EGL represents total energy head \(= \dfrac{p}{\rho g} + z + \dfrac{V^2}{2g}\). HGL represents piezometric head \(= \dfrac{p}{\rho g} + z\). The difference is the velocity head: \(EGL - HGL = \dfrac{V^2}{2g}\). Both slope downward in flow direction (energy loss). EGL is always above HGL (except at stagnation). When HGL falls below pipe centerline, pressure becomes sub-atmospheric — risk of cavitation and column separation.