How to set up source water volume as boundary condition

I have one question regarding how to set up the water source volume in petrasim/tough2.

For example, the reservoir is bottom water type. There is water driven at the bottom of the reservoir, meaning water will flow to the reservoir from the bottom to provide pressure support.  I want to set up the volume of driven water source  6 times as large as the water volume in the original reservoir.  How can I set up such boundary condition?   Thanks a lot for the advice!