In the fourth installment of our Green Your Drainage series, we’re focusing on ways to collect and reuse roof runoff with the help of rain barrels and cisterns.Unlike rain gardens and permeable paving, which are two methods for infiltrating water back into the ground, rain barrels and cisterns are about capturing water for (in most cases) non-potable reuse like irrigation. (Bonus: plants love rainwater because it does not contain the chlorine found in most municipal drinking water.)
Generally speaking, rain barrels are smaller and hold under 200 gallons of water. Cisterns are larger, holding between 200-1000 gallons and can be used for more substantial irrigation or as part of a gray water system (like toilet flushing).
Harvesting rainwater is a fairly straight forward process - downspouts are reconfigured to tie into a rain barrel or cistern through a screened inlet (to keep mosquitoes out). A drain spigot at the bottom is connected to a garden hose and an overflow outlet allows a place for excess water to go during heavy or prolonged rains. (For greater capacity, single rain barrels can also be linked together.)
Before installing a rainwater harvesting system there are a few things to consider, such as the size of your yard, type of roof and what your local rain patterns are like. Here in the Pacific Northwest, we have a rainy season and a dry season, so it doesn't usually make sense to collect rain for irrigation unless you have a series of barrels or a large cistern to hold the water until the dry summer months. Also take note of what kind of roof you have - wood shingles or shakes may have been treated with chemicals to make them resistant to rot and moss, while copper roofs and zinc anti-moss strips produce chemicals that you don't want in your garden. Even common asphalt shingles might leach various complex hydrocarbon compounds, so when in doubt, limit using roof runoff to non-edible plants.
(Image: 1.The Rain Well, 2. Sierra Club)