A swale is a ditch used in permaculture to collect water and reduce top soil erosion. Collecting water is great way to water your garden instead relying on city water. It will also help to reduce the water bill. A swale is designed to water the trees at the root instead of only from above. The swale is on contour or at level meaning that the bottom of the swale should not be higher that the other opposite end. You should also not compact or step on the soil that is leading to the trees that you will be watering from the swale. You want the soil to be able to absorb the water that is being collected. However, you will need a level sill for when the swale fills up.