Manually, the ideal way to water trees is by soaker, or irrigation hose. Sprinklers and regular garden hoses are less efficient than soaker hoses but can be used. Use organic mulch (wood chips etc.) to conserve moisture and apply water over the top of the mulch. Do not concentrate water at the base of the trunk.
Lay the soaker hose on the ground in a circle under or beyond the drip-line. Most of the trees roots spread 1-1/2 to 3 times as wide as the tree’s canopy. Some trees such as Spruce will have absorbing roots well beyond the drip-line
The best time to water is late afternoon or evening when evaporation will be less. But anytime is o.k. If using a sprinkler watering at night is not recommended.
Water well or not at all! Shallow watering will “starve” the deeper roots causing more growth of surface roots, these roots will be predisposed to freezing conditions. Less frequent deep watering is much better than frequent light watering. Let the water run slowly generally 1-3 inches of water per week in summer and every two weeks in spring/fall will keep a tree healthy. Five gallon (approx 2o litres) per square yard is about 1 inch of water.