[GRASS-user] Re: understanding r.watershed

[MS]

R.terraflow outputs catchments around sinks in the dem.  R.watershed  
should be making huge delineations in comparison.

Mark

On Feb 2, 2009, at 11:40 AM, Markus Metz <markus.metz.giswork at googlemail.com 
 > wrote:

>
>
> Georg Kaspar wrote:
>>> 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.
>>>
>>
>> so, my accumulation map contains values from -144714 to 58920 with  
>> a majority of cells between 0-100. what would be an appropriate  
>> threshold value?
> You can try standard deviation of flow accumulation (r.univar -g).
>> I already tried 5000 and it looked similar to the output I received  
>> from r.terraflow,
> The basins of r.watershed look similar to the basins of r.terraflow?
>> but when running r.watershed with depression input I still receive  
>> those null()-areas...
>>
> ... in the basins output I assume. If you really want to treat lakes  
> as real depressions, i.e. water is not supposed to leave these  
> lakes, then there will always be NULL areas around the lakes unless  
> you set the threshold to something much smaller than 100, but then  
> the stream segments and basins become meaningless because there are  
> too many...
>> by the way, this is what my region looks like:
>>
>> GRASS 6.2.3
> Rather use 6.4.0RC3 instead of 6.2.3, you should get a couple of  
> nice surprises...
>> [...]
>> cells:      466400
>>
> Not that many cells, should take just a few seconds to get the  
> basins (with 6.4.0RC3).
>
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