Twice a week may be too often. Also, the rocks around the base of the tree are forming a basin. Does water collect there, or does it run out? Tree roots extend far out from the trunk, so watering should be done around the tree past the outer circle of rocks as far as the branches extend.

A nutritional deficiency would show up on leaves in different parts of the tree, not mainly on one side.

I would say Santa Ana wind damage is a possibility, but I'm not sure about what side of the tree that would affect where you live. Where I am, wind damage happens on the northwest side of the tree, and it's been quite a while since we've had a strong wind. It's most likely overwatering.

Thanks for the insight.

Water runs out of the rock basin just a few minutes after watering is done. Good point regarding the root area...the right half of the roots get watered by the sprinklers every other day, too, outside of the rock basin.

There haven't been any strong winds recently, and the left side, which is losing leaves, is actually close to the side of the house and the side gate. I'm wondering if the sun hits the left differently than the right.

After looking at lots of websites about leaf diagnosis, it almost looks like nitrogen deficiency/overwatering, with older leaves gradually turning yellow and then falling off--so that would go along with what you're guessing. I'll guess I'll water once a week.