The English sentence sounds wrong. It needs to be "the rainfall" not just "rainfall"
I'm not sure the English sentence works?
Is it maybe: ".......how much it rains here" (?? would that have bao nhieu ?)
The English expression "to know the precipitation" does not really have a meaning. You could say "I want to know about precipitation in this area (here)." But I don't know if this is an accurate translation of the Vietnamese.