I agree it's not very reliable, but the specks in your ground are really strange. Last I used was an image map for color and opacity, just white on black, UV, and luminosity at 1-2 or so. Then an invisible (no shadow, but that doesn't make a difference) exact copy, but luminosity set to (from my head) 10-50. That illuminates the sky around that card, but it's not a nice fading glow, and hard to control. It would be easier if light sources could be populated.
The only problem with a lake I tried is that the copy illuminates the sky, but it's also very visible as a reflection. No way to separate those.
I also worked with the cloud direct and environment light inputs, but the the problem is that you can't translate the instance locations to a scalar. Something I requested some time ago, btw. And the other way around is also hard to control.
I didn't use PT, just SR, btw.
EDIT: I'm making a quick setup for you, though on this machine, so very old version. Rendering now.