kevinb,

Putting the siphon outlet under water WILL NOT hurt the siphon action.

One of the reasons siphons fail is due to air entry at the outlet working back up to the crest of the siphon. (That is why they frequently show an upturned elbow at the outlet so the end of the pipe is always "full" of water.)

The 4' lower rule of thumb is due to the fact that water flowing through pipes always loses a little pressure due to friction.

It would be difficult to say exactly how much lower your outlet pipe needs to be in your exact scenario. However, if your pipe sizing estimate falls between two different pipe diameters, I would recommend selecting the larger diameter. The larger pipe will have smaller frictional pressure losses and should sustain a siphon for a longer period.

Finally, is the main purpose of your siphon to drain the pond, or to just regulate the water level while new water is entering your pond by drawing more stagnant water off of the bottom?

I speculate that your siphon would probably operate while the pond is full due to the depth of water over the inlet. At some point as the pond level gets lower, your siphon may fail before draining the pond down to the level of the inlet pipe.

(That is low value "internet" advice since we can't see the exact situation for your siphon.)

How high does your siphon have to go at the crest of the pipe?

Good luck on your pond project!

P.S. A 1/8th acre pond is pretty small. Depending on your purposes for draining, you might be better off running in a 3" drain pipe and purchasing a small trash pump?