[GRASS-user] Re: understanding r.watershed

[Markus Metz]

Georg Kaspar wrote:
> On Thu, 29 Jan 2009 10:20:36 -0500, M S wrote:
>
>   
>> In short, r.watershed, without depression input,  will route water in
>> and *up* and out of depressions in the terrain to illustrate the
>> complete downward path.  This is why no DEM filling is necessary.  By
>> entering in known (substantial and impactful) depressions, the water
>> does not route out of it.
>>     
>
> But in my case, providing known depressions leads to areas of null()s 
> around those depressions (lakes by the way).
>   
If these lakes have an outflow, i.e. water is leaving these lakes, the 
results will be more realistic when you omit the depression input to 
r.watershed and only use the (not filled) DEM.

If you are talking about the basins output having NULL values around 
these depression, this is because your basin threshold value was too 
high, set it to a lower value. Check the current flow accumulation 
output for a reasonable threshold.