Swale refers to a water harvesting ditch on contour popularized as a rainwater harvesting and soil conservation strategy by Bill Mollison, Geoff Lawton, Jack Spirko and other advocates of permaculture.
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 just about all 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.
Swales for Food Forests
In permaculture swales are first and foremost tree growing systems and are therefore are often used to establish food forests or other agroforestry systems. In general to do so a swale is installed on the highest point where practical in a landscape and a forest system is planted in a strip along the swale berm. The forest is over time extended downgrade using edge effect to increased the forests size and production. In the case of a food forest the majority of plantings are either edible or support species.
It should be noted though that the above is but one way to establish a forest system on swale, often multiple swales are installed all at once each further downgrade then the next. Then all swales are planted out. The area between the swales known as the inner swale may either be planted to forest as well or cropped or maintained as pasture, depending on the goals of the designer.
Swales for Pond and Dam Management
Swales can also be used to drastically increase catchment for damns and ponds. By swaling along a contour you can move water that normally would not run off into a pond horizontally to fill the pond. When the pond reaches capacity, it will then back fill the swale. Swales of any size also generally have a level sill spillway. When the swale reaches capacity this spill way allows the swale to overflow. When this element is included with a dam/pond, you can actually place the that body of water's spillway far away from it. This means a large damn may actually overflow a spill way hundreds or even thousands of yards away from itself. In doing so water can be moved passively across large areas and multiple damns/ponds maintained on a single contour.
Geoff Lawton explaining swales for water harvesting.
The swales at Jack Spirko's homestead.