H. Lavee

Rock fragments in top soils: significance and processes

This introductory paper reviews various aspects of rock fragments in top soils. Such information is required for various reasons and in particular for predicting the impact of climatic or landuse changes on the response of these soils. Particular attention is paid to the definition and measurement of rock fragment content in top soils, the density of soils containing rock fragments, the spatial distribution and movement of rock fragments in top soils, the effects of rock fragments on some key hydrological processes, thermal properties of top soils, physical soil degradation, soil erosion and soil productivity.

THE IMPACT OF CLIMATE CHANGE ON GEOMORPHOLOGY AND DESERTIFICATION ALONG A MEDITERRANEAN- ARID TRANSECT

From the perspective of geomorphology, three important aspects of climate should be considered if conditions become more arid: (a) any decrease that might occur in the annual rainfall amount; (b) the duration of rainfall events; and (c) any increase in the intervals between rainfall events. These, together with increasing temperature, lead to less available water, less biomass and soil organic matter content and hence to a decrease in aggregate size and stability. As a consequence, the soil permeability decreases, soils develop surface crusts and infiltration rates decrease dramatically. Such changes in vegetation cover and soil structure lead to an increase in overland flow and in the erosion of the fertile topsoil layer. Positive feedback mechanisms may reinforce these eAects and lead to desertification. This paper considers the results of field investigations into the spatial variability of a number of ‘quick response’ variables at two scales: the regional and the plot scales. Concerning the regional scale spatial variability, results of experimental field work conducted along a climatic transect, from the Mediterranean climate to the arid zone in Israel, show that: (1) organic matter content, and aggregate size and stability decrease with aridity, while the sodium adsorption ratio and the runoA coeAcient increase; and (2) the rate of change of these variables along the climatic transect is non-linear. A steplike threshold exists at the semiarid area, which sharply separates the Mediterranean climate and arid ecogeomorphic systems. This means that only a relatively small climatic change would be needed to shift the borders between these two systems. As many regions of Mediterranean climate lie adjacent to semiarid areas, they are threatened by desertification in the event of climate change. Concerning spatial variability at the plot scale, diAerent patterns of overland flow generation and continuity characterize hillslopes under diAerent climatic conditions. While in the Mediterranean climate area infiltration is the dominant process all over the hillslope, in the arid area overland flow predominates. In contrast to the uniform distribution of processes in these two zones, a mosaic-like pattern, consisting of locally ‘arid’ water contributing and ‘moist’ water accepting patches is typical of the transitional semiarid area. Such pattern is strengthened by fires or grazing which are characteristic of this area. The development of such mosaic pattern enables most rainfall to be retained on hillslopes. Changes in the spatial pattern of contributing versus accepting waterareas can be used as an indicatorof desertification and applied to developing rehabilitation strategies. #1998 John Wiley & Sons, Ltd.

Effect of surface roughness on runoff and erosion in a mediterranean ecosystem: the role of fire

Mechanisms at the micro (patch) scale influencing runoff and erosion processes following a forest fire were investigated. The study was conducted in a Mediterranean mountainous region with a mean annual rainfall of 690 mm, Terra Rossa soil, and vegetation composed of a dense shrubland of Pinus halepensis and Quercus calliprinos. Simulated rainfall experiments were carried out on plots representing different intervals of time elapsed since a fire; i.e., immediately after fire, two weeks after fire, one year after fire. Unburned plots were also used for comparison. At a local scale, great variability was found whereby some patches produced high runoff and sediment yields following fire, while others did not. The conclusion is that the main effect of fires of low to moderate intensity is to produce a mosaic-like surface containing both rough patches in which there is almost no chance for runoff generation, and relatively smooth patches in which fire can subsequently lead to higher runoff and erosion rates.

Soil erosion and climate change: the transect approach and the influence of scale

Abstract Studies of geo-ecological processes are being made along climatological transects on similar limestone rocks at different locations across the Mediterranean. The main objectives of the research are firstly, to gain insight into the influence of climate on key-geomorphological process–pattern relationships that characterise different locations along the transect; and secondly, to obtain a better insight into the possible impact of climate change on ecosystem degradation. The paper begins by considering the possible methodological approaches that can be applied to investigate the impact of climate change on ecosystem degradation in complex ecosystems and explains why a nonlinear evolutionary modelling framework was chosen for the transect studies. The conceptual basis of the methodology is presented and the transect approach described. The research methodology takes into account the influence of climate at four different scales (landscape, slope, response unit and patch). Field research at these scales is oriented towards identifying and understanding the key processes and to identifying key parameters that can be monitored to establish change. Some results are presented from the Judean Desert transect in Israel to show how a key indicator, in this case aggregate stability, varies with temperature and to show how process–pattern features vary along the limestone transect from the area having a Mediterranean climate to the desert. Both conceptual and practical models of soil erosion need to address scale issues. In particular, as time and spatial scales change, so also does the relative significance of different processes. It is concluded that process–pattern phenomena are useful in this context and that they can be applied to the problem of up-scaling.

The effect of fire-induced surface heterogeneity on rainfall-runoff-erosion relationships in an eastern Mediterranean ecosystem, Israel

Abstract The effect of forest fire on rainfall-runoff-erosion relationships at the micro-scale (plot) and at the meso-scale (hillslope) were studied in a Mediterranean mountainous region near Haifa, Israel. Rainfall simulation experiments were carried out on several plots at different times since the fire. Runoff and sediment yield were collected during the experiments. Soil samples from the upper soil layer (0–4 cm) were collected for analysing physical and chemical properties The results obtained show that the total runoff and sediment yield varied considerably in both space and time. High spatial variability, at the plot scale, together with relatively low fire intensities and a well-structured and stable soil, were considered to be the main reasons for the very low runoff and sediment yield at the meso-scale (hillslope).

EFFECT OF SLOPE ASPECT ON SOIL AND VEGETATION PROPERTIES ALONG AN ARIDITY TRANSECT

ABSTRACT This study has investigated the effect of south- and north-facing hillslopes (SFS and NFS, respectively) on soil and vegetation properties along a climatic transect that covers four climatic zones: Mediterranean, semiarid, arid, and extreme-arid. The results indicate that differences between NFS and SFS in organic matter and soil moisture contents were significantly higher in the Mediterranean zone than in any other site along the climatic transect, particularly at the end of the rainy season. Both soil properties were higher on NFS than on SFS. Similarly, differences between the opposite hillslopes in species richness along the climatic transect were found significant only in the Mediterranean zone, where species richness was higher on the SFS than on the NFS. However, species diversity on SFS was significantly higher than that on NFS at all sites, except for the semiarid zone, where no significant difference between aspects was found. Species diversity on SFS changed along the climatic transect...

Ecogeomorphic Feedbacks in Semiarid Rangelands: A Review

Abstract The ecogeomorphic processes occurring on semiarid rangelands are reviewed, with emphasis on the source-sink relations and positive feedback loops that existed between shrub patches and intershrub areas, and the way livestock presence affected these interactions. Compared with intershrub areas, the shrub patches had a higher soil porosity, infiltration capacity, water-holding capacity, hydraulic conductivity, structural stability, and organic matter content, and lower bulk density. These differences derived from a host of processes whose intensity was less in the shrub patches, including raindrop impact, mechanical crust formation, overland water flow, soil erosion, evaporative moisture loss, and flock trampling. There was also greater shading of the soil surface; soil and litter deposition; water accumulation; microbial, fungal, and mesofaunal activities in the shrub patches. The overland flow of water carried soil and litter from the intershrub areas to the shrub patches and resulted in microtopographic modifications that tended to strengthen these source-sink relations. Grazing had an impact on these processes, not only at the shrub-intershrub scale but also within the intershrub areas, through the creation of highly compacted trampling routes. The combined role of the above ecogeomorphic processes was to maintain the rangelands functionality. Without these inter-relationships, water loss, soil erosion, and nutrient depletion would occur at the hillside scale, causing degradation of the landscape.

SPATIAL AND TEMPORAL VARIABILITY OF WATER PERCOLATION AND MOVEMENT IN A SYSTEM OF LONGITUDINAL DUNES, WESTERN NEGEV, ISRAEL

Subsurface lateral flow has previously been identified in dune areas where the average annual rainfall exceeds 200 mm. Very little is known about subsurface flow in sandy deserts with less than 100 mm of rainfall per year. The present study deals with the water regime in a system of longitudinal dunes in the Negev desert. Sixteen boreholes were dug, down to a depth of 6 m, across a sandy ridge and the adjacent corridor. Soil moisture and water movement were monitored with a neutron probe during two consecutive years. The first year had been relatively wet and in the following year the rainfall was slightly lower than the long-term average. The data obtained show that in an average year water percolation is limited to 60 cm with no lateral water movement. Deeper percolation, in the range 180–400 cm, occurs in response to rainy spells with about 100 mm of cumulative rain within two months. The process involves subsurface lateral flow and water movement by the piston effect. The lateral flow, on the flanks of the dune, is explained by differences in hydraulic conductivities within the sloping layers.

Spatial distribution of soil surface coverage on north and south facing hillslopes along a Mediterranean to extreme arid climatic gradient

Four research sites were established along a climatic gradient in Israel representing Mediterranean, xeric Mediterranean, arid and extreme arid climate conditions. The cover percentage of eight soil surface components [vegetation (annual and perennial forms), biogenic crust (mosses, lichens, cyanobacteria, and dwarf grass), stones and bare soil patches] were measured. Transects along hillslopes on each of two opposite aspects (south and north facing slopes) were established at each site (altogether eight transects). Dominance and soil surface cover diversity indices, and similarity coefficients between the transects were calculated. High similarity values in soil surface cover were found between the south facing slopes located in the xeric Mediterranean, arid, and extreme arid areas. These hillslopes are dominated by stones and bare soil patches. In contrast, lower similarity values were found between the northern slope aspects dominated by biotic components (vegetation and biogenic crusts). The study along a climatic gradient together with the comparison between northern and southern aspects provide useful information that can be utilized for evaluation of changes in soil surface cover characteristics given various scenarios of future climate regimes in that region.

The erosional response of Calcareous soils along a climatological gradient in Southeast Spain.

Abstract The erosional response of calcareous soils along a climatological gradient in Alicante, SE Spain was studied. The erosional response was defined in terms of indicators obtained from field rainfall simulation experiments and laboratory studies of soil aggregation. Measurements were made at seven sites on limestones and marls between Benidorm with an annual average rainfall of 400 mm yr −1 and Coll de Rates (annual rainfall 900 mm yr −1 ). The sites were on matorral vegetation affected to varying intensities by grazing and fire. At the seven locations, soil aggregation was studied by sieving, by the single water drop test and by a Microscan particle-sizer. Rainfall simulation experiments were made on circular 55-cm diameter target area plots using the Calvo simulator at an intensities between 35 and 59 mm h −1 . On limestone, it was found that on bare surfaces, the coarse soil aggregation increases with depth at the highest sites and decreases at the lower sites. The Benidorm surface soil has the highest erodibility. The least stable soils are at the lower intermediate sites. On marls, the aggregate stability is generally high in contrast to those under cultivation. Simulated runoff from the wetter limestone sites is very low. The driest areas have the highest runoff and sediment yields. Trends with climate suggest that the erosional responses of the soils indeed increase with aridity and temperature but local factors have a large effect, particularly at intermediate altitudes. A clear relationship like that found in Israel by Lavee on a similar lithology does not exist.