In addition to delineating watersheds (drainage basins) upstream of one or more outlet points using WhiteboxTools’ Watershed and UnnestBasins tools as illustrated in this post, WhiteboxTools has three other tools for watershed delineation – Isobasins, Subbasins, and Hillslopes. The input files of these three tools include a filled DEM raster file, a D8 Pointer raster file, and an ExtractStream raster file, all of which can be created by referring to the instruction here. Note that an outlet point shapefile is not needed since the delineation will be performed for the entire area covered by the input raster files.
Isobasins can be used to delineate nearly equal sized drainage basins for the area covered by the DEM file (Figure 1). The input of Isobasin is a filled DEM raster file. The target basin size is measured by grid cell numbers, meaning a 3 meter x 3 meter resolution DEM file with a target basin size of 100000 will generate drainage basins with an average area of about 0.9 sq km.
The output of Isobasins is a drainage basin raster file in which a single basin has the same pixel values (the pixel value is also used as drainage basin identifier). The drainage basin raster file can be converted to a polygon shapefile using WhiteboxTools’ RasterToVectorPolygons command (Figure 2).
If the option of Output basin upstream-downstream connections? is checked on in Figure 1, a CSV file denoting the basin connection relationship will be created in the same folder where the output raster file is saved (Figure 3).
The tool of Subbasins delineates drainage basins for each stream segment in the stream network identified in the ExtractStream raster file supplied (Figure 4). Instead of using a DEM file, Subbasins requires a D8 Pointer raster file as input.
After converting the drainage basin raster file to a polygon shapefile using RasterToVectorPolygons, it is clear that each stream segment of the stream network has a corresponding drainage basin (Figure 5). In Figure 5, the red dash line is the stream network, which is a line shapefile converted from the ExtractStream raster file via RasterToVectorLines.
Another tool, Hillslopes (Figure 6), is very similar to Subbasins except that Hillslopes further divides a drainage basin’s right bank and left bank areas of a stream segment as two different drainage areas (Figure 7).
Isobasins, Subbasins, and Hillslopes do not delineate watersheds based on a specific outlet location, and instead, they do the delineation for the entire area covered by the input raster files (D8 Pointer or Filled DEM), which is useful for an area-wide drainage study. It is obvious that the output of these three tools is a series of nested basins.