Keeping inlet mass flux the same as outlet mass flux in an water tank
 

Hi,
I am simulating a flow of water outside a tank. The flow is very similar to a rectangular weir with contractions. The setup is such that I am continuously pumping back the extracted water into the tank.
I am using the flowRateInletVelocity boundary condition at the inlet for velocity.
My problem is that I do not know how much inlet flow rate should I specify. Since the tank is emptying, my water level and thus my mass flow rate at outlet keeps changing. Is it possible to calculate mass flow rate at the outlet and then assign it to the inlet during the simulation using OpenFOAM. I am using interFoam for two phase simulation.

Domain: A simple tank with an opening and a collection chamber on the other side to collect water similar to rectangular weir with contractions.

TLDR: I want to maintain a constant water level in my tank so that the mass flow rate at my inlet = mass flow rate at my outlet :)