Just as a way of being a curiosity, harvesting rainwater isn't always a given, check your local jurisdiction. Certain states, and until recently Washington was one of them, would require a specific water right on your property, and a check of your deed will probably not find that, at least in water-rich Washington state. This recently changed: the Dept of Ecology put out a memo that said you don't need a water right to harvest rainwater from the roof of your home - according to one source, the average roof could harvest 53,000 gallons per year, and the memo they put out clarified that home owners can actually harvest that. It makes sense, and in their defense I don't think anyone at Ecology wanted to argue otherwise.

It all goes back to the politics of water, to times and places where who has water rights also had the power, and owned the politicians, and the cattle, wheat, etc etc. Still a big deal in some parts.