Halliday & Matthiessen (2014: 530):
If the reported clause is interrogative it typically shifts into the declarative; the declarative is the unmarked mood, and is used in all clauses that do not select for mood independently, including all dependent clauses. A yes/no interrogative becomes declarative, introduced by if or whether (he asked ‘is she coming at noon?’ : he asked whether she was coming at noon); a WH- interrogative becomes declarative with the WH- element remaining at the front (he asked ‘when is she coming?’ : he asked when she was coming).