The John Begeman gardening column in the June 8 AZ Daily Star reminds us that even the large eucalyptus and pines and other large trees in our neighborhood need a good soaking because of the extended drought. He recommends slow watering overnight under the width of the tree canopy, so that the top 2 to 3 feet of soil is moistened. Give two good soakings, about 3 weeks apart.
If you delay watering until the leaves or needles turn brown, it will be too late to save the tree.