The MODPATH program is used for calculating the advective flow pathlines for forward tracking and backward tracking particles. MODPATH has run-time settings for Discharge Options which are used to tell MODPATH what to do with particles when they enter a grid cell where water is leaving the system. Any grid cell where water is leaving the system is classified as a “sink”. For example, an extraction well is a sink, or all cells in layer 1 with evapotranspiration are sinks.
In many cases the water leaving the system through a grid cell is less than the amount of water entering the grid cell. If this difference is small, the cell is classified as a “weak sink”. Since MODPATH cannot always determine if a particle should be removed from the system when it encounters a weak sink, there are three options to control how particles should be treated when sinks are encountered.
•Particles always pass through cells with weak sinks
•Particles are always stopped when they enter cells with internal sinks
•Particles are stopped in the cells where discharge to sinks is greater than a specified total inflow to the cell [default = 5%].
The desired Weak Sink Option may be selected from the Discharge Options window shown in the following figure.
The Recharge Options are used to define how MODPATH treats the Recharge flow entering the system. The Recharge Options are:
•Recharge flux is treated as internal sources and sinks for all cells
•Recharge flux is assigned to the top face of all cells
The first option treats recharge as a distributed source entering the cell from all sides, while the second option treats recharge as though it is entering only through the top face of the cell.
According to the MODPATH reference manual, the distributed source approximation for areal recharge is usually only appropriate for two-dimensional areal flow models.
The Evapotranspiration Options are the same as described for Recharge above.
Please Note: as of Visual MODFLOW Flex version 6.0, the reference time settings have been moved to the Define Particles step so that particle release times can be defined by group.