As you all know, we're in a record-setting drought situation! Any tree planted in the last 5-10 years, and even our mature yard trees, need a little help to make it through this extreme weather. Think of the cost of replacement trees and consider watering immediately.
The very best way to water young trees is to place a 5-gallon bucket at the base of the tree with a small (about 1/4-inch) hole drilled in the bottom along the edge. Place the hole as close to the trunk as possible and fill the bucket with water at least once or twice per week. This will allow for a slow soaking of the area immediately around the trunk and give your tree the best chance for survival. In addition, make sure your tree has a properly constructed mulch ring to conserve as much of the water as possible. To keep the wind from blowing the bucket around your yard, use a piece of string to loosely tie the handle of the bucket to the trunk of the tree. Remember to remove the bucket when normal weather returns and save it to use again later if needed.
For large, mature yard trees, place a hose at the base of the trunk and set the hose to barely run. Just a trickle is all you need. Allow the water to run for at least 12 hours. Again, you are aiming for a slow soak. If you see water quickly running off, turn the pressure down a little. For mature trees, a good soak is only necessary every week or two. As with your smaller trees, mulch will help to conserve water.