[GRASS-dev] Re: [GRASS GIS] #518: negative flow accumulation with r.watershed SFD or MFD

Michael Barton michael.barton at asu.edu
Sun Mar 8 13:31:46 EDT 2009



On Mar 8, 2009, at 12:08 AM, Markus Metz wrote:

> Michael Barton wrote:
>> On Mar 7, 2009, at 8:50 PM, Helena Mitasova wrote:
>>> There is no question that the default should be kept negative,  
>>> although checking whether the result
>>> is correct would not hurt - we can look at it with our Panama  
>>> experiments, others using
>>> r.watershed could provide some helpful feedback too.
>>> But adding a flag to keep values positive actually makes sense to  
>>> me, if the flag is properly
>>> described (e.g. use positive flowaccumulation even for uncomplete  
>>> contributing areas).
> Will do, but only with a warning that incomplete contributing areas  
> can not be identified with positive flow accumulation only.
> I will also add an example to the manual on how to identify what  
> parts of what basins are incomplete:
> r.mapcalc "problems = if(flow_acc < 0, basin, null())"
>>> BUT for erosion modeling you really want to run r.watershed with  
>>> negative values - erosion models require
>>> upslope contributing area as measure of water flow and if the  
>>> watershed is not complete,
>>> water flow will be underestimated leading to underestimated  
>>> erosion rates. So the cells with
>>> uncomplete contributing area need to be excluded from the  
>>> computation of erosion, and here
>>> the negative values actually come handy.
> "accumulation
> Output map: The absolute value of each cell in this output map layer  
> is the amount of overland flow that traverses the cell. This value  
> will be the number of upland cells plus one if no overland flow map  
> is given. If the overland flow map is given, the value will be in  
> overland flow units. Negative numbers indicate that those cells  
> possibly have surface runoff from outside of the current geographic  
> region. Thus, any cells with negative values cannot have their  
> surface runoff and sedimentation yields calculated accurately."
> This is the r.watershed manual entry from grass54 through to grass7  
> (don't have access to pre-54). This description can be updated if it  
> is not clear enough.
> Markus M

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