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The pressure defined as the pressure remaining once water has begun flowing is referred to as residual pressure. This pressure is significant in systems such as water distribution networks and firefighting, as it represents the amount of pressure available to maintain flow in the system after some demand has been placed on it.

When water starts to flow through pipes, friction and other factors reduce the total pressure in the system. The residual pressure indicates the pressure that is still present to ensure that water reaches its intended destination, like a hydrant or a nozzle. Understanding residual pressure is crucial for engineers because it impacts the ability to deliver water effectively, especially in emergency situations like firefighting where adequate flow and pressure are essential for effectiveness.

Static pressure, on the other hand, refers to the pressure in a system when no water is flowing, providing a benchmark to compare against. Flow pressure is the pressure in the system while the water is actively being moved, which often fluctuates due to changing demand and system dynamics. Normal operating pressure generally describes the typical pressure conditions of a system under standard conditions, not necessarily accounting for the effects of flow. Thus, for distinguishing the condition of pressure during flow, the concept of residual pressure is the most pertinent.