A friend is worried that he'll lose his really old oak trees soon due to persistent drought. They're growing on a steeply sloped lawn right next to a large house. He expressed frustration with getting water to the appropriate depth. I have a little bit of book learnin (larnin?) but very little practical experience. Please check out my recommendations and/or jump right in.
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.
