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

Markus Metz markus.metz.giswork at googlemail.com
Fri Mar 6 10:48:46 EST 2009

Michael Barton wrote:
>> Comment (by mmetz):
>> There is a good reason *not* to add this option/flag, nicely illustrated
>> by Dylan creating this ticket. The purpose of negative accumulation 
>> values
>> is to make people wonder what on earth is going here, then figure out 
>> that
>> not the whole catchment area under study was included and expand the
>> computational region accordingly to get proper results: only positive
>> accumulation values for the catchment under study.
> IMHO, this is a very poor way to achieve this end--i.e., silently 
> performing all hydrology operations and producing completely bogus 
> values to get you to scratch your head and wonder what is going on 
> when you notice it. I suspect this is a legacy of the age of this 
> module. To make things worse, GRASS's other hydrology modules do not 
> work this way.
AFAIK, other hydrology modules also calculate what you termed 
"completely bogus" values, but there is no way to find out what values 
are bogus if all flow accumulation values are positive. The most 
conservative solution for cells for which flow accumulation could not be 
exactly determined would be to set flow accumulation to NULL and not 
give a minimum estimate.
> A much more direct way is to give a warning for each problematic basin 
> in the output:
> WARNING: part of basin XX extends beyond region extent; accumulation 
> values may be too low.
IMHO not very practical. When thousands of basins are calculated, you 
would get flooded with these warnings.
> It should be possible to turn off this "feature" of negative 
> accumulation. This is especially important for scripting. For example, 
> we use accumulation values as part of a complex, iterative 
> erosion/deposition model. 
Hmm, don't you need to know the exact flow accumulation to calculate 
erosion/deposition? Are some "at least so much, but probably much more" 
values really ok? BTW, RUSLE factors as created by r.watershed are also 
only correct for cells with positive flow accumulation.
> We are currently running this on complete watersheds created with 
> r.watershed (although an earlier version). Our goal is not to create 
> accumulation maps and in fact never see the accumulation maps but use 
> the values for additional modeling. Watershed maps get deleted along 
> the way unless a flag is set to keep them because of the very large 
> numbers of maps created to model decades or centuries of surface 
> process dynamics. The new version seems to be calculating the 
> watershed in a slightly different way, and now a tiny bit must extend 
> off the region because we are getting negative values in some 
> watersheds that were not problematic before. 
The new SFD version should produce results identical to previous 
versions. If not, I introduced a bug. The MFD version calculates 
different, improved basins compared to SFD.
> We will check this of course. But we never knew that we were getting 
> negative values until this issue came up in another context. So our 
> model values are completely bogus and we have to run this over again. 
> The slight difference in accumulation would have only a very tiny 
> effect on our results, but entire negative accumulation values make a 
> big difference. So we'll have to build in taking the absolute value of 
> accumulation (completely negating the goal of the negative values, BTW), 
Absolute values are still bogus in the sense that they are lower than 
the real values, sometimes by orders of magnitude.
> but it would still be nice to have a text informational warning about 
> which watersheds might be a problem for users of the model.
It could be possible to print out two tables like



with complete = 0 meaning that there is a problem and complete = 1 
meaning that there is no problem.

Or introduce a new output option named something like 
"incomplete_basin_parts" where the incomplete parts are assigned the 
negative value of the basin they belong to.

Markus M

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