MATERIALS AND METHODS

Study area

The study area is located in the Alentejo region (Fig.1), southeastern Portugal. The Alentejo represents the largest region of Portugal, with a total area of about 31,500 km2 hosting 5% of the entire Portugal population. Morphologically, the latter is an area with relatively low reliefs, where the elevation varies from 0 to 460 m above sea level (a.s.l.) (Fig. 2a). From a geological point of view, the basin mainly consists of metamorphic schists, greywackes, and conglomerates, distinguished by skeletal low productive soils (Chambel et al., 2007). The whole region has a typical Mediterranean – Continental climate, characterized by very hot and dry summers (Beck et al., 2018), with the highest amount of rain distributed during the winter season and drought periods (April-September) occurring during the year (Fig. S1). The mean annual precipitation ranges from 400 to 600 mm in the lowland and can reach up to 900 mm in mountain areas. In general, most of the annual rainfall is concentrated over 50-75 days (Ramos and Reis, 2001). The mean temperature is between 15.0 °C and 17.5 °C, while the potential evapotranspiration (PET) is generally higher than 1000 mm per year, causing a high water-soil deficit. Three main soil groups characterize the watershed: Leptosols are the most frequent, followed by Luvisols and Vertisols (Fig. 2c). The prevailing land cover (Fig. 2b) is represented by annual rainfed crops (wheat and oats) followed by olive groves, cork oak (Quercus suber L.) woodlands alone or in combination with Quercus ilex L. and in some cases with Mediterranean shrubs. The “montado” ecosystems, representing the traditional agroforestry system of the Iberian peninsula with a savanna-like physiognomy, is characterized by grazing animals and by an open tree canopy woodland which can vary between 20 to 80 trees per hectare (mainly Quercus suber, Quercus ilex subsp. rotundifolia L) coexisting with grasses and scattered shrubs (Pinto-Correia and Mascarenhas, 1999). The entire region is presently under a high risk of desertification due to the presence of Leptosols (Fig. 2c), shallow and extremely gravelly soils naturally susceptible to erosion (IUSS Working Group WRB, 2015), to the intensive agriculture management and the overgrazing performed in recent decades (Roxo and Casimiro, 2004; Nùnes et al., 2008). The geomorphological, soil, and land cover characteristics of the four farms are presented in Table S1.