severeness vs rigorousness

severeness and rigorousness both are nouns.

Word	Noun	Adjective	Verb	Adverb
severeness	Yes	No	No	No
rigorousness	Yes	No	No	No

As nouns, severeness and rigorousness are synonyms defined as:

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

As nouns, severeness and rigorousness are synonyms defined as:

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

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