it is interesting to ponder where this transition occurs between a siphon that relies on cohesion and one that has only repulsion to a first approximation ignoring drag effects if you take a siphon that is operating in vacuum and start lowering it into the atmosphere the liquid in the tube above the lower reservoir will be in compression and without tension if its height above the surface of its reservoir is less than the barometric height at that ambient pressure the liquid above the barometric height will be in tension thus a siphon at low pressures or an extremely tall siphon could be operating with both the chain model and the atmospheric model at the same time but as the barometric height exceeds the crest of the siphon as is typical in practical siphons there would be no more tension and the chain model would no longer apply