A swale is an intervention designed to retain water run-off on a slope. It is a long, small ditch on the contour lines, i.e. at the same elevation. Several swales can be constructed next to each other to increase their effect. Swales have a small berm on the downslope side, which often contains vegetation and acts as a retaining wall.

Swales are cost-effective and relatively easy to implement, preventing soil erosion, nutrient run-off, and flooding. They capture water which runs down the hill and let it infiltrate into the soil. Growing vegetation on the berm will strengthen the berm and the soil and will hold water and nutrients better. Trees, in particular, work well for strengthening berms and are watered by the water held by the swales. If possible, it is also an option to add stones to a berm to make it even stronger. In the Perma Atlas project, swales have been a great success in preventing flooding. Swales are similar to contour bunds and Fanya Chini and they can be used on a small or large scale. The greatest reduction observed when swales are introduced are in water runoff, soil loss, phosphorus runoff and nitrate-nitrogen runoff.