Apples falling before they ripen: when the tree sheds some of its crop

This mainly looks like early fruit drop through natural thinning. Apple trees often set more fruit than they can bring to maturity and shed some before it ripens. In your photograph, the apples are intact, with no obvious holes, rot or bite marks, and you say the tree has been watered over the past one to two weeks. That points more towards natural thinning or general stress earlier in the season than an acute problem now.

What you can do now

Clear away the fallen apples rather than leaving them beneath the tree. If the branches still carry a very dense crop, you can leave the remaining fruit if it is already approaching ripeness; September is usually late for substantial thinning. Concentrate instead on thorough, consistent watering during dry weather rather than little splashes, particularly if the ground is dry and bare as it is in the photograph.

If heavy fruit drop continues and larger, almost ripe apples also start falling, or if you later find dark patches, grub holes or brown cores in fallen fruit, there is reason to suspect other stress or pest damage. From what you show here, though, the simplest explanation is that the tree is regulating its crop, which usually does not mean anything is seriously wrong.