I do hope to dig swales to retain water. I'm curious if there are actual groundcovers that cover the whole ground surface that are useful and grow in such climates, or if using mulch is as good as it gets. (I had thought about perennial peanut and birdsfoot trefoil, but they may need more water than is available.)
The beans I use self-sow and come up when conditions are right for them suggesting that even annual plants can help moisture retention as well as provide a constant supply of organic material.
There are cactus and succulent varieties that you may be able to use too. Ice plants come to mind but some (like carpobotus) are considered invasive in some areas. Lampranthus, Aptenia and Oscularia are others.
Trailing acacia(a. redolens)?