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Using r.watershed inside QGIS, I am getting no data value along some streams for flow direction.

I tried the r.fill.dir and for the same raster it does not give any no data value for the same cells (see the image below, r.watershed is on top of r.fill.dir).

enter image description here

I tried both sink-filled and non sink filled dems.

Here are my inputs for r.watershed enter image description here

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Two issues:

You have checked "Enable Single Flow Direction". If there are real depressions in your DEM, then these cells get a zero value in flow direction. In QGIS this might be translated to NULL.

More important: What is in the HSG overland flow raster? Are there any NULL cells in that data?

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  • 1. I have provided the algorithm with already corrected DEM, so sink shouldn't be an issue right? The same cells don't get zero or nodata value in other grass direction algorithm. Also the watershed algorithm states that it does not need sink-filled DEM.
    – ar-siddiqui
    Commented Sep 29, 2021 at 11:01
  • 2. There are null values in HSG raster but very far from the cells which have got no data values. Also this should not matter because the flow direction has nothing to do with the overland flow weight, correct? It only comes into play with accumulation.
    – ar-siddiqui
    Commented Sep 29, 2021 at 11:03
  • Ok, I checked and you are correct, yes HSG was the one causing the issue. Still not sure how though, because it should only come into play at the stage of accumulation, flow direction has nothing to do with weight, in my understanding. Please explain that in the answer and I will accept. Also if you can shed light on if it does not need sink-filled DEM for SFD as well. Thanks.
    – ar-siddiqui
    Commented Sep 29, 2021 at 15:23

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