Potentiometric surface

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A potentiometric surface is based on hydraulic principles. For an example, we know that two connected storage tanks with one full and one empty will gradually fill/drain to the same level. This is because of atmospheric pressure and gravity. A potentiometric surface is the imaginary line where a given reservoir of fluid will "equalize out to" if allowed to flow. This idea is heavily used in city water supplies - a tall tower containing the water supply has a great enough potentiometric surface to provide flowing water at a decent pressure to the houses it supplies.

Without being too complicated, potentiometric surfaces explain how phenomena like Artesian wells occur. In geology, when a tilted formation receives water (from rainfall or otherwise), water enters the formation and flows downward. If the bottom of this formation is exposed to the air, this water will flow up and out of the formation in an artesian well because of the force of the water behind it that is also flowing downhill.