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Using QGIS,I attempted Africa's watershed delineation taking the following steps:

  • Downloaded DEM from earthexplorer USGS (GTOPO30)
  • Created a mosaic of the downloaded DEM
  • Reprojected the mosaic DEM to EPSG:54010 World_Eckert_ VI(For equal area world map projections)
  • Rescaled map cell size to 1km by 1km(For faster processing time) using the Align Rasters function
  • Filled depressions using Wang & Liu XXL algorithm -Created stream order using strahler order function
  • Choose strahler order threshold using raster calculator function
  • Created the basins and channels using SAGA channel network and drainage basins function

The output (Picture 1) do not look satisfactory.

How do I improve the results?

Africa_Watershed_Delineation

The goal is to have a clearly delineated River Niger Basin- the basin with thickened blue border line in picture 2

enter image description here

  • 1
    Don't reinvent the wheel. Use HydroBASINS data. – Jon Sep 5 '18 at 16:33
  • Your streams look suspiciously straight and merge at 45 degree angles which are red flags for natural flow paths and suggests a 'bad' DEM that's unnaturally homogeneous. That could be the result of some of your processing like the resampling or the fill. – Rex May 29 at 21:18
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Here are the level-3 basins provided by HydroSHEDS

enter image description here

You can download these from http://www.hydrosheds.org/download (must create an account), or you can use this link for the next 3 hours: http://www.hydrosheds.org/tempdownloads/hydrosheds-209a52bb01dc03b64a9f.zip

  • At the tail end of the Niger River Basin lies my study area(Ogale, Rivers Nigeria). I need to know the sub-basin my study area falls within. Therefore i have to determine the upslope area, River channels and flow directions using suitable QGIS algorithms. – Learner Sep 5 '18 at 17:46
  • You should include extra details and constraints in your question. My answer provides a suitable solution for what you asked. You can simply intersect your study basin with the level-3 HydroBASINS product to find which sub-basin your study area falls within. There are other basin levels if you want a finer-scale watershed delineation. – Jon Sep 5 '18 at 17:51
  • I have tried level-1-6 HydroBASINS product, i need a fine scale delineation. – Learner Sep 5 '18 at 18:02
  • It goes to level 12, which is quite fine. – Jon Sep 5 '18 at 18:09

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