Variation of rock fragment cover and size along semiarid hillslopes: a case-study from southeast Spain
Introduction
Over the past decade there has been a growing interest in understanding the spatial pattern of top soil horizon attributes such as A horizon thickness, organic matter content, amorphous Fe, electrical conductivity, pH, extractable plant nutrients, texture, soil water content and aggregate stability at the hillslope scale (e.g., Miller et al., 1988; Pierson and Mulla, 1990; Brubaker et al., 1993, Brubaker et al., 1994; Moore et al., 1993; Tomer and Anderson, 1995). Such information is crucial for understanding for instance the variation in soil erosion and in soil water availability for plants along the hillslopes, both of which control the spatial patterns of biomass production.
In most semiarid and arid environments, topsoils contain significant amounts of rock fragments. Rock fragment cover (Rc) and size (Rs) affect the intensity of various hydrological and soil degradation processes such as surface sealing, infiltration, evaporation, runoff generation, runoff energy dissipation and erosion by water (Abrahams and Parsons, 1994; Brakensiek and Rawls, 1994; Poesen and Lavee, 1994; Poesen et al., 1994; Valentin, 1994; van Wesemael et al., 1995, van Wesemael et al., 1996). Rock fragment content has also been reported to favour soil productivity especially in semiarid and arid regions (Kosmas et al., 1993; Poesen and Lavee, 1994). Until now, most studies have dealt with rock fragment cover effects on the intensity of earth surface processes operating at the plot scale. However, few quantitative data are available on the distribution of rock fragment cover along entire semiarid and arid hillslopes. Such information is crucial for understanding the vulnerability of hillslope sections to land degradation and for modelling overland flow, erosion and deposition along such hillslopes (Lane et al., 1995; Thornes et al., 1996).
Several studies in semiarid and arid environments have pointed to the existence of a positive relation between hillslope gradient and surface rock fragment cover (e.g., Yair and Klein, 1973; Martinez et al., 1979; Abrahams and Parsons, 1991; Cooke et al., 1993; Simanton et al., 1994; Simanton and Toy, 1994). Others also reported a positive relation between slope and surface rock fragment size (e.g., Carson and Kirkby, 1972; Abrahams et al., 1985; Parsons and Abrahams, 1987; Le Roux and Vrahimis, 1987). Only a few studies have quantified the relationship between slope and Rc along specific hillslopes. Simanton et al. (1994)and Simanton and Toy (1994)found a logarithmic increase of Rc with slope for hillslopes formed in weakly consolidated coarse Quaternary alluvium in semiarid rangelands of Arizona (USA). But it is unknown whether the same relation between hillslope gradient and Rc or Rs holds true for semiarid environments in the Mediterranean where shallow soils are often developed over more resistant bedrock. Therefore, a field study was set up to investigate how topography (hillslope gradient and aspect) and lithology influence the rock fragment cover and size in rangelands of the semiarid part of the Mediterranean. Currently, this area is threatened by desertification processes which are being investigated within the framework of an interdisciplinary research project, MEDALUS (MEditerranean Desertification And Land USe; Brandt and Thornes, 1996). In order to better understand how hydrology and soil erosion affect Mediterranean hillslopes, the following objectives were pursued:
- 1.
To describe and to explain the spatial variation of Rc and Rs along Mediterranean hillslopes under rangeland conditions;
- 2.
To discuss the implications of the spatial patterns of Rc in assessing the desertification risk and for modelling runoff, erosion and deposition in Mediterranean environments.
This study focuses on southeast Spain, the driest area of Mediterranean Europe. Vegetation cover is low during summer and surface rock fragments are very important in stabilising the soil surface and therefore play a crucial role in the response of the hillslopes to environmental change.
Section snippets
Study area
The study area in the Almeria Province (southeast Spain; Fig. 1) has a typical basin and range topography. The rocks are of Palaeozoic and Cenozoic age and consist mainly of micaschist, andesite and conglomerate (Delgado Castilla, 1974; IGME, 1985). Seven hillslopes ranging from 150 to 650 m length and with a local slope range of 4 to 66% were selected in order to cover different lithologies and different aspects (Fig. 1, Table 1). The hillslopes typically have convex–straight–concave profiles
Methods
The profiles of the seven hillslopes (e.g., Fig. 2) were recorded using a clinometer and a tape measure. For each hillslope, between 6 and 30 slope segments with uniform gradients were delineated between the foot of the hillslope and the steepest hillslope section (Fig. 2). Within each of these segments a representative interrill area of approximately 100 m2 was selected and within this area, orthogonal photographic slides of the soil surface were taken at six unvegetated, randomly chosen sites
Slope–Rc and slope–Rs relations for hillslopes in the Almeria province
A typical example of the relation between total rock fragment cover percentage (Rc>5 mm) and hillslope gradient as well as cover percentage of large rock fragments (Rc>25 mm) and hillslope gradient is shown for hillslope 1 in Fig. 5. Although there is some local variation in rock fragment cover percent for each hillslope segment (as indicated by the standard deviation in Fig. 5), both total rock fragment cover (Rc>5 mm) and cover of large rock fragments (Rc>25 mm) increase with hillslope
Implications
Both cover and size of surface rock fragments control the intensity of a series of hydrological and soil degradation processes such as surface sealing, infiltration, evaporation, runoff generation, runoff energy dissipation and erosion by water. Hence, the observed relationships between slope and Rc, as well as between slope and Rs, help understanding the spatial pattern of the intensity of these processes along hillslopes in the Mediterranean.
In order to illustrate this, the observed rock
Conclusions
Along semiarid hillslopes and transects of the Mediterranean, rock fragment cover and size are not randomly distributed but follow typical spatial patterns. The results of this study show that overall, Rc and Rs are largely controlled by hillslope gradient. On many hillslopes, total rock fragment cover (Rc>5 mm) increases in a convex upward curve with hillslope gradient whereas the D50 of the surface rock fragments >5 mm increases linearly with hillslope gradient. This spatial variation of rock
Acknowledgements
The research for this paper was carried out as part of the MEDALUS (MEditerranean Desertification And Land USe) collaborative research project. MEDALUS was funded by the European Commission Environment and Climate Research Programme (contract: ENV4-CT95-0118, Climatology and Natural Hazards) and the support is gratefully acknowledged. This study is also a contribution to the Soil Erosion Network of the Global Change and Terrestrial Ecosystems Core Research Programme which is part of the
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