I think you should research your particular trees and what conditions they will be comfortable in. For example an Acacia(thorny, desert type tree) requires less water than for example a Ficus tree. Trees also require certain drainage conditions. As long as your trees have quite adequate drainage they should be fine. Remember that if it rains alot it can benefit many trees that have good drainage but trees that dont will suffer because the oxygen in the sand is not being replaced by new oxygen when fresh water runs through it