One quick reality check: if the new growth is not suffering in you sun and heat, then watering is fine. As long as the new growth is fine, there could be many reasons for the leaf drop, but your plant is healthy. Older leaves can be shed because they are shaded, because the plant can't support all the leaves it has, the temps have been too cool (the tree thought it was Fall), etc. The gauge of plant health is what the new leaves are doing.