The border of this swimming-pool cover is an example of a watershed
What is a watershed? Easy, if you are standing on ground right now, just look down. You're standing, and everyone is standing, in a watershed. A watershed is the area of land where all of the water that falls in it and drains off of it goes to a common outlet.
This pool cover is an example of a watershed. All of the rain that falls into this area flows inside of the cover, in this case towards the center.