[GRASS-dev] r.watershed

[Michael Barton]

On Jan 17, 2009, at 3:04 AM, Markus Metz wrote:

>
>
> 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
>>

As I understand it, we should not break backward compatibility within  
a release or version. That means that r.watershed should have an SFD  
default for GRASS 6.4 at least and maybe 6.5 (I can't remember if this  
holds between subversion releases) and MFD could be the default for  
GRASS 7.

Michael