It is difficult to travel through the huge expanse of parched sands in the Sahara Desert, where oases are plentiful but distant from each other. Which phrase is the adverb clause?

The adverb phrase used in the sentence above is where oases are plentiful but distant from each other. It consists of a word "where" which denotes the place of occurrence that adds information about the word it modifies. Adverb phrase usually begins with subordinating conjunctions and contains subject and predicate.