The computer program MODPATH was developed by the USGS (Pollock, 1998) to calculate three-dimensional particle tracking pathlines from steady-state and transient flow simulation output obtained using MODFLOW.
MODPATH uses a semi-analytical particle-tracking scheme. The method is based on the assumption that each directional velocity component varies linearly within a grid cell in its own co-ordinate direction. This assumption allows an analytical expression to be obtained describing the flow path within a grid cell. Given the initial position of a particle anywhere in a cell, the co-ordinates of any other point along its path line within the cell, and the time of travel between them, can be computed. Documentation of the theoretical and numerical implementation of MODPATH are given in Pollock (1998), while source code and input documentation are available at the USGS MODPATH webpage.
MODPATH is currently not available for MODFLOW-USG or MODFLOW-LGR models
At the Define Particles step, you can create new Forward and Backward Particles and view Forward or Backward particles that have been imported from a Visual MODFLOW model.
•[Assign >] Point. Using the mouse, left-click on the Layer view at the desired locations for the particles. Once finished, right-click and select "Define Attributes", or click on the Finish button from the toolbox, and the following window will appear.
•Select the Particle type from the radio buttons.
•Specify the Release Time. The particle release time is the time when the particle is released from the location where it is assigned and allowed to travel with the groundwater flow. This option is only appropriate for transient flow models where the groundwater flow directions are changing with time.
•Select the vertical location for the particles; particles will be assigned to the center of the current layer by default, but can also be assigned to multiple layers.
•Click [OK] when you are finished. A new node will appear on the model tree, under Particles. Forward and Reverse particles can be shown in the Flex viewer or in a stand-alone 3D Viewer
•[Assign >] Using Data Object.... and the the following window will appear.
•From the Data Explorer, select a Points data object that represents the particle locations. If you do not have a file containing points, then you can create and digitize new points using the drawing tools. Refer to Creating New Data Objects for more details
•Click the button to insert the data object into the input field.
•If you selected 2D points, then the particles will be assigned to the current layer, and you can also include additional layers. If you selected 3D points, then the Elevation value for each point will be read and this will determine which model layer the particles will be inserted into.
A circle of backward tracking particles is typically assigned around a pumping well in order to determine the capture zone for that well.
•First, select the model layer where you want the circle to be assigned.
•Select the option [Assign > Circle] from the toolbox
•Move your mouse cursor to the location where you want the center of the circle to be located (typically on top of a well cell, at the center of the cell)
•Left-click once to create the circle
•The "Define Particles Window" will appear
•Define the Radius for the Circle
•Define the # Particles to be inserted
•Define the Particle Type, Assign to Layer, and Release time, as previously explained.
•Click OK when you are finished.
•[Delete >] By Drawing a Box, allows you to delete just a set of particles. Note that this option will only delete the particle type that is active at that time (either Forward or Backward). Before proceeding with the Delete, be sure you have the desired particle type layer as active (and visible in the 2D Viewer). To do this, click on , and move the desired layer type (Forward or Backward particles) to the top of the layer list. Once complete, select Delete > Draw in a Box, and left click with your mouse to anchor the corner of a box around the selected particles you wish to delete, then draw the outline of the box, and release the mouse button to complete the box. The particles that lie within that box will be deleted, though only for the current model layer.
•[Delete >] Current Layer deletes all particles (Forward and Backward) in the current layer
•[Delete >] All Layers, deletes all particles in all layers
There are several settings for the MODPATH run, which can be define at the "Translate" step. For more details, see MODPATH