•    If the pressure head at different sections of the pipe are plotted to scale as vertical ordinate above the axis of the pipe and all these points are joined by a straight line, which is known as hydraulic gradient line.

•    It is obtained by the sum of datum head and pressure head.

H.G.L. = datum head + pressure head

H.G.L = z + ρ / γ

•    H.G.L. is not defined properly  at the entrance of pipe because at the entrance, there is sudden drop of pressure head due to the formation of vena-contracta. The H.G.L. drawn in this portion of the pipe is shown by dotted curved line.

•    H.G.L. meets the surface of liquid level in the exist tank or ends.

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