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

Markus Metz markus.metz.giswork at googlemail.com
Sun Mar 8 03:08:00 EDT 2009

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.
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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