Question
I have close to the same question as someone else you answered.
Pump is 7.5 hp going out to 15 sprinkler heads. We got it going yesterday fine, sucking up water from a pond. There is a valve between the motor and the line going out to the sprinkler heads. The motor is centrigal water cooled so we fill the line with buckets of water before starting the motor through a hole in the pipe that then has a top that goes on it. If you put your hand on top of that hole and lift the hose that is running into the pond you can feel air  suck up. We filled the hole opening with about 30 buckets of water this morning and turned on the pump but no water at all seemed be being sucked up. Fearful we'd burn out the motor we turned off the motor after a minute or so. All lines and pipe fittings have the tephlon tape before being screwed together and all joints and fittings were screwed together tight with a pipe wrench.

Please email back any ideas. We are $20,000.00 into this and no joy with no sprinkler action today, though it worked like a charm last night. Plenty of water in the pond. All brand new system.

Answer
At the very end of your suction line, there should be a check valve (called a foot valve) that keeps water in the line when the pump is not on.  This keeps the pump primed.  You didnt mention that you had one of these, and I have never seen a pond sysem work without one.