[Tagging] Watershed or Drainage Basin relation draft proposal

[Joseph Eisenberg]

Relations of type=watershed are currently used over 2000 times and there is
a descriptive Wiki page but no proposal. (
https://wiki.openstreetmap.org/wiki/Relation:watershed)

It would be useful to have a relation that showed drainage divides (aka
watersheds) and drainage basins (the network of streams and rivers draining
into a water body or waterway)

Some people use type=waterway to associate tributaries of a river. However,
this makes it more difficult to search for the main river; one would need
to request the main_stream members only, so adding tributaries to the
waterway relation is discouraged.

There is also no approved way to specify the drainage divide for a waterway
or water body. Many mountain areas already have parts of the drainage
divide delineated as natural=ridge, so making a relation would simply
involve selecting all the ridges that divide two basins and adding a couple
more outer lines extending to the river mouth.

Having a relation for the drainage divide or watershed would be useful for
mapping natural features. Some drainage basins, such as the Great Basin in
the USA or the Amazon Basin in South America, are well known features of
the landscape, currently not well represented in the OSM database. Good
watershed data would also make OSM more useful for ecological research,
city planning, hydro engineering etc.

I have seen several watershed relations in my area (central Papua,
Indonesia). They also appear to be common in Turkey and parts of central
Asia. See: https://www.openstreetmap.org/relation/1076098
https://www.openstreetmap.org/relation/1272911
https://www.openstreetmap.org/relation/1226862

There are also a dozen relations of type=boundary, boundary=watershed

Questions: 1) Is the word watershed appropriate?
The correct British English terminology is Drainage Basin (
https://en.wikipedia.org/wiki/Drainage_basin) when referring to the network
of tributaries belonging to a river or the streams draining into a water
body. "Watershed" is the American English term, but in Britain means the
drain divide (for example, the continental divide). So far,
type=drainage_basin or =drainage_divide has not been used.

2) Should these be a relation of type=*, or boundary=*?
Type=watershed is currently used 200x more often than boundary=watershed.

We could use both: the boundary=watershed could include the ridges and
watershed lines surrounding the entire area, while type=watershed (or
type=drainage_basin) could include both the boundary and the waterways.

3) Should individual ways be added to the watershed relation, or is it okay
to add existing relations of type=waterway? It would be faster to add the
waterway relations as "child" relations. However, I have read that it is
more difficult for data users to handle parent and child relations. And
while it is easier to create a relation by selecting a couple of existing
sub-relations, maintaining the nested relations may be more difficult?

Here are 4 examples of the different options:
type=watershed with waterway=relation added:
https://www.openstreetmap.org/relation/8687462#map=12/-4.0619/138.7478
type=watershed with individual waterway members only:
https://www.openstreetmap.org/relation/8687464#map=11/-4.2105/138.7912
boundary=watershed with waterway=relation:
https://www.openstreetmap.org/relation/8687463
boundary=watershed with individual waterways:
https://www.openstreetmap.org/relation/8687500#map=13/-3.8438/138.8568

Joseph Eisenberg
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