Establish a small plastic pipe for irrigation - design the project

I have established a plastic pipe 15.8 mm in order to transfer water four times a day from a tank to an agricultural field. The system is gravity fed but there are some points of the pipe that negative pressure is developed. I cannot change the route of the pipe. I cannot change the diameter too.I cannot also use a pump.

I would like help:
- in order to determine the problematic sections (exact location) of the pipe and
- install there air release valves or other equipment (if applicable) in order the water to flow successfully.

I am attaching the route of the pipe.