The Reference Time is used by MODPATH as the time datum for the simulation of both forward and backward tracking particles. This option is only applicable for transient flow simulations.
The Release Time(s) specified for the particles, (see "Particle Release Time") will be added to the Reference Time to determine the actual time of release for each particle.
There are two Time format options for setting the Reference time:
•Prd/Stp/Rel.Tm.: Specify the Reference Time by entering the Stress Period (Prd), Time Step (Stp), and Relative time inside step (Rel.Tm).
•Absolute value: Specify the reference time as an Absolute time from the beginning of the simulation.
Note: For backward particle tracking in transient simulations, the Reference Time cannot not be the beginning of the simulation, because there are no simulation results before Time = 0 (i.e., you cannot go backwards from zero). Therefore, when simulating backward particle tracking, the Reference Time should be a time somewhere between the start and the end times of the transient simulation.