Outlet flow measurement of a mount-top drinking water treatment plant

Dear friends, A mount-top water treatment plant has DN700 steel outlet pipe with EMF(electro-magnetic flow meter). Due to the gravity flow, this DN700 outlet pipe is not full of water sometimes. And then, the EMF is not available to measure the outlet water in good accuracy. In order for EMF to improve measurement accuracy, we want to re-install or renewal this DN700 steel outlet pipe. What are your advices to measure the water flow in gravity pipe?  Thank you a lot in advance.   Best regards,   Wu Hao