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I have water reservoir at the top of my hill and I want to get free running water. I would like to go for 4 inch diameter pipe.

I have two options. 1. Dig a trench and lay the pipe from the bottom of the reservoir. This will ensure that water will flow any time due to gradient. But the problem is I have to dig a trench and in between there could be few huge boulders and it may become expensive to go for trench. 2. In this option, I can use siphon system. Cost of trenching will not be there and no risk of getting boulders while trenching. But if air gets inside, I need to flush out the air and I may need to pump water from other sources to the pipeline until the delivery pipe is filled with water.

My questions: a. Does the diameter of the pipe matters in Siphon system? Any upper limit for the diameter of the pipe (I want Practical limit and not theoretical value). I need to use 4 inch pipe size b. Is there any difference in water discharge rate between option 1 and 2 (assume that water pipe length variation between two options is around 15% (more in siphon system). Do I get same amount of water per hour in both the options?

Figure below shows the typical system I explained. enter image description here

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Large diameter pipes are fine as long as you can get the air out. The reduction in flow will be due only to the increased length, which doesn’t sound like a big deal in your case.

There are two ways the system could fail to work, both of which would be due to the pipe route going too far above the water level in the pond. Call that vertical distance from the surface of the pond up to the top of the hill H.

Failure mode #1: If you use too flimsy a pipe it will collapse under atmospherics pressure. A fire hose would have this failure for any H more than a few inches. Schedule 40 pvc is not likely to have this problem.

Failure mode #2: If H is too great (more than 30 feet is the rough figure in my head), then a vacuum will form inside the pipe at the top of the hill, breaking the water column and preventing siphoning.

I think your siphoning plan should work well if neither of those failure modes sound like a problem in your case. Hope that helps!

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