What is the best indication of available water while water is flowing from a hydrant?

In the context of fire hydraulics, residual pressure is the most effective indication of available water while it is flowing from a hydrant. When water is taken from a hydrant, especially in emergency situations, firefighters assess residual pressure to determine how much water is still available to support firefighting efforts.

Residual pressure reflects the pressure in the system after water is flowing. It indicates what remains in the system after a demand has been placed on it and gives insights into the capabilities of the water supply. If the residual pressure is adequate, it suggests that there is enough water flowing and that the supply is sufficient to support firefighting operations. Conversely, if the residual pressure drops too low, it may indicate that the supply is insufficient, which could compromise firefighting efforts.

Static pressure, total pressure, and dynamic pressure do not provide a complete picture of the water availability during flow. Static pressure is the pressure in the system when no water is flowing, which does not account for actual water movement. Total pressure includes both static and dynamic components, but again does not reflect the current capacity for flow while water is actively being drawn from the system. Dynamic pressure represents the pressure during flow but does not show what is left in reserve, which is where residual pressure is critical.