A: Well, first, the good news. Your trees are not going to die. As you expected, it’s the high temperatures and dry weather. Mature trees do not need frequent watering, but enough dry and hot weather can put the hurt on almost any species of tree. Lawn sprinkler watering once a week isn’t doing enough, and the tree is shedding less useful leaves to ensure the more productive leaves remain healthy.
Because our area is in extreme heat and moderate drought right now, we’d recommend you give your mature trees extra water every few weeks (perhaps once every two weeks), but you need to do it properly. You need to deeply wet the majority of the root area of the tree, which means using a slow dribbling hose or a low spinning sprinkler for roughly half an hour at a time per area, moving the hose end back and forth across the area until you’re certain you have watered most of the root zone. It can take several hours to water a single mature tree in this way, but you can set the hose down and just come out to move the hose occasionally.
Mature trees on properties with at least two irrigation cycles per week probably don’t need this type of extra watering. If you feel like your trees need more, once every three to four weeks is probably fine until temperatures come down.
And finally, don’t sweat it if you still lose a few leaves here and there. Some of this happens every summer, to pretty much every tree. You just don’t notice, as the occasional leaf blows down the street and your tree still looks good. It’s just a bit more than normal right now.