I have 3 x 500 gallon tanks that collect rainwater, all at level ground. I want to use them to drive a sprinkler system. Given the tank locations, I will be running a pipe from each tank to a manifold that will then feed a pump.

I calculated that when full, each tank will produce 4 psi of water pressure at the bottom of the tank. Assuming all 3 tanks are full, what would the pressure be at the manifold? I am thinking it would be 4 psi but I just wanted to make sure.

I want to monitor the water pressure at the manifold so that when it falls below a setable level, I can switch from the tanks to city water automatically.

Thanks