A common mistake in dry conditions is to over water new trees. What you're actually doing is drowning them. They shouldn't be watered at the trunk, but further from the base of the tree towards the ends of branches. This is where the majority of water is collected by the roots. To help your trees in their battle against summer drought, water deeply year-round and don't water at high pressure.
Dry ground has a harder time absorbing water, causing it to run off the surface.
The best way is to turn on your hose and let it soak into the ground around your trees. Aside from newly transplanted trees, those already under stress are susceptible to damage. Where healthy trees can recover from drought damage, the chances of recovery for weak ones are less encouraging. There is little to be done. When looking at the forests around you, keep in mind they too are affected by lack of water.