In this video from The Permaculture Research Institute, Geoff Lawton explains the simple yet elegant concept that is the swale. Formally defined by Bill Mollison as "long, level excavations, which can vary greatly in width and treatment from small ridges in gardens, rock-piles across slope, or deliberately-excavated hollows in flatlands and low-slope landscapes," the actual function of the swale is so simple and effective that it's a wonder we don't see more of them dotting the landscape.When partnered with trees, swales are an excellent tool for recharging groundwater, reducing soil runoff, capturing water for agriculture and creating rich, green landscapes in hilly areas. Appropriate to arid and humid areas, steep slopes and flatlands, urban and rural areas, swales hold water for several days, until it is gradually absorbed into the soil.
For a visual explanation about how to build a swale, check out this site. For more in-depth info, check out the writings of Bill Mollison and David Holmgren.