[Qgis-user] Watershed - Catchment area upstream of a point

[Alister Hood]

>
> Date: Wed, 9 Nov 2022 11:04:02 +1300
> From: "Tony Shepherd \(FarmMaps NZ\)" <tony at farmmaps.nz>
> To: "'Qgis Users List'" <qgis-user at lists.osgeo.org>
> Subject: [Qgis-user] Watershed - Catchment area upstream of a point
> Message-ID: <003101d8f3be$04ed8380$0ec88a80$@farmmaps.nz>
> Content-Type: text/plain; charset="us-ascii"
>
> Hi All
>
>
>
> I'm wanting to use a DEM/DTM to automatically map the watershed/catchment
> area upstream of a known point.
>
>
>
> After a bit of reading, this appears to be a not-so-simple task but
> suggestions are to use the "Upslope Area" algorithm.
>
>
>
> In my case, the program fails to finish running, and the message window
> shows the following error:
>
>              SAGA Version: 7.8.2 (64 bit)
>              Error: tool needs graphical user interface [Upslope Area]
>
>              C:\Users\Tony\Documents>exit
>
>
>
> Questions:
>
> *       Does anybody know what that error means?
> *       Is there another tool/method that is preferable to use?
>

Yes, Grass is preferable because the integration with QGIS works reliably.
It doesn't necessarily need more than two steps:

1. run r.watershed and output drainage direction and stream segments.  The
key input parameter is "Minimum size of exterior watershed basin".  You
probably also want to output unique label (a catchment map) and flow
accumulation (number of cells that drain through each cell").
2. unless your point of interest happens to be at the bottom of one of the
catchments already mapped, run r.water.outlet,  with the drainage direction
raster as input.  When doing this, view the stream segments layer - if your
point of interest is not exactly on a stream segment you need to specify
instead a nearby point that is on a stream segment.

Regards,
Alister
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