Inlet and outlet boundary condition of fluid flow

I am new to Comsol simulation.
I am trying to simulate fluid flow in porous material. The fluid gets accumulated in porous material. Only known volume of fluid is available at inlet reservoir like 5 cb.m . This liquid gets accumulated in another place where it can hold this much quantity of liquid and there should not be any deformation for the material. The flow is driven by pressure from first reservoir to the next.
How to give the inlet and outlet condition for the above problem? How to make simulation understand that only some quantity of liquid is available and the flow should stop when the first reservoir gets emptied?

Thanks in advance

Nidhin