What is the surface called that shows the water level in a tank with a glass tube?

The term that describes the surface which indicates the water level in a tank, as seen through a glass tube, is known as the piezometric surface. This surface is significant in hydraulic and hydrologic studies because it represents the height to which water would rise in a piezometer, a type of tube that measures fluid pressure.

In this context, the piezometric surface can vary depending on the pressure conditions of the water within the system, and it's particularly important for understanding the behavior of groundwater and how water moves through different layers of sediment and rock. This concept is crucial for water operators because it helps them monitor and manage the water levels and pressures in various systems, ensuring they maintain proper water supply and pressure.

The other terms, while related to fluid dynamics, refer to different concepts. The hydraulic surface does not specifically reference pressure measurements in the same way as piezometric does, the manometric surface typically pertains more to readings from manometers used for fluid pressure measurements, and the pressure surface, although closely related, is not typically used to describe what is seen in a glass tube measuring water levels.