A MULTITEMPORAL NDVI ANALYSIS FOR UNDERSTANDING THE ANTHROPICINFLUENCE ON THE RECENT EROSION DYNAMICS IN THE UPPER ORCIA VALLEY(CENTRAL ITALY)

Mediterranean environments turn out to be extremely shaped by human actions, andmany landscapes that currently look natural are the result of centuries of land domestication.Previous decadal studies have highlighted the intense land transformation that the Tuscanlandscape of the Upper Orcia Valley, in central Italy, underwent during the last century. Thearea is characterized by accelerated erosion landforms, such ascalanchi andbiancanebadlands, that have been diffusely levelled for land reclamation. A recent insight into thearchive of the Land Reclamation Authority of the area, funded in 1929, has allowed an in-depth comprehension of the reclamation works started almost 100 years ago. Consequently,the multitemporal analysis of the erosion rates shows a decreasing trend for the last decadesbut a parallel increase of mass wasting processes.In order to better understand the contribution of the land cover and use changes tothis erosion dynamics, the results of a multitemporal geomorphological survey and the analysisof the land use changes were compared to the spatio-temporal computation and analysis ofthe NDVI (Normalize Different Vegetation Index), commonly used in the ecological field as anindicator of the green biomass effects on soil erosion.NDVI was computed on Landsat and Sentinel multispectral imageries (Landsat 1-2-4-5-8, and Sentinel-2), selecting every 10 years the images acquired in spring and winterseasons, within the 1975-2021 time interval. The computed NDVI values were classified in fourcategories as i) no vegetation, ii) sparse, iii) moderate and iv) dense vegetation, interpretingthe first class as including bare lands typical of badland areas.The NDVI analysis revealed a general decrease of bare lands and a considerableincrease of dense vegetation cover. The overlay with the multitemporal geomorphologicalmapping showed a recolonisation by forests along the main riverbeds and on the badlandareas. A general decrease of sediment supply from hillslopes can be linked to the observedchannel narrowing and incision trend along the main rivers. These results can be explained bythe progressive increased land use for agricultural purposes, artificial reforestation but also bythe gradual abandonment of rural areas, which has led, recently, to the reconquest of broad-leaved forests during the last decades