Which term best describes the pressure in a water main when it is not in use?

The term that best describes the pressure in a water main when it is not in use is static pressure. Static pressure refers to the pressure exerted by a fluid at rest, meaning it is not in motion or being drawn through a system, such as during periods when water outlets are closed. This pressure remains constant and reflects the potential energy within the system due to the elevation and weight of the water in the main.

Dynamic pressure, on the other hand, pertains to the pressure that occurs when water is flowing through the system. Residual pressure is specifically the pressure that remains in a water distribution system during operation, often measured at a fire hydrant when water is flowing. Operational pressure typically refers to the pressure that is maintained during use, factoring in the demands placed on the system. Thus, static pressure is the appropriate term for the condition of the water main when it is not actively being used.