Swales are an important method of collecting surface run-off water and holding it so it soak into the soil. The soil itself is the easiest, cheapest and must effective place to store water. While some water needs to be stored in tanks for drinking or washing, most of our water requirements are to grow trees and plants, and they get their water from the soil, so we may as well store it there in the first place.
In order to capture and hold the surface run-off, swales must be constructed exactly on contour, ie level. It is important that the top of the bund wall created by the excavation is exactly level. If one part is lower than the rest, then in an extreme rainfall, the concentrated water that has been collected will commence to overflow at that point, and possibly start eroding the whole system. Even causing problems down slope.
Immediately upon constructing the swale, it is important to plant the swale with suitable plants. The bund wall can be planted to herbs and grasses which can survive drier conditions, while just below the wall is the place to plant trees which will benefit from the moisture being absorbed into the soil. At the bottom of the swale it is possible to plant species which can take occasional flooding, eg Taro. These plantings will make good use of the available moisture, and also protect the newly constructed swale form any erosion potential.
On steep slopes, shallow, small swales (only 40cms deep) can be dug by hand at a spacing of about every 10m. On more gentle slopes, the swales can be much larger and can even be built by large machines (graders or bulldozers even!). These swales can be much further apart, up to 50m. in very flat dry conditions. As a general criteria, if your swale system overflows in a normal heavy rainfall then, you need to build them deeper, or have more of them. If the sale system never overflows, even in the worst possible rainfall, then you have already overdone it a bit!
Especially on steeper slopes, swales tend to accumulate leaf litter and organic material in them. Every few years it is worthwhile scrapping this material out of the swale and using it as mulch for the nearby trees.
Many arid areas have very fragile soils` and are infrequently subject to severe rainfall. In these circumstances, think carefully before using swales, they may create a major erosion problem if not constructed very carefully and very well.
In areas with very shallow and rocky soils, it is often more appropriate to build swales by piling the rocks into low walls along the contour. These will collect silt and organic material and act as swales, and will minimize the work required and possible erosion hazards.