The following are listed by increasing complexity with regard to implementation.
One: Replace the grass with mulch and/or clover. Another thread suggested subterranean clover for its drought resistance. I would throw some of the clover seed on the ground and cover it with a couple inches of cypress mulch. Better ideas are appreciated and invited.
Two: Establish a polyculture of comfrey and other deep-rooted plants to help water get lower faster. Hopefully these plants would not consume too much water, as the lack thereof is the problem. Even just a couple of comfrey plants, located strategically between and among the trees, should be a vast improvement over the existing arrangement.
Three: Stake some straw logs (or other erosion control barrier) on contour across the narrow, steep lawn. This will keep rain and irrigation water from running straight down the lawn into the street. I wouldn't dig any basins since this would probably damage the stressed trees' roots. I would, however suggest terracing the lawn once or twice to check runoff if terracing could be accomplished without disturbing tree roots.
Four: Harvest rainwater off that large roof and send greywater to the trees.
A few suggestions that I have found useful:
1) Never used them but check out this link for Deep Tree Watering Stakes (http://www.sprinklerwarehouse.com/Tree-Shrub-Watering-s/9070.htm). The same thing can be accomplished by digging a hole near the drip line of the tree. Say 4 to 5 ft in depth with a post hole digger and bar. In the hole bury a 4" PVC with small holes drilled in it (small because you don't want dirt particles coming into your pipe). Septic PVC is cheaper than the other stuff. You can even back fill your hole with river gravel to create a sort of reservoir. I would put several of these around the tree depending on how big it is. Fill with water. You can also add compost teas, fertilizers, etc... This will allow water to get down deep where it needs to be with minimal budget.
2) Add compost under tree then mulch. I would use cedar mulch versus cypress. Cypress mulch is very unsustainable due to poor harvesting ethics. Cedar mulch is readily available in the Hill Country plus it has many beneficial properties for keeping away weeds, grass, and some insects. The compost will bring the soil back to life.
3) I agree with terracing the land since it is on a slope. Must catch whatever water falls. And the roof catchment/greywater usage is also a must for the long term sustainability.
My personal opinion is get the tree/s help as fast as possible. Water is our #1 limiting factor. Save the trees! Good luck.
Barton Parson wrote:Thanks fellas. Copy the cedar mulch. The hoserkulture is promising in that it doesn't harm the roots. I'm not sure a drought-stressed tree would survive having an augur poke holes through its roots. Maybe so, but I wouldn't risk it without lots of reassurance.
Drill holes or dig them with a post hole digger by hand near the drip line of the tree. There should be any major roots out that deep, just fibrous. Those will recover quickly once you start to squirt water, fish emulsion, kelp, etc... into the hole. Good luck and I hope that you can save the trees!