Agricultural Policy has been a major driver of Land Use Changes in Portugal, especially the south-eastern Alentejo: the commons division in the early 20 th century, autarkic policies from the fascist regime, and subsidies for forestry, cereal, and cattle production since joining the common market. This study focuses on the Serra de Mértola, an area with poor soils historically subject to land degradation. Data from the Vale Formoso Erosion Centre (1961-present) was used to compute erosion rates under different land uses representative of the study area. Higher values were found in vertical Fallow (964kg/ha/yr), Wheat (90kg/ha/yr) and horizontal Fallow (66kg/ha/yr), with lower values associated with Spontaneous Vegetation (3kg/ha/yr), Quercus (4kg/ha/yr), and Pines (7kg/ha/yr). Analysis of trends in Landsat NDVI values allowed to discriminate between land cover change and natural oscillations in weather and vegetation growth: in dry years NDVI values decrease for winter and spring, maintaining lower values throughout the year, while natural vegetation recovery shows a slow but steady increase until it stabilizes or breaks due to human action. Breaks in NDVI values were correlated with occasional ploughing in pastureland and spontaneous vegetation as well as the different moments in the agricultural year for cereal production. Three tendencies were observed: in Cultivated areas, ploughing provides higher sediment availability in valley floors, resulting in aggradation; Pastureland shows rock fragments on slopes and lower sediment availability, leading to incision on valley floors; Spontaneous Vegetation/Abandoned Land leads to a sharp decrease of sedimentary yield, with incision until its natural growth stabilizes hillslope processes.

Susceptibility to desertification increases as climate change leads to warmer and drier conditions, increasing fire risk and restricting plant growth. In Mainland Portugal, the regions with increasing aridity have also been subject to major changes in the socio-economic tissue, with population loss, larger farm areas and the overreach of irrigated agriculture to marginal lands leading to an increase in land use intensity. However, the last effort to map environmentally sensitive areas to desertification in mainland Portugal was in 2004. This study aims to apply an adapted Medalus method for mapping environmentally sensitive areas to desertification at the national level in mainland Portugal over twenty years (2000-2020). Results show both an increase in areas susceptible to desertification (58% to 74% of the country) and in intensity (50% to 59% of the area classified as ‘very highly’ or ‘critically’ susceptible). Desertification is a growing threat, especially for rural areas in inner and southern Portugal. This is mainly due to increasing aridity and land use intensity. Future studies should focus on understanding the controlling factors at the landscape and property scale, as well as including other variables such as soil carbon content and land management practices.