FWIW, the inlet depth of a siphon only effects the maximum draw down point. Since water seeks the lowest point to flow to, when you remove air from the siphon tube, you essentially set a lower water level point at wherever the outlet is. You need about a 4' drop on the outlet below the pond level to ensure you overcome water tension and friction to maintain a positive discharge PSI and good water flow.
It is actually atmospheric pressure that makes the water flow rather than a vacuum. The siphon system used on ponds is a Pascal's Siphon and atmospheric pressure causes the flow. The maximum lift for a water siphon is just over 34', or atmospheric pressure.
Flow (speed) will drop some as the pond level drops, because net head pressure drops. A pond "appears" to drain faster as it lowers only because there is far less water volume per vertical foot than at full pool due to the sloped sides of the pond, like in a V.
Last edited by Rainman; 03/31/15 01:59 AM.
