Gilbert,
I can't tell you what amount of inches we add to the rain fall. I can tell you we water based on the amount of drying out of the soil, we have learned by observation. The average gardener in our area thinks they need to water every day, even multiple times a day, because our summer temperatures can reach 110+ during the summer.
What we have learned is that the trees and plants adapt to a schedule which works with the drying out (and retreating moisture) of the soil below the surface, not based on surface moisture.
We use extreme density of planting to canopy the soil not the plants, as all edibles need full sun (in most cases). The density minimizes evaporation and cools the soil surface. We know from experience that the soil surface temperature on a typical summer afternoon in our area is about 180 degrees (along with concrete and asphalt surfaces as well as the sides of containers). The planting density includes areas which are similar to rain forest with upper story and under story plantings with certain edibles (trees and bushes and even growing ginger in the shade of trees).
If starting your garden, thick layers of mulch can substitute and aid the canopy of the soil to help start and stabilize the plantings.
For starting a garden in the desert, reclamation concepts such as Pit Basin Gardening (Zai) and barrier water catchment (cordons pierreux) are helpful. What that means is using slopes to catch rain runoff. Use barriers to catch and hold the water at the seeded or transplant area and 'pits' to create a suitable zone in the soil to help the roots take.
If the desire is to totally minimize the use of adjunct watering, concepts such as wicking principles (like growboxx)
webpage would work (or homemade version of them).
I hope I am helping the understanding of what works for a desert garden.
Barbara Kingsolver in her outstanding book "Animal, Vegetable, Miracle" famously said they moved to Virgina because they could not grow enough food in the Tucson desert. I disagree with that (the only thing in her book I disagreed with). We can, many ancient people did and many more can in the future. But understanding the water and the soil is key.
