Head loss in a fixed plumbing system follows the pump affinity equations where head loss is proportional to the square of the flow rate. This is why any plumbing system can be fit to a plumbing curve that follows this equation:

Head Loss (ft) = Cp * GPM ^2

Where Cp is the plumbing curve constant. Typical 2" plumbing has a curve constant that is close to 0.0082 while 1.5" plumbing is closer to 0.0167.

This is useful when trying to determine the operating point of a new pump on an existing plumbing system. You just have to draw the plumbing curve over the pump's head curve and where they intersect is the new operating point.