rigourousness vs severity

rigourousness and severity both are nouns.

Word	Noun	Adjective	Verb	Adverb
rigourousness	Yes	No	No	No
severity	Yes	No	No	No

As nouns, rigourousness and severity are synonyms defined as:

Other synonyms of rigourousness include asperity, grimness, hardship, rigor, rigorousness, rigour, severeness.

As nouns, rigourousness and severity are synonyms defined as:

Other synonyms of rigourousness include hardness, harshness, inclemency, rigor, rigorousness, rigour, severeness, stiffness.

rigourousness (noun)	severity (noun)
something hard to endure	excessive sternness
excessive sternness	used of the degree of something undesirable e.g. pain or weather
	something hard to endure
	extreme plainness