[@FlitterFaux]

To answer your question, we don't have a lot of information on the connection between Aura and Semblance. It may even play by different rules for different people. Some characters have semblance that can be "active" giving a temporary effect (ex: Ruby, Speed) some can be activated giving a more permanent effect, (ex:'Yang, Strength) And yet others have a passive effect that can be activated for a temporary effect. (Ex: Nora, Electricity absorption into strength) and finally, whe have been introduced to Semblances that stay active always. Ie, a passive permanent effect. (Ex: Qrow, Bad luck)

So yeah. Lots of variance on the interaction between semblance and aura. One thing that is universally agreed on is that without aura, their semblance won't/can't be activated.

Edit: Out of all of them, Qrow's strikes me as strange, because if Semblance fed off Aura like a battery or MP bar, then his would constantly be draining. Although its possible that his body passively restores it faster then it drains, but its still odd.