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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. Copyright © 2008 John Wiley & Sons, Ltd.

Preferential flow is known to influence hillslope hydrology in many areas around the world. Most research on preferential flow has been performed in temperate regions. Preferential infiltration has also been found in semi-arid regions, but its impact on the hydrology of these regions is poorly known. The aim of this study is to describe and quantify the influence of preferential flow on the hillslope hydrology from small scale (infiltration) to large scale (subsurface stormflow) in a semi-arid Dehesa landscape. Precipitation, soil moisture content, piezometric water level and discharge data were used to analyse the hydrological functioning of a catchment in Spain. Variability of soil moisture content during the transition from dry to wet season (September to November) within horizontal soil layers leads to the conclusion that there is preferential infiltration into the soils. When the rainfall intensity is high, a water level rapidly builds up in the piezometer pipes in the area, sometimes even reaching soil surface. This water level also drops back to bedrock within a few hours (under dry catchment conditions) to days (under wet catchment conditions). As the soil matrix is not necessarily wet while this water layer is built up, it is thought to be a transient water table in large connected pores which drain partly to the matrix, partly fill up bedrock irregularities and partly drain through subsurface flow to the channels. When the soil matrix becomes wetter the loss of water from macropores to the matrix and bedrock decreases and subsurface stormflow increases. It may be concluded that the hillslope hydrological system consists of a fine matrix domain and a macropore domain, which have their own flow characteristics but which also interact, depending on the soil matrix and macropore moisture contents. The macropore flow can result in subsurface flow, ranging from 13% contribution to total discharge for a large event of high intensity rainfall or high discharge to 80% of total discharge for a small event with low intensity rainfall or low discharge. During large events the fraction of subsurface stormflow in the discharge is suppressed by the large amount of surface runoff. Copyright © 2008 John Wiley & Sons, Ltd.

Soil is the largest reservoir of water in a catchment and evapotranspiration is the largest yearly output flux of water from the soil in semiarid environments at the catchment scale. Hydrologic stress is one of the typical characteristics of semiarid environments. The shortage of water affects all the biophysical processes and increases the nonlinearity in the rainfall–runoff relationship. A proper description of semiarid watersheds should include an accurate description of the wetting–drying cycle of the soil, which is critical to properly describe important processes such as the connection between the surface and the soil saturated zone, the production of runoff or the growth of plants. Evapotranspiration is typically modelled in comprehensive distributed hydrologic models (e.g. MIKE-SHE or MODHMS) using functions that relate evapotranspiration to soil moisture and plant biomass through a set of empirical coefficients, which typically adopt some default values regardless of the conditions of the area where the model is applied. In this work, using a global and a local search algorithm, a three-dimensional soil water flow model coupled to an evapotranspiration model is calibrated in a soil profile covered with grass in a semiarid environment. Four parameters for the water flow model and three parameters for the evapotranspiration model are calibrated. An existing trend between daily average soil moisture and daily maximum temperature is used as extra information during the calibration process. The results showed that the known relationship between maximum temperature and average soil moisture includes information on the seasonality of the atmospheric demand. Although total evapotranspiration is similar in all calibrations, the trade-off between evaporation and transpiration is sensitive to this extra information mainly through one model parameter. Further research is necessary to identify the value of the parameter that produces the correct trade-off. Copyright © 2008 John Wiley & Sons, Ltd.

Event-based hydrologic modeling is common practice for semiarid basins where runoff is restricted to short periods after a storm. Such models are used to predict runoff production and soil erosion in agricultural areas as well as the effects of storms on sewer systems, all in areas with limited information. Sometimes, model parameterization is done through infiltration experiments to obtain a parametric infiltration curve or using standard values in lookup tables associated with land use as it is often the case for hydraulic roughness. The model may then be used to predict soil losses or runoff production in storms of different intensities. In the present study a distributed hydrologic model was calibrated to see if rainfall-runoff events of different intensities in a single semiarid basin may have different optimal calibrated sets of parameters. To achieve this, 17 sequential events were calibrated covering a wide range of conditions and storm types in the semiarid southwest of Spain. Two parameters related to roughness (Manning's n and standard deviation of terrain micro-heights) and three related to infiltration (initial and final infiltration capacities and infiltration decay rate) were calibrated for each event. The results show that the calibrated set of parameters and their sensitivities change through time. The drift of the minima in the parameter space is partially explained by the type of storm. Hydraulic roughness and initial infiltration capacity showed the highest sensitivity to rainfall intensity, while steady state infiltration capacity showed sensitivity to information used as a proxy for the wetness state of the basin. The dynamics of the parameters and their relative sensitivities indicate that the model has to adjust itself to the different conditions of the basin so no single set of parameters characterizes the basin.

In this article, a dynamic simulation model is presented, designed to add to the understanding of commercial livestock–grass–soil systems and, especially, the processes of desertification due to overgrazing. The model has two key innovative features: (i) the joint dynamic treatment of the three mentioned stock variables and (ii) the consideration of livestock dynamics determined by economic variables. The main possible behaviours of the resulting three-dimensional system are presented and it is shown that there are significant differences with respect to the subsystem constrained to livestock and grasses as considered in the literature to date. After analysing the model qualitatively, we are able to propose some early warning indicators of the risk of long-term desertification due to overgrazing. In this article, we illustrate the use of those indicators in three hypothetical, yet likely extensive livestock farming scenarios in Spain.

An integrated dynamic model was used to evaluate the influence of climatic, soil, pastoral, economic and managerial factors on sheet erosion in rangelands of SW Spain (dehesas). This was achieved by means of a variance-based sensitivity analysis. Topsoil erodibility, climate change and a combined factor related to soil water storage capacity and the pasture production function were the factors which influenced water erosion the most. Of them, climate change is the main source of uncertainty, though in this study it caused a reduction in the mean and the variance of long-term erosion rates. The economic and managerial factors showed scant influence on soil erosion, meaning that it is unlikely to find such influence in the study area for the time being. This is because the low profitability of the livestock business maintains stocking rates at low levels. However, the potential impact of livestock, through which economic and managerial factors affect soil erosion, proved to be greater in absolute value than the impact of climate change. Therefore, if changes in some economic or managerial factors led to higher stocking rates in the future, significant increases in erosion rates would be expected.

Empirical multivariate predictive models represent an important tool to estimate gully erosion susceptibility. Topography, lithology, climate, land use and vegetation cover are commonly used as input for these approaches. In this paper, two multivariate predictive models were generated for two gully erosion processes in San Giorgio basin (Italy) and Mula River basin (Spain) using only topographical attributes as independent variables. Initially, nine models (five for San Giorgio and four for Mula) with pixel sizes ranging from 2 to 50m were generated, and validation statistics were calculated to estimate the optimal pixel size. The best models were selected based on model performance using the area under the receiver operating characteristic (AUC) curve and the generalized cross-validation. The best pixel size was 4m in the San Giorgio basin and 20m in the Mula basin. The finest resolution was not necessarily the best; rather, the relationship between digital elevation model resolution and size of the landform was important. The two selected models showed an excellent performance with AUC values of 0.859 and 0.826 for San Giorgio and Mula, respectively. The Topographic Wetness Index and the general curvature were identified as key topographical attributes in San Giorgio and Mula basins, respectively. Both attributes were related to the processes observed in the field and described in the literature. Finally, maps of gully erosion susceptibility were produced for each basin. These maps showed that 22 and 20% of San Giorgio and Mula basins, respectively, present favourable conditions for the development of gullies.

espanolLa variacion espacial es una caracteristica identificadora de la pluviometria peninsular, pero se desconoce para cuencas pequenas y so-bre todo a escala temporal de episodios de lluvia. El objetivo es analizar la variacion espacial de las precipitaciones a diferentes escalas temporales en una pequena cuenca hidrografica. Los analisis incluyen el calculo de las precipitaciones medias de la cuenca utilizando el metodo de la media arit-metica y el de los Poligonos de Thiessen. En el estudio de la variabilidad espacial de las precipitaciones a escala de evento se establecieron seis gru-pos en funcion de las cantidades diarias y de las intensidades como crite-rios. Los principales resultados muestran que los pluviometros ubicados en las zonas mas altas, registraron mayores precipitaciones, donde la orien-tacion del relieve y la exposicion a los vientos repercuten en la variabili-dad espacial. En cuanto a los episodios de precipitacion, aquellos con ma-yores intensidades presentan una mayor variabilidad espacial. EnglishSpatial variability is an identifying feature of the pluviometry of the Iberian Peninsula, but little is known for small catchment and espe-cially for the temporal scale of rain episodes. The aim is to describe and analyze the spatial rainfall variability at different time scales in a small catchment. Analyzes include the calculation of mean rainfall in the basin using the arithmetic mean and Thiessen Polygons. In the study of the spatial variability at event scale, six groups were established based on daily rainfall and intensities as criteria. The main results show that rain gauges located in the upper part of the catchment, recorded higher rainfall. Rain-fall events with higher intensities present greater spatial variability, where the relief orientation and exposure to prevailing winds affect the spatial variability. Regarding episodes of precipitation, events with higher inten-sities show a greater spatial variability.

En una pequeña cuenca bajo explotación de dehesa se investiga la producción de escorrentía superficial en vertientes. Los resultados obtenidos muestran que la determinación del umbral de escorrentía es compleja y que los factores que mejor explican la generación y producción de escorrentía en las vertientes son la precipitación total y la intensidad máxima en 10 minutos.

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.

Soil hydrology was investigated in the Guadelperalon experimental watershed in order to determine the influence of land use and vegetation cover on runoff and infiltration within the Dehesa land system. Five soil–vegetation units were selected: (1) tree cover, (2) sheep trials, (3) shrub cover, (4) hillslope grass and (5) bottom grass. The results of the simulated rainfall experiments performed at an intensity of 56·6 mm h−1 during one hour on plots of 0·25 m2, and the water drop penetration time test indicate the importance of water repellency in the Dehesa land system under drought conditions. Low infiltration rates (c. 9–44 mm h−1) were found everywhere except at shrub sites and in areas with low grazing pressure. Soil water repellency greatly reduced infiltration, especially beneath Quercus ilex canopies, where fast ponding and greater runoff rates were observed. The low vegetation cover as a consequence of a prolonged drought and grazing pressure, in conjunction with the soil water repellency, induces high runoff rates (15–70 per cent). In spite of this, macropore fluxes were found in different locations, beneath trees, on shrub-covered surfaces, as well as at sites with a dominance of herbaceous cover. Discontinuity of the runoff fluxes due to variations in hydrophobicity causes preferential flows and as a consequence deeper infiltration, especially where macropores are developed. © 1998 John Wiley & Sons, Ltd.

Soil properties and water dynamics play a crucial role in the function of oak woodland ranches and dehesas. They are largely controlled by climate conditions, terrain morphology and parent material, but also by land use and management. We review results obtained from research carried out in California and Spain on topics related to soil quality, soil degradation, and water dynamics. Of particular interest is gaining understanding of the influence of land-use and management practices. The distribution of vegetation produces spatial and temporal variation in soil properties that are described in detail. The influence of trees on soil water content is discussed and the dynamics of catchment hydrology is presented, for both California and Spanish cases. An important characteristic is high variability in precipitation, with the occurrence of prolonged dry periods (droughts) that affect water availability for plants. On ranches the effects are two-fold, influencing pasture productivity and water resources for livestock rearing. Soils in the Spanish dehesas have been subject to degradational processes as a consequence of centuries of agricultural use. Water erosion resulting in the reduction of organic matter and physical degradation is the most important phenomena. For California, with a much shorter history of plowing and livestock grazing, we present results from studies on water quality and the effects of vegetation conversion on water yield, soil stability and erosion.

Modelling the hydrologic processes of semiarid basins is specially challenging due to the specific characteristics of this type of environment. The discontinuity of the processes both in space and in time produces a highly complicated rainfall–runoff relationship. Runoff production and reinfiltration areas produce a discontinuous spatial pattern of runoff; channel flow is ephemeral and restricted to the wet season of the year or to heavy rainfalls, and many local saturated layers of soil separated by unsaturated areas can occur. The hydrological connection of the different parts of the basin produces a nonlinear and complex hydrologic behaviour because different response mechanisms of the basin are triggered as the hydrological connectivity increases. Similarly, as connectivity decreases, so does the information arriving at the monitored state variables used to calibrate the model and to understand the system. This paper describes the application of a distributed hydrologic model to describe the hydrologic behaviour of a small semiarid basin including surface and subsurface processes. The model is parameterized and calibrated to simulate a period of 71 days covering the transition of the basin from a dry to a wet state, during which most of the hydrologic processes of the basin are activated. The calibration strategy used different sources and types of information on the state of the basin and an efficient gradient based search algorithm to find the optimal values of the most sensitive parameters. The modelled hydrograph and the modelled internal description of the basin were analysed in the light of its known behaviour and the existing knowledge of semiarid hydrology reported in the literature. The results indicate that while the integrated behaviour of the basin represented by the output hydrograph can be well reproduced, the subsurface processes and the internal dynamics of the basin are less accurately simulated, mainly due to the lack of information on the geometry of the subsurface domain (soil depths and bedrock topography) and on the spatial distribution of the parameters. Copyright © 2007 John Wiley & Sons, Ltd.

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.

The /// 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. Copyright # 2002 John Wiley & Sons, Ltd.

Research on gully erosion is carried out as part of an investigation on erosional and hydrological processes in a small catchment in Extremadura, Spain. The amount of gully erosion is estimated by repeated monitonng of transverse sections since summer 1990. Data obtained so far indicate high spatial and temporal variability. High amounts of erosion are related to a channel section with active headcut retreat, whereas net accumulation occurs where the gully is already deep and wide. Present erosion, apart from headcut retreat, is related to lateral undercutting of the channel bank and bank collapsing. Temporal variability is related with rainfall characteristics and discharge production in the channel. Important sediment losses are produced by highly intensive storms. The most important gully process in the area is shown to be related with the erosive force of flowing surface waters. Furthermore it is discussed whether present active erosion is related to abandonment of cereal cultivation in parts of the catchment about 30 years ago. It is thought that reduced soil erosion on hillslopes due to the change in landuse, and hence lower sediment supply to the channel in the last decades increased gullying.

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Producción Científica, Rainfall is the key factor to understand soil erosion processes, mechanisms, and rates. Most research was conducted to determine rainfall characteristics and their relationship with soil erosion (erosivity) but there is little information about how atmospheric patterns control soil losses, and this is important to enable sustainable environmental planning and risk prevention. We investigated the temporal and spatial variability of the relationships of rainfall, runoff, and sediment yield with atmospheric patterns (weather types, WTs) in the western Mediterranean basin. For this purpose, we analyzed a large database of rainfall events collected between 1985 and 2015 in 46 experimental plots and catchments with the aim to: (i) evaluate seasonal differences in the contribution of rainfall, runoff, and sediment yield produced by the WTs; and (ii) to analyze the seasonal efficiency of the different WTs (relation frequency and magnitude) related to rainfall, runoff, and sediment yield. The results indicate two different temporal patterns: the first weather type exhibits (during the cold period: autumn and winter) westerly flows that produce the highest rainfall, runoff, and sediment yield values throughout the territory; the second weather type exhibits easterly flows that predominate during the warm period (spring and summer) and it is located on the Mediterranean coast of the Iberian Peninsula. However, the cyclonic situations present high frequency throughout the whole year with a large influence extended around the western Mediterranean basin. Contrary, the anticyclonic situations, despite of its high frequency, do not contribute significantly to the total rainfall, runoff, and sediment (showing the lowest efficiency) because of atmospheric stability that currently characterize this atmospheric pattern. Our approach helps to better understand the relationship of WTs on the seasonal and spatial variability of rainfall, runoff and sediment yield with a regional scale based on the large dataset and number of soil erosion experimental stations., Ministerio de Economía y Competitividad - FEDER (CGL2014 52135-C3-3-R; ESP2017-89463-C3-3-R; CGL2014-59946-R; CGL2015-65569-R; CGL2015-64284- C2-2-R; CGL2015-64284-C2-1-R; CGL2016-78075-P; GL2008-02879/BTE; LEDDRA 243857; RECARE-FP7; CGL2017-83866-C3-1-R y PCIN-2017- 061/AEI), Ministerio de Economía y Competitividad (FJCI-2017-33652), Organización para la Cooperación y el Desarrollo Económicos (TAD/CRP JA00088807), Junta de Castilla y León - FEDER (CLU-2018-04)