Photo Credit: Elizabeth Redmond
I've been wanting to put in a couple of rain barrels at my house this year. We put in some garden plants this weekend, and they are going to need to be watered. Rain barrels are great because they help conserve water and cut down the amount of potable water that needs to be used. Rain barrels are commercially available for around $100 (or more). These are more "decorative" (if you find a piece of plastic molded with a wood barrel pattern decorative), but with a drill, some silicone sealant, and a couple of basic parts, you can build a rain barrel of your own.
It is important to remember that this is not drinking water that you are collecting. Without further treatment, there are too many possible problems, from dust and dirt to chemicals (from roof materials) to microorganisms that may colonize an available water supply. There are rainwater catchment systems that are designed for potable water use. These are more involved, and need to have other elements in the system beyond what is being discussed here.
It is also important to make sure to prevent the standing water from becoming a mosquito breeding facility, either by closing the barrel with a screen (like a window screen) or by using mosquito dunks (a time release tablet that contains a bacterial agent that kills mosquito larvae, but do not affect people, fish, animals or plants).
Rain barrels can collect a surprisingly large amount of water. "For every 1000 square feet of roof space being used to capture rain you can expect to catch around 600 gallons from one inch of rain fall (at a theoretical 100% catch rate). Some larger roofs can easily be 2000+ square feet." (The Sietch) Conversely, if you have a 100 square foot garden, you can figure that you will want to supply 60 gallons of water for every inch of rainfall you are trying to make up. So if you collect and use five 55-gallon barrels of rainwater, that's approximately 5" of additional effective rainfall that you've supplied to your garden. With a typical 55-gallon barrel size, you are only likely to capture a fraction of the total water that falls on your roof. But this could be increased by putting barrels at several corners, to capture the rainfall at multiple downspouts.