[GRASS-user] Hydrology modules in GRASS 6/7

Blumentrath, Stefan Stefan.Blumentrath at nina.no
Tue Feb 11 05:24:34 PST 2014

Dear all,

In a little watershed-project, some of my colleagues and I are testing and experimenting with the hydrological modules in GRASS 7 at the moment. 
In general we are very satisfied. Amazingly, r.watershed in GRASS 7 managed to calculate flow accumulation and drainage direction for very big data (150,000x120,000 pixels) in a few days processing.

In that context I had a look on the available documentation and tutorials on the hydrological modules in GRASS. There are some nice tutorials on how to perform certain watershed analyses (http://grasswiki.osgeo.org/wiki/Creating_watersheds) or an overview over existing modules (http://grasswiki.osgeo.org/wiki/Hydrological_Sciences). Especially promising looked: http://grasswiki.osgeo.org/wiki/R.stream.* and http://geomorphometry.org/system/files/promo_rstream.pdf (for the latter unfortunately only a promo-version is available). 

However, what I was missing a bit was, the rationale behind the different modules and recommendations on how to chain them or choose between possible alternatives (especialy since GRASS offers so many hydrological modules). For example:
In what cases should I use r.carve, and if I use it, do I have to "fill" the terrain model in advance (given that a stream for example crosses a sink)....
When would it be advisable to use r.water.outlet and when r.stream.basin or r.basin (what module performs better for what purpose / analysis context)?
Are there possibly deprecated/outdated modules (e.g. r.fill.dir)?
Are there reasons for using r.terraflow (and not r.watershed, given that r.watershed is capable of analyzing bigger datasets than r.terraflow) and what are they?

I am not sure if the project I am working on has enough budget for testing the effect of (some) different approaches systematically and updating the GRASS wiki in that direction. If you consider it useful and appropriate  (and if we find something useful) I should try to allocate some resources in that direction.
Still, I guess developers and more experienced users of the hydrology tools already know quite a lot about pros and cons of the different modules (and approaches for using them). Therefore I would like to ask if
a) I overlooked important documentation,
b) others are interested in updating / improving the wiki in that direction too, and 
c) who can and would like contribute to collecting existing experience / conducted projects in this regards...

What do you think?


P.S.: Maybe there is also a difference if one uses GRASS6 or GRASS7?

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