Do I need a backflow device installed on a water supply line that only fills the pool? I have a valve on it and the pipe terminates above the pool.

The answer is not always a simple yes or no. In your particular circumstance, it appears as though you have a sufficient air gap from preventing any backflow. Regardless of whether or not a ball valve is on your water supply and is generally turned off, a valve is not a backflow preventer. Please contact your city inspector to verify your conditions. The city should have a plumbing inspector who is a licensed plumber. Have them make the final judgement call for you. Thank you for contacting us!