Map editor Question

[Imp]

With regards to Arid & Savanah settings realise can only have one or the other & not available for winter maps my question is are the effects widely varied or do they tend to adjust a basic set of parameters. If so is this dependant on say season theatre.
Example say Savanah on a summer map in spring & autumn tends to produce more mud in summer does little apart from change earth colour
or arid for the same tree setting as summer produces less trees.
Any rough hints would be apreciated realise I can just increase mud setting say if thats what I want but dont realy understand what arid & savanah do though I am sure have used in the past to generate nice bad going maps.