We have a couple of Seametrics mag flow meters.   http://www.seametrics.com/product/ag2000-magnetic-flow-meter   With a 6 inch meter, an increase of 5-8 gpm means that you have to go look for the drip hose that is cut or has come loose.  This is with pressure compensated, anti-siphon Jain drip hose.  The combination really is that accurate. They have to be installed right, that means with the digital display at a 45 degree angle. That keeps entrained air from causing an error.

I should have got the digital output option, they are not field convertible.  Then I could then monitor the flow remotely.