I think the answer to this is either a 'no' or a 'not very well' based on the research I've done so far but I thought I'd throw it out before I put together the test system to see if anyone else has done it.

My large pond and my small pond are only separated by the levee of the small pond. Difference in surface height is around 3 feet higher on the small pond.

I want to drain the small pond and do a reset...restocking plan has been discussed on previous posts.

The issue is that the small pond (just under 1 acre, fairly deep (average of 12 feet) has a lot of water in it. It doesn't get a ton of runoff so if I can get it down to nothing I'll be golden. By my calculations it'd take over a week with a 6" pump - that's an expensive job.

But the issue is, I think the back pressure of the large pond's water level would either prevent or seriously slow down the venturi effect of the siphon The outlet of the pipe would be something around 8 feet under the surface of the big pond.

Anyone every tried this?

Dale:

What is the gph of that 6" pump? A 3" pump moving 14,000 gph would take 11.6 days.

This pump would take 2.7 days running 24/7.
6" 60,000 gph trash pump @ Northern Tool

If the water level in the pond is only 3' higher than the other pond, you would only be able to drop the water level 3' before the levels equalized and the siphon stopped working. (If I'm understanding your ponds correctly)

Esshup....yup you're understanding right...they are 3 feet different. And since I posted this I found some details on why what I want to do won't work.

I'm leaning towards starting a couple of siphons and leave them run until I'm down as low as I can, then using a pump.

The height of inlet and outlet are irrelevant....the water levels will seek the same height. To drain the upper pond lower, you would need to drain the lower pond at the same time. As long as the siphon tube from the upper to lower never gets air in it, the siphon will continue as the lower pond level drops.

gracias rainman...I'm going to go out and start one this evening now that I have open water, just to get the process started.

I have seen water siphoned up hill it is not easy to start in that situation but it works. The one I saw like this worked by having the intake of the siphon(which was the lowest elevation) as close to the high point in the system as possible and the discharge(higher than the intake by several feet) had a really long run from the high point so the volume of the water in pipe was dramatically more than the volume of the water on the intake side. To prime the system both ends must have valves and you must have a way to pump water at the high point until the pipes are full. Then close off the inlet at the high point and open both inlet and discharge valves at the same time. The larger volume of water trying to fall the shorter distance in the discharge side will out weigh the small volume trying to fall farther on the intake side and it will flow.

This system was used for flooding feilds for waterfowl hunting and the water source it was sucking from was the river on the other side of a levy.