The loss of mechanical energy per unit mass of liquid during the movement of water is called loss of head. There are two reasons for the loss of head. The external resistance to water flow is the main external cause of head loss. The viscosity of liquid is the main internal cause and the root cause of head loss.
During the flow of the liquid, the flow resistance generated in the flow direction, the roughness of the wall surface, the shape and the area of the flow cross section, which are constant, is called the resistance along the path, or frictional resistance. The effect of resistance along the path causes loss of energy or loss of head during fluid flow. The resistance along the path is evenly distributed over the entire uniform flow section, which is proportional to the length of the pipe section and is generally indicated by 
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Another type of resistance occurs in the watershed where the flow boundary is rapidly changing. The loss of energy is mainly concentrated in the basin and nearby watersheds. The energy loss or resistance that occurs in this concentration is called local resistance or local loss, caused by local resistance. Head loss is referred to as local head loss. Usually, local head loss occurs in the inlet and outlet of the pipeline, the cross-section pipeline, and the joint of the pipeline, which is generally indicated by 
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