Swales are a passive water harvesting trench on contour, meaning they are perfectly level. Swales are a TREE GROWING SYSTEM. They are meant to grow trees and they are not meant for growing annual vegetables in. You can put annuals into the swale berms initially if you choose to but long term swales are meant to grow trees. Swales are the first step to creating a food forest or forest garden. Swales catch water and hold it passively where it soaks into the ground instead of running off of the land quickly. (see video above)
Swales will work in any environment including deserts. Slopes that are more than twenty degrees are not suitable for swales. Swales can be installed relatively fast and inexpensively. They can be laid out quickly with an A Frame level, a water level, or a transit.