What physical principle allows water to flow from a higher elevation to a lower elevation in a connected tank system?

The principle that allows water to flow from a higher elevation to a lower elevation in a connected tank system is gravity. Gravity is the force that pulls objects toward the center of the Earth, and in the case of water, this force causes it to move downward when there is a difference in elevation. When one tank is at a higher level than another, gravity acts on the water, creating a pressure difference that drives the flow from the higher tank to the lower one.

This process is fundamental to many natural and engineered systems. For example, in hydrology, water flows in rivers and streams downhill due to the pull of gravity. Similarly, in plumbing systems, water moves through pipes from elevated tanks or sources to locations lower in elevation by the same principle.

While hydraulic pressure is related to the movement of fluids and can affect how water flows, it is ultimately gravity that initiates and sustains the flow in this context. Capillary action and surface tension involve different properties of water, such as the attraction between water molecules and the surfaces they interact with, and do not play a primary role in the gravitational flow between tanks.