Newly planted trees have not established the extensive root system needed to absorb enough water during hot, dry, windy summers. Even trees two or three years old should receive special care.
Deep, infrequent watering and mulch can help trees become established. Newly transplanted trees need at least ten gallons of water per week, and on sandy soft soils they will need that much applied twice a week. The secret is getting the water to soak deeply into the soil, so it evaporates more slowly and is available to the tree's roots longer.
One way to do this is to punch a small hole in the side of a five-gallon bucket and fill it with water. Let the water dribble out slowly next to the tree. Refill the bucket once and you have applied ten gallons. Very large transplanted trees and trees that were transplanted two or three years ago will require more water.
A perforated soaker hose is a great way to water a newly established bed or foundation plantings. In sunbaked soil, you may need to rough up the surface with a hoe or tiller to get the water to infiltrate easily. It may be helpful to set a timer to help you remember to move the hose or shut off the water valve.
If you are seeing surface run-off, reduce the flow, or build a berm with at least a four-foot diameter around the base of the tree to allow water to percolate down through the soil instead of spreading out.
Regardless of the method used, soil should be wet at least twelve inches deep. Use a metal rod, wooden dowel, electric fence post or something similar to check depth. Dry soil is much harder to push through than wet.