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.