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I have the polygon of my catchment of interest, I call it catchment A (Reyran in the schema). (format *.shp).

I have also the x-y coordinate of a dam that is situated in the catchment. I have also a *.tif file that includes elevation information of the catchment A.

I would like to define and split in GRASS (QGIS) the corresponding upstream (catchment I) and downstream catchment (catchment O) that we could define in catchment A at the upstream and downstream of the dam.

I need to get the shapefile (*.shp) of the upstream catchment. How could I do it? Here is the schema of the catchments.

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  • Sorry if I am mistaken. It seems you are trying to perform r.water.outlet which creates a watershed basin over the upslope area, defined by outlet coordinates (i.e. your dam).
    – Kazuhito
    Commented Oct 23, 2018 at 10:24

2 Answers 2

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To use GRASS you need to have the raster of flow directions inside the catchement area A.

I will assume you do not have it yet and I will present all required steps as they can be perfomred in QGIS.

  1. Using r.watershed to produce the flow direction map. In the window box of QGIS has the name of Drainage direction and I call the raster dirflow. I also produced the flow streams to easily select the point for next steps.

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  1. With the use of r.outlet delinate the catchment area A.

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  1. With r.outlet delineate catchement area B (You called "I").

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The results will be 2 rasters with catchment areas A and B. Next steps are:

  1. to "substract" B from A to get the catchment area "O"

  2. to Vecotorize the raster catchments areas to produce the desired shape files.

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If you want to split the polygon manually you can use split feature tool in QGIS from Advanced Digitizing Toolbar enter image description here. You can enable Advanced Digitizing Toolbar from View → Toolbars → Advanced Digitizing Toolbar

Before using Split feature:

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After using Split Feature:

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  • In fact I dont have the common boundary between the two catchments. I have to define it somehow, for instance using the elevation information provided by *.tif. I therefore cannot do it manually.
    – Basilique
    Commented Oct 23, 2018 at 6:00
  • There is no relation between your question and your comment. If there is no defined catchment boundary, then why do you ask of how to split the catchments. You need to create catchment boundaries first, then based on the result, you can decide whether you need to split the catchment into two or not.
    – ahmadhanb
    Commented Oct 23, 2018 at 6:04
  • I first need to define the boundary and then split the two upstream and downstream catchments. I am sorry if my question was not clearly asked. Do you know how I could do these two procedures?
    – Basilique
    Commented Oct 23, 2018 at 6:06
  • In QGIS go to Processing toolbar -> Saga -> Terrain Analysis - Hydrology -> Fill Sinks (wang & liu) and use the DEM.tif as input. Then convert the Catchment raster into polygon using Polygonize tool. If after that you need to split the catchment, you can use the answer above.
    – ahmadhanb
    Commented Oct 23, 2018 at 6:16

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