declivity vs downslope

declivity and downslope both are nouns.

Word	Noun	Adjective	Verb	Adverb
declivity	Yes	No	No	No
downslope	Yes	No	No	No

As nouns, declivity and downslope are synonyms defined as:

Other synonyms of declivity include declension, declination, decline, descent, fall.

declivity (noun)	downslope (noun)
a downward slope or bend	a downward slope or bend