The movement of water both on and
above Earths surface

The movement of water on and above the earth's surface is called the hydrologic cycle, or the water cycle. Hope this helps!

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.