Being unsure of exactly what you want, you might install a recording transducer in your collector to record the water levels with time. The vented transducer does not need to be corrected for atmospheric pressure, and detailed water level data can be recorded with time, and downloaded easily into computer. You might run various tests such as turn off distribution system to check that collector is not leaking. You might run some efficiency tests to check if distribution line(s) were broken or plugged by checking water level responses and water level head in collector. A recording raingauge might help determine rate and efficiency of collection. Depending on collector and water level head, you might want to install a pressure regulator if the pressure is too high for system.