How to Water a Tree Like a Professional!
Watering a Newly Planted Tree
Above tables sourced from CSU Extension. https://extension.colostate.edu/resource/care-of-recently-planted-trees/
Watering An Established Tree
The irrigation prescription for established trees is 10 gal/diameter inch of trunk EVERY WEEK during growing season. For example: a tree with a DBH of 4″ would require 40 gallons of water per week during the growing season. This rate is the same during the dormant season, but per month, and is highly variable with deciduous vs. evergreen trees. Evergreen trees can use more during the winter, due to keeping their leaves on, and experiencing warmer winters.
Dormant Season Watering 
In Colorado winters lacking frequent moisture, water newly planted trees monthly on sunny days when it is above 40 degrees Fahrenheit and the ground is not frozen. Water early in the day, if possible, to allow water to soak into the ground before it freezes at night. These guidelines apply to established trees as well, with the increased rate of irrigation at 10 gal/diameter inch of the trunk.
Refer to CSU Extension resource https://extension.colostate.edu/resource/fall-and-winter-watering-of-plants-and-trees/ for additional information.
What to Use for the BEST Application Method?
Sprinkler application is more efficient at controlling the rate of application for best infiltration and avoids creating a sealed soil surface and run-off conditions. A fountain type, frog eye, or lawn sprinkler system is most ideal.