Aaron Yair

Physical properties of the psammophile cryptogamic crust and their consequences to the water regime of sandy soils, north-western Negev Desert, Israel

The water regime in the sandy dunal area of Nizzana, north-westem Negev Desert, Israel, is highly dependent on a fragile cryptogamic crust only several millimetres thick. This crust develops due to the presence of Cyanobacteria which agglomerate the sand grains and trap aeolian dust particles. Not only does this semi-permeable crust increase runoff but the water which does infiltrate the soil is protected from excessive evaporation. This study presents quantitative measurements of the physical properties of the crust which are important to the water regime in desert dunal areas—the granulometry, porosity and water retention capacity. The grain size distribution shows a concentration of silt and clay in the crustcompared to the sands just beneath the crust. The microporosity (measured using a mercury pore sizer) shows that approximately 40% of the access pores can be blocked by the swelling of Cyanobacteria trichomes as they absorb water, which limits rainwater infiltration. These observations concur with rain simulation experiments made in the field. An evaporation phase was simulated in the laboratory in order to quantify the water retention capacity of the crust and compare it with that of other sediments, in which the algal mat is not intact, or absent. At the end of the cycle, the crust was found to contain approximately ten times more water than the other samples.

Climate and surface properties: hydrological response of small arid and semi-arid watersheds

Abstract A positive relationship between annual rainfall and geomorphic processes (runoff and erosion rates) and environmental factors (water regime, soil and vegetation cover) is often assumed for arid and semi-arid areas with an annual rainfall in the range of 100–300 mm. This assumption disregards the fact that changes along a climatic gradient, at desert margins, are not limited to purely climatic factors. They are often accompanied by a parallel change in surface properties; especially the relative extent of rocky or soil covered surfaces that differ greatly in their response to rainfall. This raises the issue whether the change in surface properties along a climatic gradient enhances the assumed positive effects of rainfall increase or limits it. The hypothesis advanced in this paper is that runoff generation and rate in arid and semi-arid areas are primarily controlled by surface properties rather than by the absolute amounts of storm and annual rain amounts. Hydrological data collected at two instrumented watersheds, located one in an arid rocky area, and the second in a semi-arid soil covered area, support this hypothesis. The implications of data obtained for runoff generation and flow continuity under changing climatic conditions are analyzed. They point to the fact that the same regional climatic change may have different, and even opposite effects, on the hydrological response of different adjoining surface units. This response is expected to be strongly controlled by the specific local surface conditions that prevailed in the area prior to the climatic change.

Rainfall–runoff relationship over encrusted dune surfaces, Nizzana, Western Negev, Israel

A thin microbial crust covers the lower part of longitudinal dunes in the western Negev, where average annual rainfall is 95 mm. In order to study the effect of the microbial crust on rainfall–runoff relationships under natural rainfall conditions, runoff plots equipped with pressure gauges were established on opposite north- and south-facing slopes that differ in their vegetal cover and crust properties. The study covered four years (1990–94). The first two years were wet and the following two years relatively dry. One to five flow events were recorded per year. No correlation was found between runoff yield, rain amount and rain intensity. Unlike many microbial crusts reported in the literature, the microbial crust in the western Negev is not hydrophobic. Infiltration rate is high under dry surface conditions and of the order of 9–12 mm h−1 when the crust is saturated. The high final infiltration rate is explained by the occurrence of large pores that do not seal when the crust is saturated. Typical hydrographs have very steep rising and falling limbs, pointing at a limited contributing area. Most flows last less than 10 min and runoff volumes collected are, on the whole, very small. Owing to differences in crust properties, runoff is higher on north- than on south-facing slopes.

The influence of surface properties on flow and erosion processes on debris covered slopes in an arid area

Summary A study aiming to obtain quantitative data on the processes of slope runoff and erosion under arid conditions and relate them to rain and surface properties, was conducted near Elat, in southern Israel. Slope-channel relations, with regard to runoff and sediment transport, were also analysed. Data were obtained by means of three overland flow collectors, two rain recorders and one hydrometric station. The research was conducted over one rain season, during which a single channel flow and five overland flow events occurred. The results lead to the following main conclusions: 1) For a small drainage basin, the threshold values for slopes and channel flow are 3 mm and 5 mm per day, respectively; 2) No clear relation exists between the slope angle and slope runoff and an inverse relation exists between slope angle and slope erosion.

Impact of afforestation on hydrological response and sediment production in a small Calabrian catchment

Extensive afforestation using Pinus and Eucalyptus has taken place in Calabria since the early 1960s to control expansion of calanchi and biancane. In 1978 three small catchments were established near Crotone to monitor the effect of afforestation on hydrological response and sediment yield. In 1992, rainfall simulation experiments were carried out on plots in these catchments to determine more precisely the effect of tree and ground vegetation on surface runoff and erosional response. Most experiments were carried out in a logged catchment with slopes ranging from 20 to 30° and aspects from SW to NE. Results showed complex runoff generation and sediment production, reflecting the effect of microclimate and subtle variations in vegetation on infiltration characteristics. On south-facing slopes with little ground vegetation runoff generation was rapid with runoff coefficients from 27 to 37% and peak sediment concentrations reaching 83.7 g · 1−1. On north-facing slopes with good tree cover, little grass, but continuous leaf litter, runoff coefficients reached 21%, but peak sediment concentration was only 3.6 g · 1−1, while on recently logged north-facing slopes with dense grass cover the highest runoff coefficient was only 5.5% runoff coefficient, and there was virtually no sediment production. Implications of results for forest management and soil conservation are discussed.

The role of topography and surface cover upon soil formation along hillslopes in arid climates

Abstract Two north-facing soil toposequences were selected from within the northern Negev desert, Israel, where average annual rainfall ranges from 70 to 200 mm. Both slopes are composed of an upper rocky and a lower colluvial section. Similar trends were found along both slopes. A high salt content was characteristic of soils at the top of the slope; salinity decreased downslope within the rocky slope section. The opposite occurred along the colluvial slopes, with salinity increasing sharply downslope. At any location along the slopes the northernmost soil toposequence site (160 mm average annual rainfall) represents, from a pedological point of view, an environment which is far more arid than its climatologically drier, more southern counterpart. The explanation provided for the variation of soil proporties at the scale of single hillslopes and at the regional scale is the same. It is contended that water input into the soil, and therefore leaching intensity, is positively related to the ratio of bedrock/soil cover. Rocky areas have limited infiltration, thus yielding high runoff rates into adjoining soil-covered areas, and contribute to water concentration, deeper infiltration and leaching intensity. Soil or sediment-covered areas having relatively high absorption capacities will experience reduced runoff, shallow infiltration and decreased water availability for leaching. This leads over time to salt accumulation at a shallow depth. The decrease in rock/soil ratio downslope within the colluvium is therefore held responsible for the corresponding increase in salinity. Similarly, the greater salinity of the soils in the northern site is explained by the fact that its rock/soil ratio is lower than in the southern area. The theoretical and practical implications regarding the relationship between climatic change and landscape evolution in arid areas are briefly discussed.

Spatial distribution of surface conditions and runoff generation in small arid watersheds, Zin Valley Badlands, Israel

Abstract High rill density may be regarded as indicative of frequent and integrated runoff along a drainage network. However, field observations of soil development and rill geometry in small, first-order catchments (0.1 to 1 ha) of the Zin Valley Badlands, northern Negev, Israel, suggest a pattern of partial area contribution and frequent flow discontinuities along hillslopes and channels. Changing soil properties, associated with an increase of slope angle and slope length, appear to be responsible for high infiltration on the slopes, leading to nonuniform runoff generation within small drainage basins. Runoff observation, sprinkling tests, and soil analysis along ridges and sideslopes were carried out to test this hypothesis. The results confirmed that infiltration capacity on the sideslopes is significantly higher than on the ridges. Under current rainfall conditions, only extreme rainfall intensities are sufficient to generate runoff along entire slopes. The discontinuous nature of most runoff events causes erosion on ridges and deposition on slopes, which enhances soil development on the valley sideslopes, creating a positive feedback on infiltration rate and depth. This demonstrates that the links between within-storm rainfall conditions and spatial distribution of soil characteristics have to be incorporated into our understanding of landscape development in badlands.

The Role of Biological Soil Crusts on Desert Sand Dunes in the Northwestern Negev, Israel

Biological soil crusts are important microphytic communities and significantly influence both structure and processes within the ecosystem. They are built up from cyanobacteria, green algae, fungi, mosses and lichens. Various crust types could be found, depending on dune slope aspect and dewfall availability. In the sand dunes of the northern Negev they cover large areas and stabilize the sand surface against wind and water erosion. Free-living and symbiontic cyanobacteria are capable of nitrogen fixation and are important nitrogen sources in the desert sand dunes. As biological crusts enhance the surface stability and soil fertilization, they are to be considered a key factor in the protection of arid and semiarid ecosystems and, thus, in combating desertification in terms of sand dune remobilization.

Short and long term effects of bioturbation on soil erosion, water resources and soil development in an arid environment

The importance of vegetal cover on soil erosion processes has been recognized for a long time. However, the short and long term effects of faunal activity on soil erosion and soil development had been largely overlooked by geomorphologists. The study of runoff and erosion processes in the Negev desert indicated pronounced systematic differences in sediment concentration and soil erosion rates between rocky and colluvial surfaces. Erosion rates were always higher on the former than on the latter. Field observations drew attention to an intense burrowing and digging activity conducted mainly by Isopods and Porcupines. The monitoring of this activity, based on a grid system, which consists of rows 5 m wide, lasted ten consecutive years. Data obtained suggest the existence of a strong link between the spatial pattern of bioturbation and that of soil erosion. The study also examines, through feedback processes, the regulatory role of bioturbation on the spatial distribution of water availability; soil moisture and soil forming processes. Due to bioturbation two distinct environments were recognised. The rocky environment which is characterized by a positive feedback with a high water availability and low soil salinity; and the soil covered areas where a negative feedback results in low water availability and a high soil salinity.

Promoted erosion and controlled colluviation: A proposal concerning land management and landscape evolution

Abstract The upper limit of colluvial sections constitutes a marked dynamic discontinuity on slopes as is made clear by the discussion of a colluviation model. But in general little is known about the nature of processes and interactions which govern the transition between the eroded slope section and colluvial land systems. The principle of controlled colluviation, an eventual strategy for land and water management in some areas with heavily eroded soils, is explained. It is illustrated by an example from the northern Negev desert. It is argued that soil erosion can have positive effects, at least in some arid areas, and that trapping water and sediment might be an alternative for inefficient erosion control on upper slope sections.