Richard's Rainwater has made a profitable business out of selling rainwater harvesting systems. A one-inch rain on 1000 square feet of collection surface will yield about 550 real world gallons of rainwater. How much collection surface (roof) do I need? Figure on at least 1500 square feet minimum for two people here in the Hill Country with our average rainfall of 32 inches a year. How much do I need to store? Allow about 5000 gallons storage capacity for every family member. This will get you through the worst of droughts. How much does it cost? A very, very ballpark figure is $10,000 for a 10,000-gallon whole-house system including storage tank, prefiltration, pump, secondary filtration and disinfection. You can even read the first chapter of their how-to manual, Rainwater Collection for the Mechanically Challenged, for free. http://rainwater.org via worldischanging.com