Geografía e Historia

Investigations of the hydrological processes operating in a small experimental catchment representative of the dehesa ecosystem were carried out. The dehesa constitutes a system of agro-silvo-pastoral landuse, which is characterized by a Mediterranean, semi-arid climate. The study includes an analysis of the relationships between rainfall, soil water content and discharge, as well as the establishment of the annual water budget. The results demonstrate a complex hydrological response. The relationships between the factors involved and the operating processes are difficult to explain because of the decisive role played by the valley bottoms. These areas typically possess a sediment fill, and contrast with the shallow soils developed on the hillslopes. Genesis and quantity of runoff (Hortonian or saturation) measured at the outlet depend on the antecedent moisture conditions of the valley bottoms because of their water-retention capacity. Annual runoff coefficients are similar to those reported from other semi-arid areas. The analysis of the annual water budget shows that rainfall is positively related with both actual evapotranspiration and discharge. ᭧

The /// dehesas and //// montados of Spain and Portugal cover about 6 million ha and form open savannah-type woodland comprising cork (Quercus suber) and holm (Q. rotundifolia and Q. ilex) oaks. Following their decline until the 1960s, these land management practices have become valued at national and international policy-making levels for their biodiversity, aesthetic qualities and potential for tourism and recreation, but comparatively little attention has been given to consequences for soil conservation. In this paper, the soil erosion risks associated with these land uses are compared with those of selected alternative forestry and agroforestry land uses. Selected soil properties, results of rainfall simulation experiments and field observations of erosion features for (a) two /// / montado and three // / dehesa sites differing in terms of grazing pressure and tree type, (b) six alternative forestry plantations of various ages and (c) an olive grove are ranked according to relative erosion risk. None of the // / dehesa or /// / montado sites has an overall mean rank indicating an erosion risk as low as that in established forestry plantations (at least four-five years old). One (heavily grazed) // / dehesa has a relatively high overall mean rank indicating a relative erosion risk only slightly lower than that obtained for an erosion-prone, newly ploughed and planted eucalyptus plantation. It is concluded that present day // / dehesa and /// / montado variants can be prone to severe erosion if stocking rates and/or tillage frequency become excessive. The advantages and limitations of the ranking methodology used are discussed.

An assessment of the sensitivity to land degradation have been carried out for the region of Extremadura, SW Spain, by means of the modelling approach developed in the European Commission funded MEDALUS project (Mediterranean Desertification and Land Use) which identifies such areas on the basis of an index (ESA index) that incorporates data on environmental quality (climate, vegetation, soil) as well as on anthropogenic factors (management). Two maps of environmental sensitivity to degradation with different legend resolution (4 and 8 classes of sensitivity) have been made. The results of the model were validated at different spatial scales, i.e.; regional and farm scales. True field data on degradation-related variables at the different spatial scales have been used in order to undertake the validation procedures. Selected farms covering different physiographical and socioeconomic characteristics were used to gather data at farm scale. Data gathered from the National Inventory of Soil Erosion was also used to validate the model at the regional scale.

A preliminary study on water resources for livestock rearing in SW Spain is presented. The objectives of this work were to assess the availability of water for domestic animals in farms and to explain whether water shortages are controlled by environmental factors or are also related with land management. The study was carried out in 54 private farms of wooded and treeless rangelands in Extremadura. Field work, digital topographic maps and aerial photographs were used to assess the potential hydrological resources of each farm. The water needs of livestock were estimated considering the number and species of animals per farm, as well as other factors influencing their water consumption. Furthermore, semistructured interviews were carried out in order to gather information about water management. Results indicate that almost 20% of the farms suffer problems of water availability in summer and almost 50% during droughts. Different solutions are adopted by the farmers, including construction of infrastructure, bringing water from outside or moving the animals to other areas. Data indicate a positive relationship between problems of water supply and the degree of aridity. Data also point to a relationship with water consumption by livestock and the availability of water by artificial ponds.

Water erosion and its relation to livestock grazing in semi-arid to dry subhumid Mediterranean rangelands with a disperse tree cover is analyzed, based on a variety of studies carried out since 1990 in Extremadura, Spain. The dominant factors of sheetwash are rainfall intensity and soil surface cover, the latter being controlled by rainfall amounts and livestock density. The discontinuous valley bottom gullies present a complex relationship with catchment hydrology. Giving the high temporal variability of sediment losses, the influence of livestock on gully erosion is difficult to determine with short-term studies. Analysis of aerial photographs (1945-2006) of the Parapuños study catchment point to an enhanced gully activity during the last decade as a consequence of increased livestock numbers.

Gully erosion represents an important soil degradation process in rangelands. In order to take preventive or control measures and to reduce its environmental damages and economical costs it is useful to localize the points in the landscape where gullying takes place and to determine the importance of the different factors involved. The study is carried out in Extremadura, southwest Spain. The main objectives of this work are: (a) comparing two nonparametric schemes to model the potential distribution of gullies, (b) evaluating the importance of the different factors involved in gullying processes, (c) analyzing the role of prevalence in the success of the model and finally, (d) implementing and mapping the results with the help of a Geographical Information System (GIS). Two methods were used to model the response of a dependent variable (gullying) from a set of independent variables: Classification And Regression Trees (CART) and Multivariate Adaptive Regression Splines (MARS). Three different datasets were used; the first one for constructing the model (training dataset) and the others for validating the model (external datasets). These datasets are formed by a target variable (presence or absence of gullies) and a set of independent variables. The dependent variable was obtained by mapping the locations of gullies with the help of a GPS and high resolution aerial ortophotographs. A set of 32 independent variables reflecting topography, lithology, soil type, climate, land use and vegetation cover of each area were used. The performance of the models was evaluated using a non-dependent threshold method: the Receiver Operating Characteristic (ROC) curve. The results showed a better performance of MARS for predicting gullying with areas under the ROC curve of 0.98 and 0.97 for the validation datasets, while CART presented values of 0.96 and 0.66.

The capability of Artificial Neural Network models to forecast near-surface soil moisture at fine spatial scale resolution has been tested for a 99.5 ha watershed located in SW Spain using several easy to achieve digital models of topographic and land cover variables as inputs and a series of soil moisture measurements as training data set. The study methods were designed in order to determining the potentials of the neural network model as a tool to gain insight into soil moisture distribution factors and also in order to optimize the data sampling scheme finding the optimum size of the training data set. Results suggest the efficiency of the methods in forecasting soil moisture, as a tool to assess the optimum number of field samples, and the importance of the variables selected in explaining the final map obtained.

In this paper, five topsoil properties (clay, silt, sand, organic matter and bulk density) have been modeled using accessible environmental information in order to improve pedological cartography in the Extremadura region (Spain). Independent variables related to topography, climate, lithology and vegetation cover were used to generate the models. The statistical approach was based on using Multivariate Adaptive Regression Splines (MARS) to produce the models. The performance of the models was tested using Root Mean Square Error (RMSE), Generalized Cross Validation (GCV) and the r coefficient (classical regression analysis). The models presented moderate power prediction; with r ranging from 0.23 for organic matter to 0.47 for silt. The most important independent variables supporting the models were location (X and Y UTM coordinates), lithology and altitude. Finally the models were compiled and implemented into a Geographical Information System (GIS) to obtain continuous maps of target variables. I.

The paper presents a pilot research on the determination of historical soil erosion rates in rangelands with disperse tree cover using two different methodologies: 137 Cs and exposed tree roots. The study was carried out along a hillslope in a Mediterranean rangeland farm with disperse tree cover of evergreen oaks in SW Spain. Mean soil erosion rates determined with both methods were fairly similar, with 18,7 and 19,6 Mg ha-1 año-1 for 137 Cs and exposed tree roots, respectively. Although the results indicate that both methods could be successfully applied in our rangelands, future dendrogeomorphological research is necessary to determine the errors of estimation.

Iberian dehesas and montados are considered as highly valuable systems and examples of sustainable farmland practices. However, tree aging and the lack of tree recruitment are being mentioned as threads to its long-term conservation. At the same time, shrub encroaching due to land abandonment is happening in some areas, thus facilitating tree recruitment over those places. These farmland systems are highly determined by their history of land use and management. We hypothesized that the recent spatial-temporal dynamic of tree loss and tree recruitment could be adequately explained by land topographic features as final physical drivers of land use/management processes in dehesas and montados. By comparing aerial images taken in 1956 and 2009, we analyzed the temporal tree dynamics of four dehesa farms in Cáceres (Spain). Multivariate Adaptive Regression Splines (MARS) were used to build spatial models that explain the susceptibility of farm areas to tree recruitment or tree loss as de...

Rangelands in SW Iberian Peninsula occupy approximately 95,000 km2 of land grazed by millions of domestic animals. They are characterised by grasslands with varying tree density, interspersed in some areas with shrubs. The herbaceous layer is dominated by annual species and the tree layer is mainly formed of evergreen oak. Most rangelands are exploited in large farms (>100 ha), held mainly on private ownership and dedicated to extensive livestock breeding. Soil organic carbon (SOC) is an essential component of the fertility and productivity in both, natural and human-induced ecosystems. Previous research evidenced SOC is concentrated mainly in the top 5 cm soil layer, displaying large spatial variability, with higher values beneath tree canopies as compared to the open spaces. Traditional practices such as cereal cultivation, as well as an increasing number of domestic animals in the last decades, have been remarked as some of the main causes of SOC depletion in rangelands. However, a better understanding of how livestock grazing affects soil organic carbon stock is still needed.

Gully erosion plays an important role in degradation processes of Mediterranean environments. Relatively little is known about relationships between gully erosion rates and rainfall or discharge in these areas. The aims of this work are: quantifying gully erosion, analyzing its spatial and temporal variability and evaluating its relationships with catchment hydrology. The study was carried out in a small catchment