The role of soil surface conditions in regulating runoff and erosion processes on a metamorphic hillslope (Southern Spain)

Abstract

This study presents the results of an analysis into the role of soil surface conditions in the regulation of soil hydrology and erosive processes at one hillslope under dry Mediterranean climatic conditions. The methodology was based on the analysis of hillslope surface components and their hydrological and erosive function on a patch to scale by means of rainfall simulation and experimental plots. The results showed the existence of a complex eco-geomorphological system composed of a multitude of vegetation patches distributed at random on the hillslope, and where the presence of different surface conditions on the soil can have a sizeable influence on hydrological and erosive behavior. From the hydrological point of view, the runoff generation mechanisms follow a seasonal pattern depending on the moisture of the soil with a different spatial condition, with frequent hydrological disconnections between parts of the hillslope, as in other Mediterranean mountainous regions. Soil surface rock fragments, the layout of tussocks intra-hillslope and previous soil moisture as dynamic control factors in the hydrological and erosive processes are all important.

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.