Tarik,
What you show is a closed system with no inflow or outflow.
If you got a proper piece of paer instead of sketching dsomething on your phone and provided some details as to flow or describe what is happening you might get some better answers.
And answer the questions from it smoked - it's all about level of the water in the tanks.
Volume is irrelevant in this instance.
But as drawn it will be about 0.1 barg in static condition.
If flow out of the lower tank >= flow in then the pressure might drop to atmospheric.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.