My guess is the next step after water barrels is turn on irrigation sprinklers or soaker hoses connected to water barrels. So when it is not raining can flow the stored water into your garden - that is where a distant limit is reasonable. Of course then you look at california which has world record 82mi canal!
That will most likely come when we have a plumbing system, as intended by red.
Though I doubt we can expect that soon.
Electricity and plumbing would lift rising world on a whole new level. But it's difficult to implement, I assume