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A swale is a low tract of land, especially one that is moist or marshy. The term can refer to a natural landscape feature or a human-created one. Artificial swales are often designed to manage water runoff, filter pollutants, and increase rainwater infiltration.
The swale concept has also been popularized as a rainwater harvesting and soil conservation strategy by Bill Mollison, Geoff Lawton and other advocates of permaculture. In this context it usually refers to a water harvesting ditch on contour. Another term used is contour bund.
Swales as used in permaculture are designed to slow and capture runoff by spreading it horizontally across the landscape (along an elevation contour line), facilitating runoff infiltration into the soil. This type of swale is created by digging a ditch on contour and piling the dirt on the downhill side of the ditch to create a berm. In arid climates, vegetation (existing or planted) along the swale can benefit from the concentration of runoff. Trees and shrubs along the swale can provide shade which decreases water evaporation.
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