Well, yes, watering plants is what I meant by "irrigation" (if you have a separately metered service for that, which some people with sprinkler systems, at least in some jurisdictions, do)... and you're technically correct except that the trouble is that sewerage is not metered (i.e.- at least for residences there is no such device as a sewage meter, just a water meter). Because of this, it is assumed, with the possible exception of the two cases I cited above, that you do discharge as much sewage as the amount of water you consume. Is this fair? No, not really quite... but that's the way it is.