I have a 2 1/2 hp pump that pumps water from an inland lake into an irrigation system (with valve A open) and through a waterfall (when I close off the sprinkler ball valve A and open another ball valve, valve B).

Every so often the system apparently loses its prime. I have a couple of theories and wonder what is thought about this:

The waterfall part of the system is "open ended" in that the hoses open over the spill rocks slightly above the level of the pump and the water flows over the water course back into the lake. Could it be that the open pipes are allowing air back into the pump? I felt that if the pipes were full when the pump shuts and their heads are above the pump, the water would stabllize iin the system.

If the piping into the water fall is at or below the pump level, would this cause the loss of prime?

There could be a leak in the irrigation system, but I can close off that part of the system (closing valve A) and I still have the problem.

There could be a leak between the pump and before valves A and B.

There could be a leak in the intake line but this is for the most part under water with a well head in the lake. It would seem that it would just suck water into the line, not air.

Any thoughts or additional theories?

Thanks

Is there a footvalve or checkvalve on the line going to the lake?