The problem with determining water catchment areas is that many rivers are partially canalised (waterway=canal): Locks, hydroelectric power stations, etc. As a result, there is often no continuous connection between waterway=river (or =stream).
If waterway=canal is omitted from the analysis, the water catchment areas are often incomplete.
If waterway=canal is taken into account, different water catchment areas are often connected with each other.
This is a dilemma that I don’t know how to solve either.
Possible approaches:
- canalised river sections often have the same name as the river upstream and downstream of the canal sections → belongs to the water catchment area
- If the name contains the word “canal” in the respective national language and/or the name is different from the adjacent river sections at the ends of the canal, then it indicates that it is not a canalised river but a connection between different rivers (possibly different water catchment systems). - Such channels are usually not taken into account in the analysis.
- many, especially larger rivers including canalised river sections have relations of type=waterway, waterway=river in OSM. Perhaps these can be used in one way or another to determine a river system (and “normal” channels otherwise not taken into account)
I suspect that each of these variants will have various exceptions where an automatic determination of the catchment area will not work.
- perhaps an additional tag waterway=??? is needed to be inserted in places where different river systems are to be explicitly separated.