stormwater drain vs drain system

stormwater drain and drain system both are nouns.

Word	Noun	Adjective	Verb	Adverb
stormwater drain	Yes	No	No	No
drain system	Yes	No	No	No

As nouns, stormwater drain and drain system are synonyms defined as:

stormwater drain and drain system: a system of watercourses or drains for carrying off excess water

Other synonyms of stormwater drain include drainage well, storm drain, storm sewer.

stormwater drain (noun)	drain system (noun)
a system of watercourses or drains for carrying off excess water	a system of watercourses or drains for carrying off excess water