How much should I water my trees?

Please see the discussion on shrubs first, then consider that trees are much larger and their roots much wider than shrubs.

Tree roots in our desert are about 3 feet deep.  They are at least as wide as the tree canopy (the mass of leaves you see above ground).  The thirsty part of the roots are near the ends.

Palm roots stay pretty close to the trunk and don't spread out.  Palms are monocots and have very fibrous, absorbant roots.

Imagine a large doughnut drawn on the ground around your tree.  The doughnut is as wide or slightly wider than the canopy.  This is the entire area you want to wet each time your tree is watered.  If you have a couple of drip emitters right at the trunk, none of these root ends are getting water.  Not good!  You can actually rot out the trunk this way, and starve the tree at the same time.  You will need several drip emitters positioned in a ring around your tree inside this imaginary doughnut.

Your trees may need anywhere from 3-6 hours of irrigation at once, depending on the number and size of emitters you have, your soil, your drainage, and your surrounding structures.  Trees will steal residual water away from plants and other trees that receive water in their vicinity.

You can make a swell, a little bowl area with a raised ring around it, around your tree to capture and hold water.  If you have this, it's easier to use higher-volume emitters or bubblers to flood your trees and give them a good soaking in a shorter amount of time.

You can also direct rain water to your tree's root zone.  The amount of water than runs off your roof in a rain event is perhaps more significant than you think.  If it's directed to flow toward your trees, you can soak your trees really well without supplemental irrigation and deliver a satisfying amount of water.

Water your trees deep and infrequent.  Every 7-14 days in summer is adequate if they get enough water each occurrence.  In the winter, feel free to cut this back to once a month.  If you have desert adapted trees, or the varieties that you see growing wild out on the hiking trail, consider that these trees may survive without supplemental water at all.  After they're established, you can train them to "harden off" and live on rain.  Respond to what you see above ground by feeding what's below ground.

Thick, vulnerable canopies are in many cases due to improper pruning and too much water.  Shallow roots are due to frequent, shallow water.  Both contribute to the loss of so many trees in our winds.