MODPATH (Pollock, 1994, 2012) is a separate program that uses the output of MODFLOW to perform particle tracking. The pane for this program is on the MODFLOW Packages and Programs dialog box under Post processors.
The user specifies starting locations for one or more particles and MODPATH will track the movement of those particles due to advection. MODPATH does not take dispersion or retardation into account. Particles can be tracked either forward or backward. One use of MODPATH is to estimate the recharge area of a well. To do this, the user can start particles in every cell and note which ones end up at the well or start a large number of particles at the well and track them backwards in time to their starting locations.
The user can specify a large number of options that affect how MODPATH operates. In version 5 of MODPATH, some of these are specified in the main MODPATH input files whereas others can be specified by the user either interactively or in a "response file." ModelMuse only supports the "response file" method. However, if desired, the input files generated by ModelMuse can be used in an interactive session with MODPATH. In version 6 of MODPATH, all the options are specified in input files.
Response file options | Simulation file options
ModelMuse supports MODPATH versions 5 and 6. Use these radio buttons to select which version you wish to use.
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MODPATH can produce three different types of output files.
•End point files record the initial and final locations of the particles
•The pathline file contains the starting coordinates of a particle and the coordinates at every point where a particle enters a new cell or a confining layer. In addition, coordinates of intermediate points are recorded whenever the cumulative travel time corresponds to (1) a point in time for which a data point was requested by the user, or (2) the end of a MODFLOWtime step in a transient simulation.
•The locations of all particles are computed for a specified point in time and recorded in the time series file. The procedure is repeated at each point in time for which output is requested. Time series mode produces a series of particle locations in the time series file that are stacked in time. When points in the time series file are plotted, the effect is to follow the progress of a group of particles as a series of snapshots in time. |
When particles are tracked into a cell from which evaporation occurs with the EVT package, the evapotranspiration can be treated either as in internal sink or as occurring at the top of the cell. IEVTTP is used to choose which option applies. (There is no provision in MODPATH version 5 for determining how evapotranspiration in the ETS, UZF, RES, SFR, LAK, MNW1, MNW2, or STR packages will be treated.)
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When particles are tracked into a cell to which recharge is applied with the RCH package, the recharge can be treated either as in internal sink or as occurring at the top of the cell. IRCHTP is used to choose which option applies.
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If this checkbox is checked, the user can specify a zone in which the particles will stop. The data set "Modpath_Zone" is used to define the zones. The zone in which the particles stop must have a number of 2 or higher.
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If this checkbox is checked, the user can specify a zone in which the particles will stop. The data set "Modpath_Zone" is used to define the zones.
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The user can choose to record all particles or only those that enter the zone designated above. (This option is only present in MODPATH version 5.)
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If the user chooses to have particles stop when they enter a zone, this is the zone in which they will stop. The data set "MODPATH_Zone" is used to define the zones. The zone in which the particles stop must have a number of 2 or higher.
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Weak sinks are cells in which some water leaves through a boundary condition such as a general-head boundary and some water leaves through one of the cell faces. The user must decide how particles that enter such cells should be treated. The particles can either (1) pass through the cell, (2) stop in the cell or (3) stop in cells in which the fraction of the flow leaving through the boundary condition is above a threshold. Option 3 is only available in MODPATH version 5.
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When the fraction of flow leaving through boundary conditions at a cell exceeds this threshold, MODPATH will stop any particles entering the cell. (This option is only present in MODPATH version 5.)
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In transient MODFLOW simulations, the user can specify a reference time. This need not be at the beginning of the simulation. Tracking time is defined to be 0 at the specified reference value of simulation time. Tracking time measures the accumulated time during a particle-tracking analysis. The value of tracking time is always positive, regardless of whether particles are tracked forward or backward.
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The user can choose to have MODPATH track particles either forward or backward in time. When using backwards tracking the you typically set the reference time to the end of the simulation and set the release time to zero.
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Version 5 options
If this checkbox is checked, MODPATH will check the water budget in all cells. Any cells in which the water budget fractional error exceeds the Error tolerance will be listed in the MODPATH listing file.
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If this checkbox is checked, the final locations of the particles will be printed in the MODPATH listing file.
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If MODPATH detects that a composite budget file larger than MAXSIZE will be created, it will prompt the user to either quit or to go ahead and create the large composite budget file. If this checkbox is checked, MODPATH will create the large budget file.
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This option allows MODPATH to terminate when a specified time has been reached. For steady-state models, the maximum time is specified. For transient models, the tracking time is specified.
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COMPACT indicates that MODPATH should generate endpoint, pathline, and time series files as text files using the global node number to indicate cell location. If COMPACT is omitted, the cell location is designated using the row-column-layer grid indices (as in previous versions of MODPATH).
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BINARY indicates that the endpoint, pathline, and time series files will be generated by MODPATH in binary form. This keyword also is required by MODPATH-PLOT in order to correctly read binary versions of these files.
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When checking the water budget in cells, MODPATH will check if the fractional error in each cell is greater than the Error tolerance. If so, the cell will be listed in the MODPATH listing file.
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When backwards particle tracking is used, only a single release time can be specified.
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When the maximum time or maximum tracking time is reached, MODPATH will terminate.
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BeginPeriod and BeginStep are the stress period and time step of the first time step in the block of time steps that will be processed. ModelMuse calculates BeginPeriod and BeginStep from the Beginning time. If any stress period is a steady-state stress period, MODPATH requires that BeginPeriod and BeginStep both be 1 so Beginning time is not specified.
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EndPeriod and EndStep are the stress period and time step of the last time step in the block of time steps that will be processed. ModelMuse calculates EndPeriod and EndStep from the Ending time.
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MAXSIZ-- is the maximum allowed size (in bytes) of the Composite Budget File.
If MAXSIZ = 0, the program uses a default value that is set in the MODPATH main program. The default value can be changed by resetting the MAXSIZ value in the MODPATH main program and then recompiling the source code. If a transient MODPATH run will generate a composite budget file larger than MAXSIZ, a warning message is issued and an option is provided to either quit or proceed. If the Allow unlimited budget file size check box is checked, MODPATH will proceed.
The default value of MAXSIZ is 15,000,000 bytes.
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Version 6 options
WeakSourceOption indicates whether particles pass through or stop in cells that contain with sources.
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StopOption indicates how the particle-tracking simulation should be terminated.
1 = For forward tracking simulations, stop at the end of the MODFLOW simulation. For backward tracking simulations, stop at the beginning of the MODFLOW simulation.
2 = Extend the initial or final steady-state MODFLOW time step as far as necessary to track all particles through to their termination points.
For forward tracking simulations, this option would have an effect whenever the final MODFLOW stress period is steady-state. For backward tracking simulations, this option would have an effect whenever the first MODFLOW stress period is steady-state. If all MODFLOW stress periods are transient, option 2 produces the same result as option 1.
3 = Specify a value of tracking time at which to stop the particle-tracking computation.
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StopTime is a specified value of tracking time at which to stop the particle-tracking computation. StopTime is always greater than or equal to 0.
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BudgetOutputOption indicates the budget checking option.
1 = No budget checking
2 = A summary of cell-by-cell budgets is printed in the listing file.
3 = A list of cells is specified for which detailed budget information is summarized in the listing file.
4 = Trace mode is in effect.
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TraceID identifies the particle for which trace data will be generated when BudgetOutputOption = 4 (trace mode).
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RetardationOption indicates if a retardation factor array will be read.
1 = Retardation factors are not read or used in the velocity calculations.
2 = An array of retardation factors is read and used in the velocity calculations.
If used, the retardation factors should be greater than or equal to 1. The velocities calculated by MODPATH will be equal to the unretarded velocities divided by the retardation factor. The retardation factors are specified in the data set "Modpath_Retardation."
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AdvectiveObservationsOption indicates if advective observations are computed and saved as output.
1 = Advective observations are not computed or saved.
2 = Advective observations are computed and saved for all time points.
3 = Advective observations are computed and saved only for the final time point.
Advective observations can only be used when the SimulationType is time series.
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When particles are tracked into a cell which interacts with a ETS, UZF, MNW2, Reservoir, SFR, or Lake boundary, the flow can be treated either as in internal sink or as occurring at the top of the cell. These combo boxes are used to choose which option applies.
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Output times
When MODPATH is creating pathline or time-series files, the user can choose to have the locations of all the particles reported at certain designated times. Either of two methods can be used. The user can choose to have uniformly spaced times or the user can choose to list the times individually.
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When the user chooses to use uniformly spaced times for output, this is the spacing.
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When the user chooses to use uniformly spaced times for output, this is the maximum number of times at which output will be generated.
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When the user chooses to specify times for output individually, this table is used to list the times. The Number of times can be changed to increase or decrease the number of rows in the table or the Add  , Insert  , or Delete  buttons below the table can be used to insert new values or remove existing ones. The user can copy a column of numbers from a spreadsheet and paste it into the table and the number of rows in the table will increase to accommodate the number of rows of data on the clipboard.
The output times are specified relative to the reference time. (For steady-state simulations, the reference time is the beginning of the model. For transient simulations, it is the user-specified reference time.)
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