The role of soil surface conditions in regulating runoff and erosion processes on a metamorphic hillslope (Southern Spain): Soil surface conditions, runoff and erosion in Southern Spain
Introduction
The eco-geomorphological system under dry Mediterranean climatic conditions is characterized by the co-existence of areas with vegetation and areas of bare soil, with implications in the hydrological behavior of the same, more complex, and in which the presence of different surface conditions or components (vegetation, rock fragments, litter, crusts, rock outcrops etc.) are a key factor in determining the hydrological functioning of hillslopes.
The spatial variability of soil surface characteristics, along with the existence of patches of vegetation, is controlled by the distribution of tussocks, shrubs and bare soil (Lavee et al., 1998). This supposes different responses to rainfall and, therefore, different water conservation and distribution patterns. On some patches of bare soil, small and medium-sized stones, especially when they are embedded in the topsoil, generate surface runoff; but where there are shrubs, tussocks or small stones accumulated on the soil surface, infiltration processes are dominant, both directly from rainfall and from overland flow coming from adjacent areas (Lavee and Poesen, 1991). Therefore, there is a mosaic or a pattern containing patches that generate runoff or infiltrate surface flow, distributed over hillslopes either grouped together or at random.
There may be a clear threshold between the arid eco-geomorphological system controlled by abiotic factors such as dissolved salt content or the formation of mechanical crusts, from the humid eco-geomorphological system controlled by biotic factors such as the development of vegetation, microbial activity and the production and decomposition of organic matter. However, this threshold is not so clearly defined for semiarid areas. The Mediterranean climate zone, having an annual precipitation of 450 to 700 mm, may belong to the semiarid or humid type system, and depends on the degree of human interference through introducing fires, grazing or deforestation. In semiarid areas, the size of water-accepting patches decreases, due to both less direct rainfall on the patches and less water supply from runoff contributing patches. Plants at the lower part of the accepting patches will die. Such a mechanism causes a gradual increase in the size of the contributing patches and the disappearance of all shrubs from the drier upper part of a hillslope. As the erosion rate increases, more stones and bedrock outcrops will appear on the surface. Most of the hillslope, if not all, will become a runoff contributing area, and semiarid systems may turn into arid systems (Lavee et al., 1998).
Along a hillslope, therefore, there are many patches without vegetation cover and with fragments of rocks, stones and litter, or just bare soil, which have different functions in the surface redistribution of water through their distinct hydrodynamic behaviour. The effects of surface components on overland flow and infiltration have been addressed in several studies (Poesen, 1986, Poesen et al., 1990, Lavee and Poesen, 1991, Brakensiek and Rawls, 1994, Bunte and Poesen, 1994, Valentin, 1994, Moustakas et al., 1995, Katra et al., 2007). Some relationships were observed between rock fragment size, infiltration rates and overland flow yield. Valentin, 1994, Martínez-Murillo and Ruiz-Sinoga, 2007 also showed that the rate and yield of overland flow generation were higher in the presence of ‘partially embedded’ rock fragments than ‘on top’ rock fragments. This was attributed to the continuity of overland flow in the vicinity of ‘partially embedded’ rock fragments on the ground surface, thus delaying infiltration into the underlying soil (Poesen and Lavee, 1994, Katra et al., 2007). These hydrological processes affect soil water content after rainfall depending on soil physico-chemical properties and soil retention capacity, and determine how long the water remains in the soil.
Several studies on the hydrological behaviour of surface components were conducted in limestone or granite areas (Poesen, 1986, Poesen et al., 1990, Lavee and Poesen, 1991, Poesen and Ingelmo-Sanchez, 1992, Ingelmo et al., 1994, Valentin, 1994, Moustakas et al., 1995, Bergkamp, 1998, Calvo et al., 2003, Buttafuoco et al., 2005, Katra et al., 2007). These researches found that shrubs affect soil properties through soil–vegetation interaction that resulted in higher infiltration rates and greater soil water retention capacity than in inter-shrub areas, and consequently higher soil moisture content after rain and overland flow events (Sarah, 2002, Katra et al., 2007). Other studies showed how the soil under rock fragments is subjected to microclimatic conditions of relatively moderate soil temperatures compared to soil without vegetation, which enhances biophysical feedback (biotic activity–soil aggregation) and, in turn, pedo-hydrological properties (Danalatos et al., 1995, Li and Sarah, 2003). This improvement, together with specific hydrological processes such as stone flow, may enable relatively high soil moisture content to accumulate under rock fragments at the end of a rainfall event and during the subsequent days (Katra et al., 2007).
This study presents the results obtained on a monitored hillslope in dry Mediterranean climatic conditions in Southern Spain, and the objectives being: i) to evaluate the state of degradation by analyzing the soil surface components of the hillslope, ii) to determine the surface hydrological processes and their consequences from the point of view of erosion and iii) to establish the factors which regulate such processes. By means of this evaluation, information about the hydro-geomorphological dynamics on hillslopes with metamorphical and impermeable geological substratum, lithologies which are not very frequent in the scientific literature relating to the hydrological and erosive processes in Mediterranean environments has been offered.
Section snippets
Materials and methods
The methodological approach applied in each field site is based on: i) the definition of the eco-geomorphological units of the hillslope and determining their principal surface components, defining homogeneous areas, and ii) characterising the hydrological functioning and the water erosion processes of those surface components.
Intra-hillslope variability of soil surface components
Three eco-geomorphological units were delimited on the hillslope (Fig. 2A). Unit A with a high level of vegetal cover (> 90%) of shrub-like species (Quercus coccifera) and woodland areas (Quercus suber) which covered the soil surface with litter. Unit B with a lower degree of vegetal cover (60–90%), constituted by Cistus monspeliensis and Genista umbellata, with soil partially covered with litter and rock fragments. Unit C with vegetal cover of less than 50%, composed of Lavandula stoechas,
Discussion
The different hydrological behaviors observed with soil surface components both at the patch scale and the experimental plot implied the existence of areas which generate runoff (bare soil) and infiltrating areas (scrubland) as was observed by Cerdà, 1997, Calvo et al., 2003, with a variable implication in the water erosion processes. If from a functional point of view the bare soil was regulated by abiotic factors (quantity and disposition of the rock fragments on the soil surface), the
Conclusions
The hydrological processes on hillslopes in dry Mediterranean climatic conditions vary in space due the surface cover (rock fragments and plants) and time (season) due to the contrasted climatic conditions. Hydrological disconnection is frequent along the slope because of infiltration of the surface runoff. This results in low soil water erosion. This was found also on limestone permeable parent material. Our contribution from metamorphic and impermeable geological substratum confirms that the
Acknowledgments
The authors are grateful to the Cuenca Mediterránea Andaluza of the Agencia Andaluza del Agua and the III Plan Andaluz de Investigación, the Regional Government of Andalucía, for their financial support that enabled this study.
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