[GRASS-user] flow accumulation values

Markus Metz markus.metz.giswork at gmail.com
Thu Aug 25 10:54:59 PDT 2016

On Thu, Aug 25, 2016 at 4:07 PM, Ken Mankoff <mankoff at gmail.com> wrote:
> Is this MFD or SFD? If MFD, flow may split and re-join.

Exactly. With MFD, flow may be distributed to several downstream cells
and the accumulation value can drop when following the extracted path.
This is is most prominent when a river flows into an ocean. Paths,
however, follow Horton's stream network logic and can not split, only

> 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.

r.stream.extract is the preferred stream extraction tool.

Markus M

>   -k.
> 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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