''I love her so much''
In this sentence, 'so' is an adverb. It modifies the adverb 'much.'
So can also be used as a conjunction or a pronoun.
'a' and 'little' modify the noun 'water' therfore they are being used as
Note that articles are always adjectives.
I don't believe an adverb ever 'adds and and explains' a noun.