If you are not ready to install a green roof yet, you can achieve some of the same benefits with a low-cost solution: a rain barrel, which captures rainwater that would otherwise run off of your roof.
One of the main benefits is to reduce the amount of stormwater runoff. Typically rain water flows off roofs, driveways, parking lots and other "impervious surfaces" into storm drains, which discharge either into community sewer systems or into nearby streams. In the first case, rain over-burdens sewers, leading to overflows that can contaminate public swimming beaches. In the latter case, rushing stormwater can erode stream banks, introduce pollutants and ruin habitat for fish and other aquatic life.
A second benefit is for your wallet. You can use water captured in rain barrels to irrigate your lawn and garden, saving on utility bills. (In water-stressed regions, or during droughts, this water-conservation technique may be a necessity, given the imperative to conserve water.)
The drier the climate, the more water gets dumped outdoors, according to research by Aquacraft. In the desert Southwest, as much as 60% of home water usage occurs outdoors, whereas the figure is as low as 20% in wetter regions, like the Northeast. In summer, not surprisingly, the percentage of water used outdoors spikes, as we water lawns and gardens.
As Environmental Work Group recently pointed out, if we use tap water to irrigate, we're essentially wasting our own tax dollars, which have gone toward treating that liquid to drinking water standards.
Commercial rain barrels can be attractive, and can be built to prevent mosquito breeding and make hooking up gardening hoses easy. But you can also make your own rain barrel. The Environmental Protection Agency offers a how-to pdf.