What happens to static pressure once water starts flowing from a hydrant?

When water starts flowing from a hydrant, static pressure is transformed into residual pressure. Static pressure refers to the pressure in the system when there is no flow, while residual pressure is what remains in the system when demand (flow) is created. As water is drawn from the hydrant, the demand causes a drop in pressure because the water is in motion, moving away from the source. The flow of water creates a situation where the pressure is no longer static; instead, it reflects the reduced availability of pressure at that point in the system, hence becoming the residual pressure. Understanding this relationship is crucial for fire service operations, particularly when calculating water flow and ensuring sufficient pressure for firefighting efforts.