I don't think the explanation needs to be so complicated. A place need not be arid desert to be described as "dry". Even in well watered Britain there are place names incorporating the word dry, e.g., Dry Drayton. People of non-European descent in the Americas usually had surnames imposed upon them, sometimes by Christian missionaries, sometimes by the civil authority. A slave would not normally have a surname, but on being freed would require one. So the abolition of slavery in any nation on the American continent, North or South, would lead to a mass adoption of surnames, not necessarily having anything to do with a person's ancestry. We know that some slaves who were freed took the names of their former owners. I've also heard of some who, on being baptised, took the surname of their sponsor or godparent, usually a European. But I imagine many were just given a name at random, or allowed to choose one.