Dry Farming is a term used to describe growing food crops without modern irrigation, that is, using only or mostly the rain that falls from the sky in a given area. It was the typcial way of farming for most of the history of agriculture.
When considering buying trees with the intention of dry farming them, make sure that you choose trees that do not demand a lot of water. You can use resources like CA Rare Fruit Tree Growers local members and website http://www.crfg.org/ and The California Backyard Orchard http://homeorchard.ucdavis.edu/The_Big_Picture/Tree_Selection/ to help select appropriate varieties. If you want to be more thorough, consult http://www.water.ca.gov/wateruseefficiency/docs/wucols00.pdf, A Guide to Estimating Irrigation Needs of Landscape Plantings in CA. But remember, this resource does not consider the option of dry farming when making these estimates.
It's also important to pay attention to the tree's root stock and select varieties that are cultivated for our area.
One helpful technique for successful dry farming of fruit trees is keeping mulch on the ground around the trees. Organic mulch is recommended. Keep the mulch at least 6 inches away from the tree so that the base of the trunk and the root crown are exposed.
When training young fruit trees to be dry farmed, they do need to be watered during the establishment period, the first three years. Yet the way they are watered will help insure that they are successfully dry farmed or not. It's important to hold off on any watering after the rainy season until the trees show signs of slight stress (not extreme stress). This allows their roots to stretch and reach for possible water, and to develop strong and far reaching root systems. These root systems will be one advantage the trained trees will have to succeed in a dry farmed environment. Deep infrequent watering also promotes healthy root systems.
Deciding to stop watering trees that have gotten used to receiving shallow and frequent irrigation will generally not be a recipe for successful dry farming and is not recommended.
Read more about Dry Farming in this article about Conserving Water in the Garden by local landscape designer Joan Bolton.