Jersey, in your example you are correct because that outlet end is not much lower that inlet level. This is due to the pressure of the volume of water is helping with the outflow. BUT, I will guarantee you that if you lower the outlet hole significantly, the flow will increase dramatically because thare will be much more water inside the inclosed system.
I my work, I have syphoned many ship gyro containers filled with liqued to do an annual overhaul and the higher I can get the container above the end of the syphon tube the quicker it empties.
I think we all are correct. the pressure of the water in the pond is relevent, also the height difference of inlet and outlet.