[GRASS-dev] r.watershed

[Markus Metz]

Helena Mitasova wrote:
> [...]
> Just run it with elevation in nc_spm. The MFD result is actually more 
> realistic because it simulates the existing
> lakes but you would have to do additional processing if you need a 
> stream network 
Have you looked at the stream segments output (option stream)? That is a 
stream network AFAICT, similar to SFD stream output, and can be 
vectorized with r.thin followed by r.to.vect, same as the SFD stream 
output. Using MFD flow accumulation instead of MFD stream segments to 
get a stream network with different stream segments is however very 
difficult.

> [...]
>>> 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?
>
> when you need to extract vectorized stream network from lower 
> resolution data (30-90m) and you need the network
> to create a single channel through lakes, SRTM is a good example as is 
> the elevation DEM in nc_cpm_08.
See comment above, the stream output does give you a single channel 
through a lake, also with MFD. I tried to make sure the stream segments 
output of MFD can be as easily vectorized as the stream segments output 
of SFD.
>
> | Should a paragraph be added to the documentation when SFD is 
> preferred over MFD?
>
> yes, some comments on when to use which option would be helpful.
>
> Helena
>
>>>
>>> So whoever has some examples, scripts with r.watershed please run it 
>>> to see whether
>>> it behaves as expected.
>>>
>>> Helena
>>>
>
>