I am trying to replicate my erosion model in ArcGIS for Desktop in QGIS 2.4.

I tried to calculate the upstream distance along the flow path for my agricultural field using QGIS. I did the equivalent in ArcGIS with the tool Flow length and it is simple and effective.

I see two potential tools that can deliver a similar result in QGIS, r.flow (GRASS) or SAGA. The first tool, r.flow gives me a flow length path but the result make no sense and some output cannot be saved. The second tool flow path length with SAGA, I don't understand what is the seed and I don't see the option to calculate the upstream distance.

Arcgis Flow length (Value 0 to 840 meters, black to white): enter image description here

QGIS r.flow (GRASS) (Value 0 to 6.5, black to white): enter image description here

Here my input for the tool r.flow (GRASS) in QGIS 2.4

  1. LiDAR elevation, I did the treatment Fill Depression.
  2. Aspect raster.
  3. Barrier, I made a raster contains all 0 value. (I'm not very sure here, I didn't find so much information for this input).
  4. Calculate for upstream length.
  5. Others parameters are the default ones.

Suggestions for the appropriate tool to use are welcome.

  • What is the problem with r.flow for you? For an overview, see grasswiki.osgeo.org/wiki/Hydrological_Sciences#Flow_calculation – markusN Oct 25 '14 at 20:40
  • By flow length, I mean for each pixel i want to know the length of flow in the drainage network. I edit my post to show you the difference in my result. – FatAl Oct 25 '14 at 22:40
  • It looks to me like you need to fill the topographic depressions in the dem beforehand. – WhiteboxDev Oct 26 '14 at 13:06

It is very difficult to know exactly where things went wrong without knowing details about the inputs that you used for each tool. However looking at the two images that you embedded into your post, it looks to me like the common problem of running a flow path analysis on a non-hydrologically preprocessed DEM, i.e. a DEM for which you have removed all of the topographic depressions and flat areas through either depression filling or breaching. The ArcGIS tool requires an input of a D8 flow pointer raster, which clearly you provided from a depressionless DEM. I think you may have provided the r.flow tool input the original DEM which contained depressions. Take the following images as examples:

enter image description here

enter image description here

The first image is a maximum upslope flow-length grid calculated from a depressionless (breached) DEM and the second one was derived from the original DEM, which contained numerous artifact topographic depressions. You'll notice that the flow paths have been severely truncated because as flow encounters a depression or flat area, there are no downslope grid cells for the flow path to continue towards. Just as is the case with your data, the maximum flow path value in the grid is considerably less in the second raster as a result of this flow path truncation. I think you need to double-check that you input a hydrologically corrected DEM into the r.flow tool. I know from experience that this is one of the most common errors when performing DEM-based hydrological analysis. Otherwise, it is possible that you simply provided the r.flow tool the wrong input, e.g. the same D8 flow pointer that you provided the ArcGIS flow length tool (note r.flow requires a DEM and not a flow pointer as an input). Again this would be a fairly common mistake and the effect would be identical, i.e. truncated and discontinuous flow paths which is what you seem to have. I hope that helps.

  • Impressive answers and images as always! – Joseph Oct 27 '14 at 9:39
  • @user33272 Okay then, what were your input parameters to the r.flow tool? Which raster did you input? – WhiteboxDev Oct 27 '14 at 17:15
  • I edit my initial post for explain you my input. – FatAl Oct 28 '14 at 17:39
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    @user33272 Sorry I couldn't be more helpful then. The GIS can be downloaded here uoguelph.ca/~hydrogeo/Whitebox/download.shtml and the tool is called Maximum Upslope Flowpath length (there are also tools for the mean upslope flow path length, the downslope flow path length, the upslope area, etc.). The tool requires an input D8 flow pointer (calculated using the D8 Flow Pointer tool). The GIS is free and open source and cross-platform. Let me know if you have any problems. – WhiteboxDev Oct 28 '14 at 18:56
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    @user33272 Sorry, I should have said that the tools of interest are contained within the Hydrological Tools toolbox and the Terrain Analysis => Flow Path Terrain Attributes toolbox. And you'll need Java 8 installed on your system. – WhiteboxDev Oct 28 '14 at 19:00

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