New flow direction attribute for sub-catchment layer in QGIS

Question

I have a polygon layer in QGIS with self-created sub-catchments. I also have a DEM, a flow-direction raster and a raster with the corresponding stream network and stream order. I would like to create a new attribute in the polygon layer that specifies the flow direction from one polygon to the next.

sub-catchments and stream network:

sub-catchments and flow direction: For example, SE_NR is a unique ID for a polygon feature and SE_IN is the next polygon feature in terms of flow direction. What I have is SE_NR and I want to get SE_IN.

SE_NR       SE_IN
882682001   882682006
882682003   882682006
882682006   882682005
882682007   882682005
882682005   882682004
882682002   882682004
882682004   882683006
882681001   882681002

Answer 1

Even though you did not tag your question with GRASS, I would mention that the GRASS module r.stream.order captures into the streams attribute table the ID's for next_stream and the prev_stream for each stream reach. You could then overlay the streams with the catchments, to get these stream attributes into the catchment attribute table.

As an aside, note that it is possible that a basin has two outlet drainage points (with two different downstream catchment ID's). This situation is rare but it can happen, for example, when a stream encounters a large island. Flow can split around the island, creating two separate downstream catchments.