Some of our garden vegetables, once established, can withstand going without much precipitation because they have long roots that are able to seek out moisture. Carrots, parsnips, salsify, various endive, parsley, strawberry spinach, garden sorrel, etc. all seem to do really well in this situation. Many other plants can also survive and produce on very little water, with a little help. The trick, in our garden, is to plant these vegetables as deeply as possible and space them far enough apart so they do not compete with each other for water. Tomatoes and peppers are perfect for deep planting as they will root along the buried parts of their stems.
I have been playing around with the idea of growing plants with no water for a few years now. One of the best candidates for a certain experiment I have been working on has been a cherry tomato plant. No water, as in no water from me after the initial planting. A small cherry tomato plant with a good root system was chosen as it would seem to be closer to its wild South American counterparts and having a smaller fruit would, in theory, need less moisture. Each year I have saved the largest seeds off these plants and productivity seems to be increasing.
In this particular trial a hole around 3' deep is dug in an area of dry hardpacked earth. It is then filled with a bucket of raw kitchen scraps whose moisture the plants roots will reach in about a 1 1/2 months when water is most needed in our area.
A half foot of rich composted soil is then added. The plant is placed into the hole, surrounded with more rich soil, damp leaves or grass are also added to help hold in the moisture. At this point the plant receives the only water I will give it other then what, if any, mother nature provides for it.
More dirt covers the leaves and as the plant grows I continue to add leaves and dirt three or four more times during the first month until the plant has about a foot of this dirt covered mulch above the soil line. The moisture will be held in, the plants roots are deep and as they grow will eventually have access to the moist rotting kitchen waste.
Productivity is less then if it was watered regularly but our plants have grown tall and healthy producing exceptionally sweet cherry tomatoes. It is a little more effort to plant this way, but I do know that if faced with a water shortage I would still be able to grow tasty tomatoes.
I am also considering trying some small water wicking beds for certain crops on the outskirts of my garden. There is a lot of information on this creative subject at http://scarecrowsgarden.blogspot.com/search?q=water+wicking+beds.
One of the best books that I have read on the subject of growing food with little moisture and many other helpful topics is Steve Solomon's book "gardening when it counts." I highly recommend it to everyone.