[GRASS-user] flow accumulation values

[Markus Metz]

On Fri, Aug 26, 2016 at 9:41 AM, Micha Silver <micha at arava.co.il> wrote:
>
> Hi Markus
>
> ------ Original Message ------ Subject: Re: [GRASS-user] flow accumulation
> values Date: Thu, 25 Aug 2016 19:54:59 +0200 To: Ken Mankoff, Micha Silver,
> Grass-user From: Markus Metz
>
> 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
> join.
>
>
> So what would be the correct way to get total flow accumulation for each
> stream segment?
> Do I have to revert to running r.watershed with the SFD flag?

Probably yes. For MFD, streams can be more than one cell wide, and you
would need to draw a cross-section at the beginning and end of each
stream segment, then sum up all stream cells for each cross-section.

>
> 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.
>
> OK, but a flow accum raster is required for running r.stream.order (which
> outputs flow_accum) and AFAIK, the only way to get a flow_accum raster is
> with r.watershed.
> ??

Correct. Extracting a stream network needs flow accumulation and
elevation as input. Thus the workflow would be 1) r.watershed to get
flow accumulation, 2) r.stream.extract to get the stream network, 3)
r.stream.order.

Markus M

>
> Best,
> Micha
>
>
>
> 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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