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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