Also, what does this excerpt from Wikipedia mean
However, since with open channel flow, the water surface is open to the atmosphere, the pressure term between two points has the same value and is therefore ignored.
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Because head pressure is the weight of the water standing on top of the region being measured. Even though you can visualize an open channel as a slope you will not see a difference in pressure if you measure one foot deep at any point on the slope; this is because you are only measuring the weight of the water above the measuring point.