Soil degradation phenomena, including water erosion and physical and biological processes, have already been reported in rangelands of southwestern Spain. The increasing numbers of livestock since 1986 have been highlighted as one of the key causes. The main goal of this work is to analyse the effects of the excessive number of animals on soil quality and pasture production on privately owned farms dedicated to extensive ranching. Soil properties and surface cover, pasture production, rainfall and land management variables such as livestock density were analysed during a period of 3 years (2008)(2009)(2010)(2011). The study was carried out in 22 livestock enclosures selected from ten farms distributed throughout the Spanish region of Extremadura. The occurrence of bare soil patches and water erosion processes, as well as an increase of mean bulk density in the soil layer from 5 to 10 cm in depth, was observed in the enclosures with animal stocking rates exceeding 1 AU ha À1 . Indications that confirm the negative effects of increased bulk density on pasture production and quality were also found.

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An assessment of sensitivity to land degradation has been carried out in the Extremadura region, SW Spain, by means of a modelling approach developed by the European Commission funded MEDALUS project (Mediterranean Desertification and Land Use) which identifies such areas on the basis of an index (Environmentally Sensitive Area index, ESA index) that incorporates data on environmental quality (climate, vegetation, soil) as well as anthropogenic factors (management). Two maps of environmental sensitivity (ES) to degradation with different legend resolution (four and eight classes of sensitivity) have been made and tested by comparing classes of the legends with an extensive number (2690) of true field data gathered from plots distributed all over the study region. Independent variables of validation consisted of nine degradation-related types of data and the method tested the performance of the whole model and the statistical separability among classes of sensitivity, as well as the capability of the variables in delimiting the classes. Results showed a good performance of the whole model to both, the map of four and eighth classes of sensitivity. Separation among classes of sensitivity showed a slightly different behaviour of both maps, identifying transitional classes in the map of eight classes where classification could be improved in terms of the ranges of ESA index values assigned to the different classes.

Gully erosion plays an important role in degradation processes of Mediterranean environments. In this paper aerial orthophotographs were used for (i) analysing the evolution of a valley bottom gully and its relation with land use and vegetation cover, (ii) exploring the role of land use and vegetation cover on the coefficients of the equation S ¼ aA Àb (where S is slope at the headcut and A is drainage area), which is based on the topographical threshold concept and is commonly used to predict gully initiation. The study was carried out in a small catchment (99Á5 ha) located in the southwest of the Iberian Peninsula. Gullies and headcuts were mapped together with land use and vegetation cover using aerial photographs for the years 1945, 1956, 1989, 1998, 2002 and 2006, which had to be digitized and orthorectified in advance. The results showed an increase of the area affected by gullying from 695 m 2 in 1945-1009 m 2 in 2006, reaching a maximum of 1560 m 2 in 1956. Gullying was closely related with land use, especially with the amount of cultivated areas within the catchment and also with grazing intensity. No clear relationship was found between the evolution of the gullied area and rainfall amounts. Finally, the values of the exponent b obtained for different headcuts and different dates (close to 0Á4) were similar to those proposed by other authors for gully erosion caused by Hortonian overland flow in semiarid environments.

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