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I'm delineating some coastal subbasins using open source software. I have asked a similar question before (Delineating coastal subbasins using SAGA GIS or GRASS in QGIS?). I found that using SAGA Fill Sinks (Wang & Liu) in QGIS creates watersheds according to what I expect. the problem is I don't find any documentation about how this tool creates the watersheds.

I looked for in the tool documentation (Module Fill Sinks (Wang & Liu)), but it doesn't say how the tool works. Most of the tools used in SAGA, GRASS and other softwares create watersheds based on a previously created channel network, but here the only input is a DEM.

Does anyone know how this tool create watersheds?

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If you look closely at the tool documentation, there's some references info . The problem is that the text needs to be bought or to have some student account access.

Wang, L. & H. Liu (2006): An efficient method for identifying and filling surface depressions in digital elevation models for hydrologic analysis and modelling. International Journal of Geographical Information Science, Vol. 20, No. 2: 193-213.

The way the tool works seems quite clear to me. It seeks to preserve downward slope by optimizing the pit removal.

The method was enhanced to allow the creation of hydrologic sound elevation models, i.e. not only to fill the depression(s) but also to preserve a downward slope along the flow path. If desired, this is accomplished by preserving a minimum slope gradient (and thus elevation difference) between cells.

The documentation refers to the other modules developped by the same person, where you might find some additional info on each detailed operation. This module does what the other modules do. You will probably have more info on each detailed module :

This is the fully featured version of the module creating a depression less DEM, a flow path grid and a grid with watershed basins

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  • I checked Wang & Liu paper, but I don't fully understand how the algorithm delineates watersheds, I don't want to completely understand the algorithm, but I would like to know how it compares to other watershed delineating algorithms. Citing Wang & Liu paper "Besides depressionless DEM and the locations of surface depression, our method can simultaneously generate flow direction and watershed partition with one pass of processing. The watershed partition is obtained by successively labelling nodes on each tree of optimal paths during the least-cost search."
    – Ernesto561
    Commented Sep 18, 2018 at 2:55
  • The minimum area for the watersheds cannot be chosen, nor the watersheds based on a threshold like Strahler order or number of cells of a flow accumulation raster. What I like about this algorithm is the way it delineates coastal subbasins, but I don't understand how the the size of the watersheds is chosen, or what represents the delineated watersheds.
    – Ernesto561
    Commented Sep 18, 2018 at 3:00
  • I see ! I guess the easiest way would be to contact the author (Volker Wichmann) : sourceforge.net/u/userid-1223006/profile
    – gisnside
    Commented Sep 18, 2018 at 7:35

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