Please have a look on the attachment. In Fig 1 the water is stationary and has an equal level all along. As they say that water seeks its own level and the reason for this is the fluid pressure due to gravity. In Fig 2 the water is flowing from left to right. The height of column water in the tubes vary which represents pressure differential because of varying speeds. Is the height of water in the tubes affected by whether the tubes are exposed to atmospheric pressure or have vacuum? I believe that it does matter. The atmospheric pressure finds its way everywhere even in underground water reservoirs. The flowing water in the pipe is already affected by atmospheric pressure. Therefore, the pressure at point C is only due to the weight of column of water and the atmospheric pressure gets cancelled with each other. Please see Fig 2. The water in a column is raised to almost 10 m height by atmospheric pressure therefore when the tubes have a vacuum, their height should be enough to compensate for 10 m height plus additional increase in height due to the pressure of flowing water. Thank you.