Analyzing anthropogenic impact on river morphodynamics in the Upper Orcia Valley (central italy) through multitemporal NDVI assessment

Mediterranean environments are significantly shaped by human activities, and the morphology of rivers often reflects centuries of human intervention for land development. Previous studies, spanning decades, have highlighted the drastic river adjustments in the Italian landscape. These adjustments often reflect the man-induced land use changes within the drainage basin. The analysis of the rivers in Tuscany site (central Italy), has revealed profound changes, especially in terms of channel bed narrowing and incision. This work present the fluvial system scale analysis of the anthropogenic impact in the Upper Orcia Valley (southern Tuscany), that undergone a huge land reclamation, as evidenced by the archives of the Land Reclamation Authority, established in 1929. Accelerated erosion landforms, such as calanchi and biancane badlands, have been extensively reduced by this intervention. Consequently, a multitemporal analysis indicates a decreasing trend in erosion rates over the recent decades, accompanied by an increase in agricultural and forested areas and a narrowing of river channels. To comprehend the role of land cover and land use changes in river dynamics, the results of a multitemporal geomorphological survey and an analysis of land use changes were compared with spatio-temporal computation and analysis of the NDVI (Normalized Difference Vegetation Index), commonly used in ecology to gauge the impact of green biomass on soil erosion. NDVI was computed using Landsat and Sentinel multispectral imagery (Landsat 1-2-4-5-8 and Sentinel-2), selecting images acquired every 5 years in May, corresponding to the Start of Seasons (SOS) month, within the 1975-2021 timeframe. The results revealed a general decrease in bare lands and a significant increase in dense vegetation cover. The overlap this data with multitemporal geomorphological mapping demonstrated a recolonization by forests along main riverbeds and in badland areas, indicating a reduction in sediment supply from hillslopes, possibly causing the observed channel narrowing and incision trends along main rivers. These findings can be attributed to increased land use for agriculture, artificial reforestation, and the gradual abandonment of rural areas, leading to the recent reconquest of broad-leaved forests.