From a DEM of a watershed, how is possible to calculate the longest flow path?
I know this questions has been asked before but I haven't found a complete solution yet.
[GIS] Longest Flow Path in QGis
flow-maphydrologyqgis
Related Solutions
As long as you 'drew' the lines from upstream to downstream in a consequent fashion, you can do this through the styling options.
Right-click the shapefile layer, select Properties. Then select 'style' from the left hand side menu. Where you set the linestyle, on the top right you can choose a symbol line.
Then you can select which symbol (I used the default 'arrow'), and set rotation to 90 degrees to tilt it sideways. You can then adjust intervals, size, .. Note you can also set them to only be on the start or endpoint of a line, in case you made individual lines where you want arrows, rather then replacing a full line with an 'arrow line'.
I applied this on an overview of waterways in my area, with ortho imagery in the back, gives me this result:
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:
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.
Best Answer
After a long and painful process, I found a way to do it combining GRASS GIS and QGIS with GRASS.
Here is how I did it:
In QGIS load the DEM raster.
Using GRASS for QGIS run the r.watershed command (from the processing toolbox) setting an appropriate threshold.
In my case, I always try to make the watershed no bigger than the “acceptable area limit” for the Rational Method, which normally is 2 to 5 km², so (for example) using the ASTGTM2 DEM with a cell resolution of approximately 30m x 30m, the threshold should be (considering 2 km² of watershed): 2000000 / (30*30) = 2222 ≈ 2200.
Save all the outputs of the r.watershed command, specially Drain Direction and Streams.
Using the results r.watershed command, pick the desired outlet point coordinates and run r.water.outlet command. The result will be a raster with the complete basin that flows directly to your desired outlet point.
Clip the original DEM raster with using the basin result from step 3. This can be done by transforming the basin raster to a vector polygon and then clipping the DEM with that polygon or masking the DEM with the basin raster.
Once again, run the r.watershed only this time using the clipped DEM from step 4. The only result useful for the analysis will be the Drain Direction, the rest is optional.
This is the tricky part. Now you have to open GRASS GISS (the standalone program. It gets installed automatically with QGIS).
Create or open a mapset.
Two GRASS addons should be loaded before doing anything else: r.stream.distance and r.lfp. To do this in the console panel type g.extension and when asked for the new command type r.stream.distance, click execute and then repeat with l.lfp.
Restart GRASS GIS
Now load the Drain Direction raster from the clipped DEM, this is done with the command r.in.gdal and selecting the Drain Direction raster file.
Set up the correct GRASS region using the command g.region and using the Drain Direction raster as the input.
Once done, run the command r.lfp, the input raster should be the Drain Direction of the basin and the coordinates of the output point.
This point should be located on a stream from the Streams raster result of the r.watershed from step 3.
The result of the r.lfp command is a raster with the longest flow path of the basin. This raster can be transformed to a vector line with the command r.to.vect.line.
Maybe my whole process can be done more efficiently but this is how I manage to do so.
Hope someone someday find it helpful.