Quantitative geomorphological and environmental analysis requires the adoption of well–defined spatial domains as basic mapping units. They provide local boundaries to aggregate environmental and morphometric variables and to perform calculations, thus they identify the spatial scale of the analysis. Grid cells, typically aligned with a digital elevation model, are the standard mapping unit choice. A wiser choice is represented by slope units, irregular terrain partitions delimited by drainage and divide lines that maximise geomorphological homogeneity within each unit and geomorphological heterogeneity between neighbouring units. Adoption of slope units has the advantage of enforcing a strong relation with the underlying topography, absent in grid cell–based analyses, but their objective delineation is a challenge. A given study area admits different slope unit maps differing in number and size of units. Here, we describe a parametric delineation for slope units, using the r.slopeunits software in GRASS GIS.

The r.slopeunits software implements an algorithm that defines a mosaic of SUs bounded by drainage and divide networks based on iterative extraction of the boundaries of hydrological “half-basins” from terrain information available from a DEM. To single out individual SUs from the half-basins, r.slopeunits assumes that homogeneity (heterogeneity) within (between) SUs is controlled by the variability of terrain aspect in each SU. The procedure begins with the delineation of a set of a few large half-basins characterised by a large upslope contributing area, which is iterated - for half-basins not meeting size and homogeneity criteria - with decreasing values of the contributing area, until the entire study area is covered by a mosaic of SUs, of different sizes and shapes.

The aim of the short course is to enable the attendees to use r.slopeunits in their laptops. We will provide a Linux virtual machine (intended for VirtualBox, running on any operating system) containing the software and necessary libraries. Attendees will need basic understanding of GIS concepts, and they will be guided through the specifics of processing within GRASS GIS for execution of r.slopeunits. They will be guided through a complete example of slope unit delineation, starting from the import of the DTM into the GRASS GIS environment to the production of slope units in vector layer format.

Organized in cooperation with NhET (Natural hazards Early career scientists Team)