[Tagging] Irrigation: ditches, canals and drains

[Joseph Eisenberg]

It looks like our long discussion a while back didn't lead to
substantial changed to the wiki pages for waterway=drain,
waterway=ditch and waterway=canal.

I was checking on how many waterway=ditch were tagged as being used
for irrigation, and decided to slightly updated the pages. I've marked
all features as "De facto", and slightly adjusted the descriptions and
pages to suggest that:

- A waterway=drain is lined by concrete or similar materials and is
used to remove superfluous water
- A waterway=ditch is not lined and is usually used for drainage, but
can (controversially) be used for irrigation.

I note that the page suggested irrigation=yes, but actually there are
4 options to showing that a ditch or canal is used for irrigation:

1) irrigation=yes was the first tag used, starting it 2011. As of May
2019, it has been used 200 times with canals and 2000 times with
ditches. Most of these tags were added between 2011 and 2013.

2) service=irrigation was used starting in 2012, and as of May 2019 is
used 12,000 times, mainly with canals but also with several hundred
ditches

3) usage=irrigation was introduced in 2018 and usage is increasing
quickly. It is currently used 6,000 times.

4) About 2000 hundred canals are tagged canal=irrigation and a few
ditches are tagged ditch=irrigation or irrigation=ditch

The tagging suggests that waterway=canal is more commonly used for
irrigation features, but there certainly is significant usage of
waterway=ditch for irrigation:

overpass-turbo.eu finds 3605 ways tagged waterway=ditch plus one of
the irrigation related tags: https://overpass-turbo.eu/s/JqW

Comparisions: 10687 ways tagged waterway=canal and one of the
irrigation tags: https://overpass-turbo.eu/s/JqX

Waterway=drain is  specified for irrigation on 1298 ways:
https://overpass-turbo.eu/s/JqY

So it looks like a number of mappers consider waterway=ditch
appropriate for use with irrigation features, and there is even some
use of waterway=drain for irrigation, which surprises me.

What, then, should be the distinguishing characteristic between
waterway=canal and waterway=ditch or =drain? Width or importance or
navigability, or should we still mention the usage as the main
difference?

- Joseph E