I would like to add tunnel=headrace to the waterway=drain because that's how they are called. Just google "headrace tunnel" and this is exactly what you will get.
The pressurized pipeline that goes into the hydro power plant is called "penstock": https://en.wikipedia.org/wiki/Penstock That's why I would like to tag those pipelines as man_made=pipeline + pipeline=penstock. "Penstock" by itself means that the pipeline is pressurized, but we can add "pressurized=yes" just to be safe (waterway=pressurized is a funny tag to me). Using waterway=drain for underground rivers is not very accurate. I think a new waterway tag is needed here, because this is a new concept. I suggest waterway=subterranean_river. And if it has a siphon, add subterranean_river=syphon. So, I made an image with my suggestions, changes are bolded and underlined (I love your images, they are worth a thousand words :) https://imgur.com/a/obdNd Janko Mihelić
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