Available flow from hydrants is estimated by determining the percent drop between the static and which type of pressure?

The correct choice is the residual pressures, as they provide crucial information for estimating the available flow from hydrants. Residual pressure refers to the pressure that remains in the system when water is flowing. When assessing the flow from a hydrant, firefighters typically measure the static pressure (the pressure in the hydrant with no water flowing) and the residual pressure (the pressure while water is being drawn from the hydrant).

By calculating the difference—or the percentage drop—between the static and residual pressures, operators can estimate how much water is available for firefighting efforts. This is vital for determining if the hydrant can provide sufficient flow to meet the needs of the operation.

Dynamic pressures are related to the flow of water, but they measure pressure during operation rather than reflecting the remaining capacity once flow has started. Atmospheric and absolute pressures do not apply to this scenario as they do not provide direct insight into flow capabilities from a fire hydrant. Therefore, the concept of residual pressure is essential for understanding how much water can be delivered from the hydrant during firefighting operations.