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 infiltration basins 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, however they increase transpiration, so their net effect on the hydrologic cycle is probably to reduce infiltration.
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- Merriam-Webster. "Swale." Online Dictionary. Retrieved 2009-09-21.
- U.S. Environmental Protection Agency (EPA). Washington, DC (1999). "Storm Water Technology Fact Sheet: Vegetated Swales." EPA Document No. 832-F-99-006. September 1999. Archived July 22, 2009, at the Wayback Machine.
- "Water Harvesting: Microcatchment Contour Bunds". Food and Agriculture Organization. Retrieved 2009-11-26.
- "Soil contour bunds" (PDF). United Nations Office for Project Services. 1998. Retrieved 2009-11-26.
- Michigan State University Extension."Wetlands of the Great Lakes Open Shoreline and Embayed Wetlands." Retrieved 2009-09-21.