Residual pressure is defined as:

Residual pressure refers to the pressure that remains in a water distribution system when water is actively flowing through it. This measurement is crucial for firefighters and water supply systems because it indicates the availability of water and the effectiveness of the pressure during operation, such as when fighting a fire.

When firefighters open a hydrant or a nozzle, water flows, and the pressure drops from the static pressure (the pressure in the system when no water is flowing). The residual pressure helps assess how much pressure is available for effective firefighting and ensures that sufficient water supply can be maintained during operation. This indicates the state of the system under real operational conditions rather than ideal no-flow situations.

In context, the other options describe related concepts but do not accurately capture the definition of residual pressure: pressure with no water flowing would refer to static pressure, the amount of water available is more aligned with flow rates, and the pressure at static sources again pertains to the pressure with no demand. Therefore, understanding residual pressure is vital for effective firefighting operations.