Grass roots are the key to having a lawn that will survive the stresses of summer drought. Our warm season grasses have deep roots. This helps them survive the heat of the summer, when other grasses either need massive amounts of water or just die off completely.
The grass roots are the powerhouse of a lawn. On warm season grasses they will go down many feet, as opposed to cold season (UK type grasses) that only go down a few inches. The more roots you have underground the better the chances of the grass surviving drought.
If you keep the roots happy then you will have a good lawn. What's above the ground reflects what is below the ground.
See soil testing for more information.