A risk map for gully locations in central Queensland, Australia

In central Queensland, Australia, relatively little is known about where gullies occur ('gully presence'). This is despite a general acceptance among scientists and politicians that gully erosion in the region is an ecologically important process, exacerbated by grazing pressure. We aimed to create a risk map of gully presence for a 4.86 × 10⁶-ha area of central Queensland dominated by grazing and thought to be particularly prone to gully erosion. We achieved this by using (i) light detection and ranging (lidar) technology (vertical accuracy < 0.15 m; spatial resolution 0.5 m) to observe topography on transects at eight selected sites within the study area, (ii) object-oriented classification to derive gully presence from lidar observations and (iii) a random forest to model the relationship between gully presence and a set of readily available explanatory variables (comprising soil, topography, and vegetation information; finest spatial resolution 25 m) and (iv) extrapolating the model to unsampled locations. Cross-validation indicated that the predictive ability of the model was modest, with an average area under the receiver operating characteristic curve of 0.62 (where 1.0 is a perfect model and 0.5 is no better than chance). The greatest risk of gully presence was associated with areas of large topographic variation, and where, coincidentally, there was relatively little long-term vegetation cover. Ultimately, however, we acknowledge that the quality of the map is limited by the small area of observed lidar data relative to the study area, the relatively coarse spatial resolution of the explanatory variables and the possibility that gully presence is the result of different processes at different locations.