You are right about why to water.  Here are the scientific reasons.  First, trees in the forest don’t lose water as quickly because their roots and part of branches are shaded by other trees. Second the forest tree roots don’t have as much competition for water. Grass/Turf grabs up water before trees can get enough. Mulching can also help trees retain moisture. Nature’s way of “mulching” is to retain all the dead leaves and rotting materials at the ground surface which keeps down weeds and helps to hold moisture in the soil.

Right now we are in a reduced water situation as spring rains have been almost nonexistent. So you are smart to keep watering trees now and again in the fall if there is reduced rainfall.

The following website gives more details: 

https://extension.umn.edu/planting-and-growing-guides/watering-established-trees-and-shrubs

You are welcome!