[GRASS-dev] r.watershed

[Markus Metz]

Helena Mitasova wrote:
> just a quick note from the first run - it may be better to have SFD as 
> default so that the same command as in grass63
> gives the same result. E.g. running
> g.region rast=elevation
> r.watershed elevation thresh=10000 accum=accum_10K2 
> drain=draindir_10K2 basin=basin_10K2 stream=rivers2
>
> gives an MFD result that some people may consider weird when compared 
> with the old r.watershed
> (although it is perfectly fine as MFD result)
This is true, the same command gives a different result. But some other 
people may consider the SFD result weird, e.g. with elev_lid792_1m in 
nc_spm_08. As an example imagine a user who wants to compare the output 
of different modules for flow accumulation and runs r.terraflow and 
r.watershed with default settings. With DEMs like elev_lid792_1m, MFD is 
needed to get halfway realistic results, and r.watershed with SFD 
produces here really weird results. I haven't found yet a testing 
dataset where I would prefer the SFD results over the MFD results.

So the argument to have SFD as default is that default settings produce 
results identical to the previous version.
The argument to have MFD as default is to keep default settings similar 
to r.terraflow and to use the mode as default that is likely to produce 
the most realistic results (still debatable).
>
> running the same with r.watershed -s gives the same, more "normal" 
> looking result.
> So people who have r.watershed in scripts and expect/need SFD result 
> would need to add -s,
> or will be puzzled by the different result.
I'm not that much of a hydrology expert, when would SFD results be 
needed? Should a paragraph be added to the documentation when SFD is 
preferred over MFD?
>
> So whoever has some examples, scripts with r.watershed please run it 
> to see whether
> it behaves as expected.
>
> Helena
>