Resto Shaman Resurgence question

So I'm trying to maximize my heals by trying a few different builds (have been doing full spirit, with sub par output).

I am now trying full crit, and after re-reading the Resurgence description I have a question:

Does resurgence proc from every tick of Riptide? or just the initial cast?

If anyone knows for sure, I'd appreciate an answer. Thanks.