It is very hard to be explained in just a few words.
Generally speaking, you can set up such like Seepage BCs at the bottom of the pit, meanwhile, combining with transient inactive elements to represent each pit stage.
If necessary, modulation functions could be used to control the activation of BCs in mine dewatering.
The flow rate budget through those BCs could be recorded in budget history charting during the simulation.
We have written a relevant training course to demonstrate all the processes involved. If you would like to know more details, please contact mike.de@dhigroup.com