water system vs reservoir

water system and reservoir both are nouns.

Word	Noun	Adjective	Verb	Adverb
water system	Yes	No	No	No
reservoir	Yes	No	No	No

water system (noun)	reservoir (noun)
a facility that provides a source of water	tank used for collecting and storing a liquid (as water or oil)
a river and all of its tributaries	lake used to store water for community use; a lake (natural or artificial) used to store water and used as a source of water
	anything (a person or animal or plant or substance) in which an infectious agent normally lives and multiplies
	a large or extra supply of something