The discharge is given by the formula
Q = μA√2gh.
Q = discharge of syphon in cubic metres per sec.
μ = a coefficient of reduction of discharge which varies between wide limits.
A = the minimum cross-sectional area of the syphon in square metres.
g = value of acceleration due to gravity.
h = the fall, or the difference of level in metres between the water surfaces in the supply basin A and in the small tank e.
As soon as the supply basin surface falls, the opening of the aeration tube becomes uncovered and air is drawn into the syphon. But until the surface has fallen some centimetres the supply of air is not sufficient to cause the syphon action to stop completely, and thus the escape varies gradually from the maximum discharge to zero as the water surface falls a few centimetres till it reaches its original level.
In certain cases it is possible to do without the aeration tube, especially when the fall used in the syphon is not great and when it is possible to arrange matters so that the velocity of the water flowing past in front of the syphon is small.
The syphon with a width of 3 metres escapes 8 cubic metres per sec. of water.