Deep root watering is a great way to maintain healthy trees and prepare them for a long winter. Watering should be done at the drip line and no deeper than 8 inches into the ground. As a guideline, the tree may need about ten gallons of water for every inch of the trees diameter. For example, a tree that is four inches in diameter will need at least 40 gallons of water. Surface watering can be done with a drip hose, however, deep root watering is more effective and more water efficient. Newly planted trees or shrubs will need some watering a couple of times per month for the first two years of growth. Established trees should be fine being watered once per month from October through March.
More information on maintaining your trees during the winter can be found here: http://www.denverwater.org/Conservation/TipsTools/Outdoor/TreeCareGuide/