Albert Solé-Benet

Hydrological and erosion response of a badlands system in semiarid SE Spain

A long term monitoring program is being conducted to study runoff and erosion processes in a semiarid badlands environment (Tabernas Desert, SE Spain). The first six years of data from an instrumented experimental area with nested microcatchments are presented. The overall area is composed of a complex mosaic of soil surfaces with contrast hydrological and erosion behaviour. At microcatchment scale, runoff and erosion are controlled by the types of soil surfaces: small bare microcatchments had the highest runoff coefficients and the highest erosion rates, while those completely covered by vegetation had the lowest. Rainfall intensity significantly affected water and sediment budgets. The effect of antecedent soil moisture could only be observed when soil was near saturation and a few millimetres of additional rainfall were sufficient to produce Horton-type runoff, but it was very difficult to separate this from the effect of surface crusts formed in the first minutes of rainfall. Most of the rainfall events were below the threshold for producing runoff although they were important for sediment preparation through weathering. Small magnitude, low-intensity rainfall events along with protective plant cover over half of the total surface, are the main factors explaining low overall erosion rates at microcatchment scale.

Effects of soil and vegetation on runoff along a catena in semi-arid Spain

Runoff and infiltration were investigated on abandoned fields of patchy vegetation in semi-arid Spain during 15 months of natural rainfall and by rainfall simulations. The aim was to ascertain sources and sinks of runoff and the effects of soils and plant cover. Soils of the catena developed from mica schists of the upper hillslopes, fan deposits of the lower hillslopes, and an alluvial terrace at the bottom. Runoff from natural events were from three sets of three pairs each of 10 × 2 m runoff plots. The pairs of each set had different densities of plant cover; the sets were vegetated with tussock grass, Stipa tenacissima, a shrub, Anthyllis cytisoides, and a bush, Retama sphaerocarpa. Nineteen natural rainfall events of intensities up to 18 mm/h produced 400 mm of rain during the study period. Because the rainfall threshold for runoff production was about 20 mm, only eight events produced runoff. The rainfall simulations used a sprinkler that produced 50 mm/h of rain for 30 minutes; runoff was recorded each minute in 0.24 m2 bounded plots. The depth and structure of the soil mantle provide the main controls on runoff rates, which are lowest on the lower fan deposits and highest on the thin upslope soils. The river-bank terrace, with a surface covered by crusts and mosses, also yields relatively high runoff. In general, vegetation density varies inversely with runoff. Nevertheless, shrub and bush litter favor runoff, as does a particular spatial distribution of individual plants on the hillslope. Settling of the upper few centimeters of soils of the alluvial fan following cessation of cultivation 15 to 40 years ago has produced a near-surface compacted layer favoring shallow subsurface runoff. Apparently contradictory results between runoff plots and rainfall simulations are the result of differing processes.

Topographic controls on the spatial distribution of ground cover in the Tabernas badlands of SE Spain

Abstract The relationships between the spatial distribution of ground-cover and terrain attributes were examined in the Tabernas badlands (SE Spain) in order to understand the terrain-dependent driving forces of the spatially heterogeneous ground cover. Ground cover was mapped in the field and terrain attributes were derived from a 1-m resolution Digital Elevation Model (DEM). The association of spatial distribution of the landforms resulting from a regionalisation (using a nonhierarchical classification of the topographic overlays) and the ground-cover pattern was proved. From the analysis of relationships between terrain attributes and proportional abundance of ground-cover types, it was found that ground cover is arranged along topographic gradients: plant-covered surfaces are more abundant on low slope angles, concave slopes, relatively large contributing areas and with low length slope factor values. Unvegetated surfaces show contrary trends and lichens are associated with intermediate conditions. Relationships with local terrain attributes, such as slope angle or elevation, are more pronounced than those with terrain attributes related to sediment and water transfer, such as contributing area, wetness index or length slope factor which could be explained by the heterogeneity of runoff that is usually shorter than the hillslope length. The relationships established between the spatial distribution of ground-cover types and terrain attributes provide the basis for future development of a tool for mapping spatial distribution of ground cover in similar areas from only topographic information.

Soil–geomorphology relations in gypsiferous materials of the Tabernas Desert (Almerı́a, SE Spain)

A detailed pedological study in an apparently homogeneous badlands area of gypsiferous mudstones in the Tabernas Desert (Almeria, SE Spain), with an annual precipitation of 200 mm, has been shown to be composed of different soil units belonging to different stages of soil development. Twenty-four soil profiles in four topographic transects within a small instrumented catchment have been described and analysed, along with over 100 probings and observations. A complementary approach to ascertain the relationships of soil-units with topography made use of a 1-m resolution digital elevation model (DEM) and derived terrain attributes. Moreover, the relationships with soil cover, surface hydrology and erosion have all contributed to understanding pedogenic and evolutionary processes. The five soil units identified correspond to distinct topographic positions, from steep S-oriented slopes with incipient soil development under bare surfaces (Epileptic Regosol), to moderately sloping, N-oriented soils, fairly well developed below a dense cover of annual and perennial plants (Haplic Calcisol). Both the spatial distribution and the topographic position of soil units favour gypsum and salt washing processes and gypsum accumulation is restricted to higher positions with very small contributing areas and minimum overland flow and thus reduced leaching. Gypsic horizons and Gypsisols, while previously described in the area associated to gypseous rock outcrops, are now described associated to gypsiferous mudstones.

Weathering of a gypsum-calcareous mudstone under semi-arid environment at Tabernas, SE Spain: laboratory and field-based experimental approaches

The weathering of a Late Miocene gypsum-calcareous mudstone outcropping in large badland areas of SE Spain, under a semi-arid Mediterranean climate, was studied by means of two experimental approaches. Field and laboratory experiments were carried out to reproduce, though in accelerated form, some of the weathering conditions of the consolidated mudstone. In the laboratory, three sequences of 5, 10 and 20 wetting–drying cycles were produced on undisturbed blocks of fresh mudstone samples. At the end of the three sequences, samples were analysed for their micromorphology, elemental and soluble salt chemistry, and total mineralogy. Unweathered dry samples, as blanks, and permanently wet samples were also analysed. In the field, two small plots of freshly exposed mudstone were monitored over 3 years for their response to natural weathering in terms of morphological changes and sediment output. The porosity was increased by a few wetting–drying cycles, as assessed by significant increases in water absorption capacity of the mudstone. A combination of three factors is responsible for mudstone weathering: repeated cycles of wetting–drying, the presence of geologically-induced cracks and fissures, and dissolution–crystallisation of relatively soluble minerals, gypsum being the most abundant within this category. A few wetting–drying cycles were sufficient to reveal ion migration (specially Na+, Ca++, Mg++, SO4−, HCO− and Cl−) within the mudstone, explaining mineral dissolution. In the field, surface weathering rates from 0.7 to 8 mm year−1 were measured. Weathering rates were found to be proportional to the number of rainfall events during the sampling periods, confirming what was found in laboratory conditions, namely, that the number of wetting–drying cycles has the greatest influence on weathering. These weathering rates might be considered as the probable range of incision rates under present semi-arid conditions.

Evaluation and Selection of Indicators for Land Degradation and Desertification Monitoring: Methodological Approach

An approach to derive relationships for defining land degradation and desertification risk and developing appropriate tools for assessing the effectiveness of the various land management practices using indicators is presented in the present paper. In order to investigate which indicators are most effective in assessing the level of desertification risk, a total of 70 candidate indicators was selected providing information for the biophysical environment, socio-economic conditions, and land management characteristics. The indicators were defined in 1,672 field sites located in 17 study areas in the Mediterranean region, Eastern Europe, Latin America, Africa, and Asia. Based on an existing geo-referenced database, classes were designated for each indicator and a sensitivity score to desertification was assigned to each class based on existing research. The obtained data were analyzed for the various processes of land degradation at farm level. The derived methodology was assessed using independent indicators, such as the measured soil erosion rate, and the organic matter content of the soil. Based on regression analyses, the collected indicator set can be reduced to a number of effective indicators ranging from 8 to 17 in the various processes of land degradation. Among the most important indicators identified as affecting land degradation and desertification risk were rain seasonality, slope gradient, plant cover, rate of land abandonment, land-use intensity, and the level of policy implementation.

Evaluation and selection of indicators for land degradation and desertification monitoring: types of degradation, causes, and implications for management

Indicator-based approaches are often used to monitor land degradation and desertification from the global to the very local scale. However, there is still little agreement on which indicators may best reflect both status and trends of these phenomena. In this study, various processes of land degradation and desertification have been analyzed in 17 study sites around the world using a wide set of biophysical and socioeconomic indicators. The database described earlier in this issue by Kosmas and others (Environ Manage, 2013) for defining desertification risk was further analyzed to define the most important indicators related to the following degradation processes: water erosion in various land uses, tillage erosion, soil salinization, water stress, forest fires, and overgrazing. A correlation analysis was applied to the selected indicators in order to identify the most important variables contributing to each land degradation process. The analysis indicates that the most important indicators are: (i) rain seasonality affecting water erosion, water stress, and forest fires, (ii) slope gradient affecting water erosion, tillage erosion and water stress, and (iii) water scarcity soil salinization, water stress, and forest fires. Implementation of existing regulations or policies concerned with resources development and environmental sustainability was identified as the most important indicator of land protection.

The influence of local sources on the mineral content of bulk deposition over an altitudinal gradient in the Filabres Range (SE Spain)

Mineralogical characteristics of bulk deposition have been determined at seven sites along an altitudinal gradient in the southern face of Filabres Range from 1770 m altitude to sea level, over a period of 8 months, the rainiest in this semiarid area. X ray diffraction and scanning electron microscopy were used for identifying and semiquantifying minerals. The aim was twofold: to ascertain the contribution of mineral particles to both the high pH and alkalinity of bulk deposition, the highest in Europe, and to use mineralogy of bulk deposition as a possible fingerprinting technique to identify the source of insoluble particles. The pH, alkalinity, and electrical conductivity of rainwater increase down the altitudinal gradient as precipitation decreases, indicating a clear increase in alkaline particles, in agreement with the presence of calcite, aragonite, dolomite, and gypsum, which also explains high pH and alkalinity of the bulk deposition. Moreover, pH and conductivity of rainwater are somewhat more related to E-SE winds than to W-NW ones. In addition to carbonates and gypsum, seven silicate minerals were found. Some minerals, like feldspars, chlorite, and aragonite, are neither related to site nor to rain events. Some others, like quartz, high-crystalline illite, paragonite, and kaolinite, are only related to rain events. Finally, a few minerals, like low-crystalline illite, palygorskite, paragonite, and to some extent calcite and gypsum, are site related. Some minerals have been interpreted to come from local sources, e.g., high-crystalline illite, gypsum, and paragonite; others, like palygorskite and low-crystalline illite, smaller in size, may come from distant sources.

Unveiling soil degradation and desertification risk in the Mediterranean basin: a data mining analysis of the relationships between biophysical and socioeconomic factors in agro-forest landscapes

Soil degradation and desertification processes in the Mediterranean basin reflect the interplay between environmental and socioeconomic drivers. An approach to evaluate comparatively the multiple relationships between biophysical variables and socioeconomic factors is illustrated in the present study using the data collected from 586 field sites located in five Mediterranean areas (Spain, Greece, Turkey, Tunisia and Morocco). A total of 47 variables were chosen to illustrate land-use, farm characteristics, population pressure, tourism development, rainfall regime, water availability, soil properties and vegetation cover, among others. A data mining approach incorporating non-parametric inference, principal component analysis and hierarchical clustering was developed to identify candidate syndromes of soil degradation and desertification risk. While field sites in the same study area showed a substantial similarity, the multivariate relationship among variables diverged among study areas. Data mining techniques proved to be a practical tool to identify spatial determinants of soil degradation and desertification risk. Our findings identify the contrasting spatial patterns for biophysical and socioeconomic variables, in turn associated with different responses to land degradation.

Badlands in the Tabernas Basin, Betic Chain

The complex badland landscape at Tabernas results from a combination of relief amplitude generated by tectonic uplift since the Pliocene and reactivated several times during the Pleistocene, the properties of the Tortonian sedimentary rocks and a predominantly arid climate. The landscape is dominated by deep incision of the main river systems, which continues in part of the headwater tributaries, and characterized by contrasting slope morphologies and a variety of microecosystems. The Tabernas badlands exhibit a diversity of landforms resulting from the combination of multi-age soil surface components that allow a variety of processes to operate at different rates. These are dominated by rilling and shallow mass movements on south-facing hillslopes. On old surfaces and north-facing hillslopes, where biological components are present, overland flow with variable infiltration capacity and low erosion rates prevail. Incision in the gully bottoms occurs in the most active areas.