Which factor causes a decrease in pressure as water moves through a pipe?

The decrease in pressure as water moves through a pipe is primarily attributed to friction loss. This phenomenon occurs due to the interaction between the water and the internal surfaces of the pipe. As water flows, it encounters resistance from the walls, which leads to energy being dissipated as heat and results in a gradual reduction in pressure along the length of the pipe.

Friction loss is affected by various factors, such as the pipe material, diameter, flow rate, and the length of the pipe. A smoother surface will generally result in less friction, while a longer and narrower pipe will contribute to a greater loss of pressure.

Understanding friction loss is crucial for fire safety and hydraulic calculations, as it plays a significant role in determining the effective pressure available at nozzle outlets during firefighting operations.