Based on what actual evidence? I ask that question with full respect, but established trees are adaptable with root systems evolved to function at various levels of the soil profile without any serious consequence if a part of the profile dries out- fine feeder roots are dying and forming constantly. Orchardists in the east have grown fruit trees without irrigation for centuries, although short droughts are common. In our forests, the rich highly organic top layer dries out every summer in normal years and from spring on, major fine root growth starts on top and works downward as the season progresses.
How are you monitoring your trees' needs for water? Perhaps your specific soil doesn't allow the roots to go very deep and isn't proper orchard soil.