By catchment I meant cistern, its the same thing. Everyone has been harvesting rain for thousands of years for household use, it may be time to revisit US water rights law and think them through again. We haven't always had thousands of gallons flowing out of our taps. If average rain fall is a half or a quarter of what it was when the law was developed to protect the watershed then hydrologists and engineers - and politicians - have to rethink priorities. People are shocked when they don't have legal rights to harvest rainwater coming from their own roof, but it goes back to gravity and elevation and principles like the farmer downstream or downhill needs water too. If your cistern is collecting what will otherwise turn into storm water, you have to consider downstream effects. In the PNW with 38 inches of rain per year mostly that's endangered salmon runs, for now. If we hit 22 per year avg rivers and lakes subside and we have salinization issues too - and no more fish. It can be a big deal for someone to lift out thousands of gallons a year, much less a city of 70,000 doing the same thing. Better learn from our agricultural areas and mete out water in newly arid areas before it becomes a bigger thing.