I am coming off my well hydrant with 3/4 inch 6 ply hose running downhill about 100 yards. I want to use an impact head sprinkler. Should I have enough pressure. Would it help to neck down the hose to 5/8 on the last section.

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Not enough details to answer your question. What PSI is the pump set at? Is this a single sprinkler head? What's the GPM of the sprinkler head? Back to you, Tom

Pressure is determined by the head, not the diameter... meaning the higher the source is above the outlet.. the higher the pressure will be (in addition to any line pressure at the source) assuming the outlet isn't unrestricted..

It pressure would be the same between two side by side pies.. on 1/2" and the other 6" as an example from a common source.

The limiting factor is the flow rate... that however is limited by the nozzle size of the sprinkler head as that is likely the greatest restriction..

However was mentioned above... there is more information needed for all parts of the answer.