I don't know the answer to this... but, I would suspect you could use a gravity flow calculator for a rough estimate. I Googled up this one... maybe it will get you close?
You should be able to do a search and find a calculator. It mostly depends on the amount of vertical drop vs the horizontal distance the water travels and the size of the pipe. The "steeper" the pipe is the more differential in pressure is created by the weight of the water.
I do not have a siphon system (bottom draw) and mine is just a surface draw pipe. But it does have several feet drop and when my NRCS guy designed the pond he did calculate its flow when it goes into siphon mode (inlet fully covered and all air expelled from pipe). It would flow lots more than just a horizontal pipe. And if it gets covered with water you can tell it is drawing water and exiting like it was under pressure.
Good luck with the calculations. An on line calculator should be out there. You will need to know the figures to plug in. Pipe diameter, vertical fall and pipe length or horizontal distance.
I used the calculator and guessed my pond at 40ft of 8" pipe with a 6' drop and came up with about 448 cu ft per minute times 7.5 gal. per cu ft equals 3360 gpm as an example.
I'm a civil engineer and I can tell that the Hazen Williams calculator applies to pipes flowing under pressure, and it grossly overstates gravity flow situations such as culverts and pond outlets.....the correct answer could be as little as 10% of what the calculator shows. Also, your question cannot be answered without a sketch of the pipe with pertinent dimensions and info as to allowable water depth at the inlet to the pipe, etc. Sorry to rain on the parade, but it's better to know the truth than to be making decisions with bad info.
Roundy, a lot of information is needed to estimate the flow of your siphon.
In "normal" mode, the pipe acts like any normal, gravity drain pipe, but when in siphon mode, the weight of the water, head pressure, pressurizes your pipe and flow increases dramatically.
Contrary to what was said about where the inlet is, the inlet location has no bearing on flow. Head pressure, (and pipe pressure) is determined by total height of the water level to the outlet, vertically.
With a 50 foot long pipe, and a 10 foot drop using smooth bore PVC and a minimal roughness coefficient, I'd conservatively estimate typical discharge flow around a minimum of 5000 GPM in "full siphon" mode, and about 500 GPM, maximum, in normal gravity drain mode....a 100 foot long pipe, but with 5 feet more of drop (15'), your flow would drop about 10% due to friction.
Here is what our eight inch syphon pipe looks like running full. The flow is able to add about a foot and a half to the little pond before it flow out its emergency outflow. Little pond is a half acre pond.
I have an 8" Sch 40 PVC pipe the NRCS calculated a "peak outflow" of 4.72 cfs (2,118 GPM) at an effective height of 19 feet and 113 foot long pipe. I assumed that is in siphon mode but not sure.
This reminds me I need to haul in more broken concrete to dump in the discharge area before it eats out a big hole this spring.