Most of whatever is now dry and unpopulated, including in the US is so because moisture is being pumped out of there by the atmosphere and sun. Not because someone forgot to drive some water there.
It is being moved out of there in such amounts that no economically possible diversion of water flow can overcome this. Cf the crazy and eventually buried USSR project of diverting north-flowing rivers water to the Kazakhstan from 80s.
That could not work even in theory, where the talk was about thousands of cubic meter per second and you suggest that some toilets can make this work. That is just not possible. Maybe if the climate actually changes, some of those lands will become good enough for agricultural use. But not now.
It is being moved out of there in such amounts that no economically possible diversion of water flow can overcome this. Cf the crazy and eventually buried USSR project of diverting north-flowing rivers water to the Kazakhstan from 80s.
That could not work even in theory, where the talk was about thousands of cubic meter per second and you suggest that some toilets can make this work. That is just not possible. Maybe if the climate actually changes, some of those lands will become good enough for agricultural use. But not now.