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).
User who selects to run r.watershed with this flag apparently knows
that he has
uncomplete watersheds and will be getting negative values that may
not be useful
for his application, so there is no need to tell him that he has a
problem - he would already know it
and for some reason wants to ignore it. I often found myself running
mapcalc abs on the accum
output for various reasons.
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.
Helena
On Mar 6, 2009, at 12:35 PM, Markus Metz wrote:
Michael Barton wrote:
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.
Are people calculating so many basins that thousands would be
along the region extents?
It is technically possible to calculate thousands of basins,
therefore the code must consider that. IMHO, the code must consider
all technically possible scenarios, you never know what a module is
used for, and I would like it to be very generally applicable and
not restricted to certain scenarios.
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.
It depends on how much difference there is. A couple cells would
not make much difference, 100's would make a difference. But I'd
still like to know which ones are a problem of course. I'm not
suggesting to make absolute value of accumulation a default (not
permitted in 6.5 anyway), but simply making it an option that the
user could exercise
Now I'm confused: negative values tell you where there is a
problem, but you don't want negative values, only positive values,
but then there should be additional, new output telling you where
the problems are?
Coming up with my technically possible scenarios: let's assume a
basin threshold of 10,000, that gives a max accumulation value for
exterior basins of 10,000. 10 cells within an exterior basin have
negative flow accumulation, that's only 0.1%. The absolute flow
accumulation value is probably very low, whereas the real flow
accumulation value can be anything, also >>> 10,000. Certain
calculations will be very different. The exterior basin is in this
case in reality an interior basin, but this can only be found out
by expanding the computational region.
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.
This is the issue. It happened when we tried MFD. I'm happy for
the better calculations and we'll redo our watershed boundaries.
But this points out some of the problems.
To be precise, the calculations are only better if MFD is regarded
as more accurate than SFD.
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.
If this is better than warnings, that's OK too. I just think that
there might be a better way to do this than only by making the
accumulation values negative.
Maybe Helena can give some tips?
I personally am biased, I like the concept of the original
r.watershed with all that information in the output too much:-) See
also negative drainage direction, IMHO very useful.
Markus M
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