keyword to indicate that boundary names may be provided with the list of particle release points.

  * `PRINT_INPUT` keyword to indicate that the list of all model stress package information will be written to the listing file immediately after it is read.

  * `exit_solve_tolerance` the convergence tolerance for iterative solution of particle exit location and time in the generalized Pollock's method.  A value of 0.00001 works well for many problems, but the value that strikes the best balance between accuracy and runtime is problem-dependent.

  * `LOCAL_Z` indicates that "zrpt" defines the local z coordinate of the release point within the cell, with value of 0 at the bottom and 1 at the top of the cell.  If the cell is partially saturated at release time, the top of the cell is considered to be the water table elevation (the head in the cell) rather than the top defined by the user.

  * `EXTEND_TRACKING` indicates that particles should be tracked beyond the end of the simulation's final time step (using that time step's flows) until particles terminate or reach a specified stop time.  By default, particles are terminated at the end of the simulation's final time step.

  * `TRACK` keyword to specify that record corresponds to a binary track output file.  Each PRP Package may have a distinct binary track output file.

  * `FILEOUT` keyword to specify that an output filename is expected next.

  * `trackfile` name of the binary output file to write tracking information.

  * `TRACKCSV` keyword to specify that record corresponds to a CSV track output file.  Each PRP Package may have a distinct CSV track output file.

  * `trackcsvfile` name of the comma-separated value (CSV) file to write tracking information.

  * `stoptime` real value defining the maximum simulation time to which particles in the package can be tracked.  Particles that have not terminated earlier due to another termination condition will terminate when simulation time STOPTIME is reached.  If the last stress period in the simulation consists of more than one time step, particles will not be tracked past the ending time of the last stress period, regardless of STOPTIME.  If the EXTEND_TRACKING option is enabled and the last stress period in the simulation is steady-state, the simulation ending time will not limit the time to which particles can be tracked, but STOPTIME and STOPTRAVELTIME will continue to apply.  If STOPTIME and STOPTRAVELTIME are both provided, particles will be stopped if either is reached.

  * `stoptraveltime` real value defining the maximum travel time over which particles in the model can be tracked.  Particles that have not terminated earlier due to another termination condition will terminate when their travel time reaches STOPTRAVELTIME.  If the last stress period in the simulation consists of more than one time step, particles will not be tracked past the ending time of the last stress period, regardless of STOPTRAVELTIME.  If the EXTEND_TRACKING option is enabled and the last stress period in the simulation is steady-state, the simulation ending time will not limit the time to which particles can be tracked, but STOPTIME and STOPTRAVELTIME will continue to apply.  If STOPTIME and STOPTRAVELTIME are both provided, particles will be stopped if either is reached.

  * `STOP_AT_WEAK_SINK` is a text keyword to indicate that a particle is to terminate when it enters a cell that is a weak sink.  By default, particles are allowed to pass though cells that are weak sinks.

  * `istopzone` integer value defining the stop zone number.  If cells have been assigned IZONE values in the GRIDDATA block, a particle terminates if it enters a cell whose IZONE value matches ISTOPZONE.  An ISTOPZONE value of zero indicates that there is no stop zone.  The default value is zero.

  * `DRAPE` is a text keyword to indicate that if a particle's release point is in a cell that happens to be inactive at release time, the particle is to be moved to the topmost active cell below it, if any. By default, a particle is not released into the simulation if its release point's cell is inactive at release time.

  * `dry_tracking_method` is a string indicating how particles should behave in dry-but-active cells (as can occur with the Newton formulation).  The value can be "DROP", "STOP", or "STAY".  The default is "DROP", which passes particles vertically and instantaneously to the water table. "STOP" causes particles to terminate. "STAY" causes particles to remain stationary but active.

  * `release_time_tolerance` real number indicating the tolerance within which to consider consecutive release times coincident. Coincident release times will be merged into a single release time. The default is epsilon x 10<sup>11</sup>, where epsilon is machine precision.

  * `release_time_frequency` real number indicating the time frequency at which to release particles. This option can be used to schedule releases at a regular interval for the duration of the simulation, starting at the simulation start time. The release schedule is the union of this option, the RELEASETIMES block, and PERIOD block RELEASESETTING selections. If none of these are provided, a single release time is configured at the beginning of the first time step of the simulation's first stress period.

##### Block: DIMENSIONS

  * `nreleasepts` is the number of particle release points.

  * `nreleasetimes` is the number of particle release times specified in the RELEASETIMES block. This is not necessarily the total number of release times; release times are the union of RELEASE_TIME_FREQUENCY, RELEASETIMES block, and PERIOD block RELEASESETTING selections.

##### Block: PACKAGEDATA

  * `irptno` integer value that defines the PRP release point number associated with the specified PACKAGEDATA data on the line. IRPTNO must be greater than zero and less than or equal to NRELEASEPTS.  The program will terminate with an error if information for a PRP release point number is specified more than once.

  * `cellid` is the cell identifier, and depends on the type of grid that is used for the simulation.  For a structured grid that uses the DIS input file, CELLID is the layer, row, and column.   For a grid that uses the DISV input file, CELLID is the layer and CELL2D number.  If the model uses the unstructured discretization (DISU) input file, CELLID is the node number for the cell.

  * `xrpt` real value that defines the x coordinate of the release point in model coordinates.  The (x, y, z) location specified for the release point must lie within the cell that is identified by the specified cellid.

  * `yrpt` real value that defines the y coordinate of the release point in model coordinates.  The (x, y, z) location specified for the release point must lie within the cell that is identified by the specified cellid.

  * `zrpt` real value that defines the z coordinate of the release point in model coordinates or, if the LOCAL_Z option is active, in local cell coordinates.  The (x, y, z) location specified for the release point must lie within the cell that is identified by the specified cellid.

  * `boundname` name of the particle release point. BOUNDNAME is an ASCII character variable that can contain as many as 40 characters. If BOUNDNAME contains spaces in it, then the entire name must be enclosed within single quotes.

##### Block: RELEASETIMES

  * `time` real value that defines the release time with respect to the simulation start time.

##### Block: PERIOD

  * `iper` integer value specifying the 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 applies only to that stress period.

  * `releasesetting` specifies time steps at which to release a particle. A particle is released at the beginning of each specified time step. For fine control over release timing, specify times explicitly using the RELEASETIMES block. If the beginning of a specified time step coincides with a release time specified in the RELEASETIMES block or configured via RELEASE_TIME_FREQUENCY, only one particle is released at that time. Coincidence is evaluated up to the tolerance specified in RELEASE_TIME_TOLERANCE, or epsilon x 10<sup>11</sup> by default, where epsilon is machine precision. If no release times are configured via this setting, the RELEASETIMES block, or the RELEASE_TIME_FREQUENCY option, a single release time is configured at the beginning of the first time step of the simulation's first stress period.

          ALL
          FIRST
          LAST
          FREQUENCY <frequency>
          STEPS <steps>
          [FRACTION <fraction>]
  * `ALL` keyword to indicate release at the start of all time steps in the period.

  * `FIRST` keyword to indicate release at the start of the first time step in the period. This keyword may be used in conjunction with other RELEASESETTING options.

  * `LAST` keyword to indicate release at the start of the last time step in the period. This keyword may be used in conjunction with other RELEASESETTING options.

  * `frequency` release at the specified time step frequency. This keyword may be used in conjunction with other RELEASESETTING options.

  * `steps` release at the start of each step specified in STEPS. This option may be used in conjunction with other RELEASESETTING options.

#### Example Input File
```
    BEGIN OPTIONS
      STOPTIME 10000.
      BOUNDNAMES
    END OPTIONS
    
    BEGIN DIMENSIONS
      NRELEASEPTS 3
    END DIMENSIONS
    
    BEGIN PACKAGEDATA
      1    1  3    1250. 10250. 400.  relpt_001
      2    1  3    1255. 10250. 390.  relpt_002
      3    1  23   1250.  9450. 400.  relpt_003
    END PACKAGEDATA
    
    BEGIN PERIOD 1
      FIRST
    END PERIOD
```