Application of RUSLE model for the assessment of soil erosion in the western sector of Palermo Mountains (north-western Sicily)

Abstract

The aim of this work is to study the impact of vegetation cover on water erosion phenomena by exploiting an integrated approach to the assessment of soil loss rates. The investigated area is located in the western portion of the mountainous group named “Monti di Palermo” and occupies an area of approximately 475 km2; this area, which extends from sea level up to about 1,300 m, is mainly characterized by the presence of lithosols and luvisols laying above carbonate and terrigenous substrates. The assessment of erosion intensity was indirectly obtained by applying the RUSLE model, that allowed to estimate soil loss produced by rill-interrill erosion phenomena. Starting from topographical and thematic maps, pluviometric data and field surveys, a GIS grid layer, for each of the erosion control factors, was defined; by exploiting ArcView GIS 3.2 spatial analysis tools, all the factors’ grids were overlayed and multiplied to produce a grid map, made up of 89,001 square cells, showing the spatial variability of predicted erosion rates. The RUSLE application, here presented, distinguishes for the approach used to define the C factor (cover-management factor), which was calculated using a landscape and plant biodiversity map, rather than a land use one; this map was constructed by identifying and detecting plant communities with a phytosociological approach. The mean annual soil loss, which average value for the entire area is equal to 9.16 t ha-1 year-1, shows a spatial variability that is mainly controlled by the geographical distribution of topographic and cover-management factors. Sever soil erosion rates (>40 t/ha-1 year-1), in effect, fall on abandoned fields, grasslands and garrigues areas, characterized by high altitude values and steep slopes, often affected by fire. In order to detail the spatial variability of the C factor, the analysis was focused on the “Vallone San Vincenzo” river basin, that extends for 33 km2 in the NW sector of the “Monti di Palermo” region. In this area, analysis of high-resolution orthophotos and field surveys of the phytosociological associations allowed to discriminate, within vegetation categories, some sub-categories related to percentage of area covered by canopy of trees and undergrowth, type and height of vegetation; these sub-factors permitted to define a more detailed and reliable cover-management factor layer. By intersecting the latter with the others erosion factors layers, a mean annual soil loss map of “San Vincenzo” river basin was produced. The two differents approaches of evaluating the C factor result in different average values of mean annual soil loss rate in the “San Vincenzo” river basin; the second method provides an average value of soil loss equal to 9.3 t ha-1 year-1, which is about 10% lower than the correspondent one (10,6 t ha-1 year-1), obtained by applying the first method. This work reveals the possibility to define the C factor by exploiting phytosociological surveys; the latter permitted to comprise the effect of vegetation canopy and ground covers in reducing soil loss, providing a more comprehensive and multidisciplinary approach in evaluating water erosion.