Geo-environmental factors controlling gully distribution at the local scale in a Mediterranean environment

The study was conducted with a view to identifying the primary factors controlling the formation and evolution of gullies in Central Portugal. A total of 38 permanent gullies were identified in an 11 km2 area. Their mean length, maximum width, and mean width were 105 m, 6.8 m, and 3.8 m, respectively, based on orthophotograph analysis. The gullies are located in an area of highly weathered granites which led to the formation of deeply weathered mantles. To understand gully distribution, 11 geo-environmental controlling factors were selected. Group Analysis (GA) identified two groups of gullies. These are differentiated not only by morphological aspects but also by geo-environmental elements. One group (Gg1) is characterised by being larger in size, and these tend to be located on hillslopes with a higher slope. The second group (Gg2) contains the smaller gullies that are located on the lower slopes of hillsides. Principal component analysis (PCA) identifies as controlling factors the slope, topographic ruggedness index (TRI), and vegetation gain (VG), after the October 2017 wildfire. Although this wildfire affected all the study area, the creation of higher burn severity mosaics positively enhanced the drainage and accelerated the gully erosion of Gg1, as well as the location of the nearest roads and paths, which emerge as a primary factor in the initiation and enlargement of this group of gullies. This study helps to better identify the current gully erosion conditions, and this can provide an important reference for risk assessment and control of gully erosion. Given the size of the gullies and the likelihood that recurring forest fires will affect the study area, some vital gully erosion control practices should be implemented urgently.