Ask Mr. Smarty Plants

We think you are already doing a bunch of the right things for xeric landscaping during our extreme heat and drought in Austin. The perforated PVC pipe you are considering is ordinarily used for "French drains" in which an area does not drain well because of a slope or a roof draining directly onto it. It is used horizontally, and usually buried in a fine gravel to permit that extra water to disseminate over a wider area, and not drown the roots of the plants. We think it would be more trouble and expense than it is worth to drive those pipes vertically into the ground. In the first place, as you pointed out, your soil is caliche-it doesn't take well to having anything drilled down into it, including an oil rig.  In the second place, we're concerned that there would be damage to already established roots as the pipe goes down, and root damage is the last thing you want. And don't forget, most roots, including those of very large trees, are in the upper 12 inches of soil. If you drive, say, a 36 inch pipe down there, the water is going to rush toward the bottom by the force of gravity, with probably very little coming out in the actual root area, and the rest going down to sit out of range of roots in the subsoil. 

As far as we can see, the bubblers and soaker hoses are probably the most efficient method you can use. Right now, a lot of people are asking about watering the roots of their large trees, and we have been suggesting that they run sprinklers on the root area out away from the trunk. On a large tree, this can be fairly far out, even beyond the dripline or shade line of the canopy. However, you do lose water to evaporation when you use sprinklers. While we recommend pushing a hose down into the soil around the roots of bushes, and letting the water dribble very slowly, that is really not practical with a large tree. You could use some combination of soaker hoses and sprinklers to cover a lot of the root area of trees and then, certainly mulch is always a good thing over extended roots. It helps in several ways: if the shade is deep, you're probably not going to have much luck growing anything under that tree, and mulch is attractive and protects the roots from heat as well as holding in moisture. Do not heap the mulch around the trunk, that will make it vulnerable to fungi and rots. 

And we really, truly do believe that it will rain again. The job now is keeping things alive until it does.