In Portuguese is there a way to distinguish between 'It hasn't rained much today' (pres. perf. - it's still that day) and 'It didn't rain much today' (past simple - said that night/evening)?
No, there is not. There is no tense in Portuguese corresponding to the Present Perfefct. We use the Pretérito Perfeito (choveu) for Past Simple (it rained) or Present Perfect (it has rained).
Yes. At four or five in the afternoon, it would certainly possible to use the simple past when you consider the day is almost over. If, however, there are many remaining hours in the day, we would more than likely use the present perfect. The language is quite flexible concerning the present perfect tense.
The hover hint says that you can conjugate this. How can I or you or we rain? Or does the word have other meanings?
That doesn't sound right in English. "It is has not rained much today" is way too convoluted. Did it or did it not rain today? ;)
'It did not rain much today' has the same word count as 'It has not rained much today' but is a bit simpler, I agree - but would Duolingo?