What is the best way to: How to know when to water my apple tree?

a). When you see the leaves start to wither, and the branches are kind of sagging, you should water your apple tree. You can sort of tell just by looking at it, if the tree is shrinking rather than growing, it probably should be watered.; b). On a young sapling, to two years, twice a week no matter what. On mature trees, check the soil and when it's dry down to 8 inches, then water, and water deeply to promote deep roots. That way you will get rewarded with a prolific harvest in the fall.;
Answer:
b).