I am a hydrologist and there are 2 important pieces of information I need for modeling. I need to know the longest flow path for each sub catchment in my area of interest and I need to know the elevations of point specific distances along those paths.

As an example, I have a shapefile with specific boundaries that represent my catchment and an elevation raster. I would like to be able to determine the longest path that water flows in each of these defined areas. Once I have a shapefile containing the longest flow paths for each catchment, I would like to find the elevation at 10% and 90% the length of each flow path and the distance between each point.

I have not found either of these functions in anything except the Arc Hydro tool bar for ArcGIS. I know that the individual tools for creating these things exist in QGIS ( through SAGA and GRASS ) but I am new to Python and not a programmer in any respect. I am hoping these tools are in an existing package and I am just too new to QGIS/GRAS/SAGA to find them.

I have been using QGIS for about 18 months.

1 Answer 1


If you have QGIS 2.0, you will find these tools in the processing toolbox. If you cannot see the toolbox, then you can activate it under: View--> Panels --> Toolbox

go to the bottom and change from simplified interface to advanced interface. You should now be able to see the Grass and Saga tools.

  • I have looked extensively in the sextant tool bar and these functions do not exist. Individual components needed to generate these things exist ( D8 direction grids, drainage basin, flow direction, etc ) but there is no tools to use this information together to determine longest flow path in the default qgis 2.0.1 install. I have even tried to get tauDEM tools to run but they never produce a file. Always gives me an error saying the output file can not be opened.
    – G.Oxsen
    Nov 14, 2013 at 19:45
  • 1
    You will have to generate the different components, streams, and flow direction yourself. It boils down in the end to two tools: r.watershed elevation=elev drainage=dirs streams=stream treshold=<some value> r.stream.order streams=streams dirs=dirs hack=longest where the first stream will be the longest See this website for more information: link These tools should be part of the standard installation of QGIS
    – mace
    Nov 14, 2013 at 20:05
  • I will read up on these tools and the link you provided and report back with my results.
    – G.Oxsen
    Nov 14, 2013 at 20:49

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