Slope dynamic processes in a medium-low permeability catchment in Central Italy

Purpose: At the catchment scale, land cover changes can affect the slope dynamic, exacerbating the effects of intense rainfall. In this framework, the present study analyses rainfall-runoff transformation in a marly-calcareous-arenaceous watershed in Central Italy (La Bruna basin, 32 km2). The area is characterized by medium to low permeability soils, and the slopes are affected by different processes, such as runoff and shallow landslides. As recently reported by [1], runoff depth plays an important role when calculating the stability of shallow soil slopes. In the land-use scenario of the selected basin, we present runoff generation thresholds based on event rainfall depth and antecedent soil moisture content. Methods: The following methods and datasets are used: - Multi-temporal land-use changes by aerial photos and Corine Land Cover datasets. - Landslide inventories to collect information on the number, type, and distribution of slope movements. - Rainfall depth by 3-gauge stations and satellite products (e.g., GPM IMERG). Runoff values are derived from river discharge data using the Local Minimum Method. - Satellite soil moisture data from SSM1km Copernicus product (ESA), referring to the first 50 mm of soil (each 3-8 days since January 2015). Results: The multitemporal aerial photo investigation shows a gradual increase in forested areas between 1954 and 2000, especially in the mountainous sector of the basin, combined with a decrease in cultivated areas. During the 1990-2018 period, the temporal analysis of the Corine Land Cover datasets shows drop-in olive cultivation, with a 10% increase in non-irrigated arable land. The land occupied by anthropic settlements remained unchanged, while a significant reduction in coniferous forests was observed in favour of mixed forests during 2006-2012. Landslides are represented mainly by rotational/translation movement in agricultural land areas, covering about 12% of the area and are triggered by short duration and intense precipitations. Moreover, intense rainfall also affects the runoff processes and slope dynamics, leading some areas to be affected by severe erosion. Exploiting a database of 64 rainfall events that occurred within the 2015-2021 period an updated runoff generation threshold for the La Bruna watershed was derived and tested on five events that occurred in 2022. The derived threshold may be used in areas with similar characteristics. Conclusions: An integrated study about the slope dynamics in the La Bruna catchment highlighted that even more than 10 mm/5 minute rainfall deeply affected the slope dynamics. Identifying the runoff generation threshold can help understand runoff depth's role in triggering shallow landslides. The analysis allowed us to define the runoff generation threshold through event rainfall depth and the antecedent soil moisture content data at the event scale.