The hydraulic grade line lies below the energy grade line by an amount equal to the velocity head at the section. The two lines are parallel for all sections of equal cross sectional area. The distance between the pipe centerline and the hydraulic grade line is the pressure head, or piezometric height, at the section. The line showing the pressure head, or piezometric height, at any point in a pipe. The slope of the hydraulic grade line is known as the hydraulic gradient. The hydraulic gradient is the slope of the water surface in an open channel.