Please use the English version when there are differences:
https://wiki.openstreetmap.org/wiki/Tag:waterway%3Ddrain
There: "For artificial waterways created for carrying useful water or for transport consider using waterway=canal. "
I used ditch/drain for some time before, but when i came across waterway=canal & irrigation=yes I looked at the english version and changed to canal.

In contrary your mass edit of levadas in Madeira is not correct.