The mountain areas are barren, but the valleys are fertile since they are irrigated daily. What is the dependent clause

Hint: A dependent clause cannot be a sentence, but it contains a subject and verb. It is not a complete thought but it usually starts with a marker word like "since" or "because".

There are three clauses in that sentence. Two of them are independent clauses; that is, each could be a sentence on its own. One of them is a dependent clause; it cannot be a sentence on its own. I've separated them below. Which one cannot be a sentence on its own?

The mountain areas are barren,
but the valleys are fertile
since they are irrigated daily.

The dependent clause in the sentence is "since they are irrigated daily."