When we talk about headloss in pipelines (which is defined with Darcy-Weisbach or manning or hazen-Williams equations) we talk about friction between liquid and pipe wall and we do not talk about moving molecules of water on each other. Does not it (moving liquid surfaces on each other)result friction in fluid as headloss? This phenomenon can affect both on headloss in steady state condition and transient condition. Would any one explain about? Thanx in advance.