It can be easy to dismiss trees, especially the larger ones, as self-sufficient, but that is not the case. If you are experiencing extended periods without rain then you should be watering your trees 2-3 times per week.
The best way is to give the soil a good, deep soaking by removing the nozzle from a garden hose and setting it on the ground. Ideally it should be placed half way between the trunk and the tips of the branches. Let it sit for about 5 minutes, then repeat 2-4 times around the tree, depending on the size.