I am trying to compute the accumulated drainage area (flow accumulation) using the GRASS' r.terraflow algorithm in QGIS (QGIS 3.20.3 and GRASS 7.8.6RC2 were installed using the OSGeo4W Network Installer).
Upon inspection of the output raster with the Identify Features tool it seems as if some raster cells got floating numbers assigned to them (e.g. 54318.957, where large drainage areas contribute), whereas the majority of the cells contains integer numbers as one would expect.
I wanted to inquire about possible causes for these results and value any suggestion.

1 Answer 1


I believe this is due to how r.terrflow choosed the flow direction algorithm. From the man page:

If flow accumulation of a cell is larger than the value given by the d8cut option, then the flow of this cell is routed to its neighbors using the SFD (D8) model. This option affects only the flow accumulation raster and is meaningful only for MFD flow (i.e. if the -s flag is not used)

So in some cases the MFD algortihm is chosen, and then you can get fractions of cells in the flow accumulation.

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