The amount of pressure remaining in a water distribution system when water is flowing is called:

The term that describes the amount of pressure remaining in a water distribution system while water is actively flowing is referred to as residual pressure. This concept is crucial in understanding how water supply systems function under various conditions, particularly during firefighting operations where water needs to be drawn from hydrants or other sources.

Residual pressure is measured at a specific point in the system while water is being drawn off, thus it reflects the pressure available that can actually be used for firefighting efforts. It accounts for the dynamic effects of flow, including friction losses and any other factors that may reduce the pressure when water is in motion.

In contrast, static pressure refers to the pressure in the system when no water is flowing, providing the maximum potential pressure available. Elevation pressure relates to changes in pressure due to differences in elevation within the water system, while flow pressure typically refers to the pressure at the outlet of a nozzle or device delivering the water during an active flow situation. Understanding these differences is essential for firefighters, as they must assess the residual pressure to ensure there is enough water supply for effective firefighting operations.