overspill vs runoff

overspill and runoff both are nouns.

Word	Noun	Adjective	Verb	Adverb
overspill	Yes	No	No	No
runoff	Yes	No	No	No

As nouns, overspill and runoff are synonyms defined as:

overspill and runoff: the occurrence of surplus liquid (as water) exceeding the limit or capacity

Other synonyms of overspill include overflow.

overspill (noun)	runoff (noun)
the occurrence of surplus liquid (as water) exceeding the limit or capacity	a final election to resolve an earlier election that did not produce a winner
the relocation of people from overcrowded cities; they are accommodated in new houses or apartments in smaller towns	the occurrence of surplus liquid (as water) exceeding the limit or capacity