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One-third of all residential water use in the United States is devoted to irrigation, according to the US Environmental Protection Agency (EPA); that adds up to an average of more than 8 billion gallons of water each day for home lawn and landscape irrigation (and that's an awful lot). While not having a lawn or garden is probably the greenest way to go, when it comes to water use, at least, eschewing them in favor of xeriscaping or a rock garden may not match your "green" aesthetic. If that's the case, we recommend getting some (or all) of the water you need for outside watering by harvesting it from your roof.
But before you can build a rain barrel or decide to add a downspout bog garden, we recommend crunching a few numbers to see how much water you can expect to harvest. First, you'll have to figure out how much water you'll require. This varies with location, weather, season and application, of course, but generally, you can figure 125 gallons for 1000 square feet of lawn, and 118 gallons for an hour of average garden watering. Now, when it comes to harvesting, 1,000 square feet of roof will give you 600 gallons for every inch of rainfall. Since each state has different climates and precipitation patterns, check out these average precipitation maps to see what you can expect in your neck of the woods; going by our numbers, if you have the same area on your roof as you do in the yard, an inch of rainfall will net you a little more than four good waterings.
Many locales have restrictions about how you use the water you harvest -- it isn't safe for drinking without treatment, and often can't be used indoors without a permit -- but using your roof's rainwater harvest is a great way to keep your garden growing (or your lawn green) without having to pull it from your local municipal source. So if you don't happen to know the square footage on your roof, or your lawn, grab a tape measure and a calculator and take the first step toward harvesting rain from your roof.
Difficulty level: Moderate
More about rainwater harvesting at Planet Green and TreeHugger:
TreeHugger at Rainwater Harvesting Workshop
Rainwater Harvesting, Personalized-
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