A lot of water gushes down your gutter pipes during a rain storm. A typical suburban roof has about 1,000 square feet of rain-catching surface that could be put to good use with the strategic placement of a barrel or two. If a meteor strikes the nation's capital and we're cast into anarchy, it's a good bet your city water will stop flowing after the food riots subside. Assuming it rains where you live, you could provide drinking water for a small family with a couple of good rain barrels.