Isochrone along direction map of watershed

Question

I am interested in producing a isochrone map given a raster speed map and a direction map (like the output of r.watershed), but I do not see how to produce it.

I have studied r.walk and r.cost wich calculate accumulated cost on a friction map, but it is not intended to work with a direction map.

I have been trying to see if there is a way to produce the inverse of an accumulative map like the result from r.drain but backword, so given the outlet and the direction maps, assign a cumulative cost for each cell of the catchment area from the sarting point (downstream).

I am familiar with GRASS and QGIS, but any solution with a free software is welcome.

Answer 1

Here how "Isochrone Variable Speed" from SAGA works:

1. First you need to load your filled DTM into SAGA
2. Then you need to run the "Slope, Aspect, Curvature" tool as shown in the two pictures below. (only the slope layer will be used in the "Isochrone" tool)

3. Then click on "Geoprocessing" ; "Find and run tool" and look for "catchment"

4. Select the catchment area method you want to use

5. Select in "grid system" the parameters associated to your DTM and use your DTM in the "elevation grid" line

6. In "method" use "Needs more memory but is quicker" (this step might be very long, if it is the case I would advice you to add directly a catchment area layer calculated in QGIS)

7. Once your catchment area layer has been added to your saga project (as shown below), double click on it to add it to the map.

8. Then go in "Geoprocessing" "Terrain Analysis", "Hydrology", "Dynamics", "Isochrone Variable Speed"

9. Once you have set up the inputs, click on the black arrow as shown below:

10. Once the arrow is selected you can directly click on your outlet to run the "isochrone variable speed" tool (if your drainage basin is big it might take a few minutes for the results to be loaded)

You should get something like that:

Isochrone output (in hours):

SAGA GIS is not as intuitive as QGIS to use but it doesn't take too long to get familiar with ! Let me know if you have some troubles running the tool ! :)

MAJ QGIS 3.22 and < :

The Isochrone Variable Speed tool is now available in QGIS within the SAGA core plugin

Answer 2

There's another trick you might try: You can invert the DEM, so that ridges become valleys and stream outlets become the peaks. Then run r.watershed, and the resulting flow accum should be highest at the original "upstream" mountains, and lowest at the original drainage points.