One reason is to filter pollutants before they enter a natural body of water like the Fishkill Creek. Another reason is to filter water of pollutants before they reach a man-made storm sewer system. If run-off from a rain event reaches a creek or sewer before filtering through soil it is very likely the water run-off will be polluted and in turn pollute the body of water which it enters.
And still another reason might be to protect a drinking water resource. When water filters through the earth it becomes cleaner before entering the ground water. Ground water, also called an aquifer, is a source of drinking water. When run-off is channeled through the earth rather than entering streams and sewers directly, the aquifer fills up with cleaner water. Run-off that enters a storm system directly never has the chance to infiltrate into the ground, and will never add drinkable water to the aquifer. This is just one way wells go dry.
One online source Wikepedia states that a rain garden is "a planted depression that is designed to absorb rainwater runoff from impervious urban areas like roofs, driveways, walkways, and compacted lawn areas." The source continues to tell us that, "this reduces rain runoff by allowing stormwater to soak into the ground as opposed to flowing into storm drains and surface waters which causes erosion, water pollution, flooding, and diminished groundwater."
A book, Creating Rain Gardens, written by Cleo Woelfle-Erskine and Apryl Uncapher, tells us that "a rain garden is a living water treatment system" and after water filters through soil "the water will be clean and cool by the time it ends up in the river".
Link to Rain Gardens Across Maryland (52pg PDF)
UConn, Office of Communications: Rain Gardens in Connecticut (PDF)
UConn, iPhone Application (i-Tunes), useful for rain garden basics, design, plant selection and installation.
New York State Stormwater Management scroll down & review pages 5-71 to 5-79 for Rain Gardens & pages 5-98 to 5-103 for Rain Barrels (PDF)
Please email comments about any of the above discussion topics and if noteworthy they will be posted.
Rain barrels can be used to catch the water run-off from buldings. For an example, put one under a leader pipe of a house or shed to store water for later use. This way when a vegetable garden needs watering you will have some on hand. Check out the following websites for proper installation and more information. The following links are for educational purposes only.
ArlingtonEcho Outdoor Education Center
SpruceCreek Rain Saver