[GRASS-user] r.watershed

Robl Jörg Christian Joerg.Robl at sbg.ac.at
Wed Jun 24 06:32:58 PDT 2015

Dear Moritz,

Thank you for testing and reproducing the strange behavior or r.watershed and D8.
At least it is not a problem of a library on my computer!

Any help is highly appreciated!


-----Ursprüngliche Nachricht-----
Von: Moritz Lennert [mailto:mlennert at club.worldonline.be] 
Gesendet: Mittwoch, 24. Juni 2015 11:30
An: Robl Jörg Christian
Cc: grass-user at lists.osgeo.org; Markus Metz
Betreff: Re: [GRASS-user] r.watershed

On 23/06/15 10:32, Robl Jörg Christian wrote:
> Dear GRASS Gurus,
> During the last days I've discovered a strange behaviour of 
> r.watershed and D8. The used DEM is the result from a landscape 
> evolution model and there are no depressions. I cannot use MFD as the 
> D8-FlowDirections input data of a variety of tools I've written during the last years.
> I got obviously wrong values for the flow accumulation of r.watershed 
> and D8. The number of cells in the Flow Accumulation grid at the 
> outlet point was significantly smaller than the area of catchment 
> analysed catchment (spatial resolution in north south and east west 
> direction is
> 1) also computed with GRASS using r.water.outlet and a conversion to 
> an Area. I checked Flow Accumulation at the the outlet with an 
> alternative code of a colleague and the contributing drainage area was 
> exactly the value of the catchment size computed with r.water.outlet.
> I further found some deviations in the flow direction grid. In some 
> cases the flow routing leads to the lowest neighbouring cell which 
> however might not be the steepest decent. r.watershed chooses 
> sometimes a diagonal flow direction while the the local channel 
> gradient would be greater in horizontal direction.
> I started some test runs with very simple geometries. I've defined a 
> ramp with r.mapcalc that dips in north direction and I expected that 
> the this will also be shown by the flow direction of r.watershed. 
> However, this is not the case. The synthetic DEM features a 
> watershed!!! See attachment.  It seems that r.watershed fills the region (without need).
> This strange behavior disappears in by applying the multiple flow 
> direction algorithm.
> I have tested with the latest versions grass70 , grass71 and grass645.
> Here is a test case (grass71). I would be great if somebody could 
> reproduce the problem and show me a workaround.
> ---------------------------------------------------------
> g.region w=0 e=1000 s=0 n=1000 res=1 -p r.mapcalc "DEM = 1000 - y()"
> r.watershed -s elevation=DEM at PERMANENT accumulation=ACC drainage=DIR

After a first rapid look I can confirm the issue, but don't have an explanation, nor a workaround.

MarkusM, you know r.watershed best. Could you have a look ?


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