It was nice to see this as a feature in the project. Rain Water Harvesting is fairly simple and can be accomplished with either purchased barrels, or with barrels built from recycled bulk foods drums. A simple search on Rain Water Harvesting will turn up several forums and resource pages. It would be an excellent segment for "Ask This Old House." I believe this was also addressed on a Carribean house project several years ago where the roof was designed to collect and channel the water.
Many people are unaware how many thousands of gallons of water fall on their roof each year. A 1500 sq ft roof which receives 35 inches of average rainfall is a source of almost 30,000 gallons annually. Harvesting rainwater decreases the amount of water that a municipality has to treat both as potable water and as potential waste water at the sewer plant. It would have been nice if the show had discussed some real numbers in terms of this project. Is there a rain barrel located at each downspout? What is the size of the cistern? Were there any issues of water diversion away from the wetland.* Does the system pay for itself over time with the water it recovers? The owners had a sizable garden, and a 1500 gallon cistern could provide them with 50 gallons a day during a 30 day summer drought. It is also possibly to link barrels for additional storage.
* While many communities encourage residents to limit storm water runoff and to utilize rainfall on their property, it must also be recognized that water which enters a stream in one state may very well be the water that another person in another state downstream depends on for their potable supply, or for irrigation. Where you live might be under some sort of restriction based on the ultimate destination of that rainwater.