downslope vs declivity

downslope and declivity both are nouns.

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

As nouns, downslope and declivity are synonyms defined as:

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

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