[GRASS-user] flow accumulation values

Ken Mankoff mankoff at gmail.com
Thu Aug 25 07:07:02 PDT 2016

Is this MFD or SFD? If MFD, flow may split and re-join.

I also noticed that r.stream.extract finds different flow-paths than r.watershed, which complicates comparisons between the two. The work-around for this is to use a mask to force r.stream.extract to find the flow-path you want.


On 2016-08-25 at 13:35, Micha Silver <micha at arava.co.il> wrote:
> I noticed something today that I don't understand. When I run
> r.watershed and calculate the flow accumulation raster, I expect that
> along each stream, in the direction of flow, accumulation values will
> always be higher from one cell to the next. But I see this is not the
> case. Sometimes, along a stream, the accumulation will drop for one
> cell, then "jump up" again a few cells downstream.
> See the attached image. The green squares are clipped from a flow
> accum grid. And the numbers are flow accum values. The arrow is
> general flow direction, and the circles show examples of a sudden drop
> in flow accum.
> I'm aware of the "edge of map" behavior where out of region accum gets
> a negative value. That's not the issue here, since the sample in the
> attached image is from right in the center of the region, no off map
> flow is involved.
> This problem surfaced in a script I've prepared to calculate total
> flow accumulation for each stream reach. After running the addon
> r.stream.order I add columns to the streams vector map for X-Y of the
> end points and total flow for each reach, then I use v.what.rast to
> get the flow accum at each stream reach end-point. But the values I'm
> getting are strange. Sometimes a downstream reach shows lower total
> accum than the previous, upstream segment.
> Maybe someone can shed some light?
> Thanks, Micha

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