Hello Runoff (also spelled with a hyphen - run-off) is a noun and it means the draining away of water (or substances carried in it) from the surface of an area of land, a building or structure, etc. • the water or other material that drains freely off the surface of something. But, in your sentence "When plants are irrigated from above the surface, much water evaporates or runs off", "runs off" is a phrasal verb.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.