Well, what you described involves some fair amount of changes to the firmware, which is pretty difficult to do without any programming experience. If I were to do it, it’s probably easier to directly hook up the distance sensor to OS3.0 (which has some spare GPIO pins), such that it can read the sensor directly. If you use a separate Arduino, then OS3.0 needs to communicate with that Arduino in some way and it can complicate things. Then, if you are going completely sensor driven (sounds like what you want is to use the sensor to detect how much water is consumed and you want to pump a certain amount of water to each zone), this will be quite a departure from the time-based scheduling algorithm that the controller uses by default. So this is going to involve some amount of changes to the scheduler to switch from time-based algorithm to sensor-based algorithm. You should definitely hire a programmer if you want to get this done.