One of the methods used to control the flow of water into water cisterns and tanks is the use of ball (aka float) valves. Put simply, a float on the water inside a cistern or tank is attached to an arm which operates a valve which controls the inflow of water – as the water level in the tank drops, the arm falls and opens the valve to allow water to enter the tank; then as the water flows in, the water level rises, as does the ball thus moving the arm which then closes the valve shutting off the water flow.