From KDVR.com (Dan Daru):
90-percent of our plants and trees are not indigenous to Colorado, and they’re going to need more water to stay growing. About 20 to 30 inches of moisture a year.
Myth buster: trees and shrubs do not go dormant in the winter. They grow all year long, with spring and fall the most important times of the year. Giving your landscape a good, thirst quenching drink of water right now will give it a fighting chance this winter, and for years to come.