Ask Mr. Smarty Plants

There is no question that even old, well-established trees are going to need some extra help during the kind of drought we are having right now. Many trees have been saved in past droughts by additional  watering in late summer. As the underlined article suggests, professional arborists suggest deep watering, and the amount of water applied would depend on your soil type, e.g., rocky or clay.  The trees might also benefit from additional fertilization.  Furthermore, rees suffering from drought stress are often more susceptible to insect and fungal attack.

This University of Illinois Extension website Dealing with the Drought affirms that watering even drought-resistant trees like the oak is essential. It points out that a 90 ft. oak tree will be a lot worse to lose than annual flowers or vegetables. We don't recommend watering at the base of the trunk, but watering farther out, with a hose or sprinklers, probably about every two weeks. An excerpt from this website will tell you why:

"Our 90-foot oak example could have roots at least 40 feet beyond the tree’s
drip-line. The same goes for most shrubs. So, apply water at the tree or
shrub’s drip-line, not at the trunk."

Those roots may be pretty deep, and part of the protection of the trees, but much of the root system is in the top 12 inches of the soil. So, putting the water out there helps to avoid fungus and rot at the base of the trunk.