Haim Tsoar

Spectral reflectance of biogenic crust developed on desert dune sand along the Israel-Egypt border

Abstract The effect of biogenic crust on imagery acquired by spaceborne sensors is demonstrated. The crust consists mostly of microphytes such as cyanobacteria. The macrophytes (higher vegetation) on the sand dunes are sparse and have a relatively low spectra! reflectance response. However, since a considerable ponton of the ground is covered by this biogenic crust, (which has a different spectral reflectance from that of the mobile sands), a sharp brightness contrast is created between the two areas. It can be concluded that the well-known contrast between Sinai (Egypt) and the Negev (Israel), that has long drawn the attention of many observers, is not a direct result of vegetation cover but is caused by an almost complete cover of biogenic crust in the Negev, and a lack of this crust in Sinai, due largely to mans activities.

The effect of microphytes on the spectral reflectance of vegetation in semiarid regions

The normalized difference vegetation index (NDVI), which is derived from satellite sensor images, is widely used as a measure of vegetation and ecosystem dynamics, change in land use, desertification, and climatic change processes on a regional or global scale. Surprisingly, in semiarid regions, relatively high values of NDVI were measured in landscapes where little, if any, photosynthetic activity of higher plants exists. We tested the hypothesis that the high NDVI values may be caused by the photosynthetic activity of microphytes (lower plants), consisting of mosses, lichens, algae, and cyanobacteria, which cover most of the rock and soil surfaces in semiarid regions. We found that the spectral reflectance curves of lower plants can be similar to those of the higher ones and their derived NDVI values can be as high as 0.30 units. We conclude that, in semiarid environments, the reflectance of lower plant communities may lead to misinterpretation of the vegetation dynamics and overestimation of ecosystem productivity.

Linear dunes - forms and formation:

Most geomorphologists and geologists do not differentiate the various types of linear dune and refer to them as one dune type mainly in the terms of linear, longitudinal or seif dunes. In recent morphodynamic and genetic classifications of dunes, all types of linear dunes are lumped under the heading of longitudinal dunes (Wasson and Hyde, 1983; Hunter et al., 1983). This review, however, substantiates three different simple linear dunes - lee dunes, seifs and vegetated- linear - each of which has a different shape and mechanism of formation and elongation.

Elongation and migration of sand dunes

Two distinct processes are known to act on dynamic dunes, the process of migration by erosion on the windward side and deposition on the lee side, typical for transverse dunes, and the process of elongation typical for linear dunes. These two processes are determined by wind direction relative to the dune alignment. This article reviews the assertion that linear dunes experience lateral displacement in addition to elongation. Fieldwork on vegetated linear dunes (VLDs) and GIS work on seif dunes indicates no lateral migration for these dunes. Linear dunes can shift laterally only when a slip face, formed on the lee side, reaches the plinth of the dune. The winds from both sides of the seif dune are never symmetric; usually winds from one direction are more dominant and effective. The outcome is the formation of peaks and saddles along the dune. The strongest winds create a slip face on the lee side of the peak segments of the dune, oblique to the dune alignment, which reaches the base of the dune and displaces the peak downwind along the dune alignment. The internal structure of the seif dune is formed mostly by this dominant wind direction and gives the impression that the dune has shifted laterally. On the other hand, there are cases in which the wind directions relative to dune alignment fall between those of transverse and seif dunes. In such cases, both processes act on the dune, which subsequently experiences migration as well as elongation.

Dune formation under bimodal winds

The study of dune morphology represents a valuable tool in the investigation of planetary wind systems—the primary factor controlling the dune shape is the wind directionality. However, our understanding of dune formation is still limited to the simplest situation of unidirectional winds: There is no model that solves the equations of sand transport under the most common situation of seasonally varying wind directions. Here we present the calculation of sand transport under bimodal winds using a dune model that is extended to account for more than one wind direction. Our calculations show that dunes align longitudinally to the resultant wind trend if the angle θw between the wind directions is larger than 90°. Under high sand availability, linear seif dunes are obtained, the intriguing meandering shape of which is found to be controlled by the dune height and by the time the wind lasts at each one of the two wind directions. Unusual dune shapes including the “wedge dunes” observed on Mars appear within a wide spectrum of bimodal dune morphologies under low sand availability.

What determines the spectral reflectance of the Negev-Sinai sand dunes

Abstract The difference in spectra reflectance between the north-western Negev desert and the adjacent Sinai sand dunes has attracted the attention of many scientists. Remote sensing analysis of three Landsat Multi-Spectral Scanner (MSS) images acquired in the summer of 1984, 1987, 1989, followed by intensive field work, indicate that the area is not homogenous and is undergoing a quick recovery from the intensive grazing that it suffered between 1968 and 1982. The outcome is a gradual decrease in the brightness of the Negev between 1984 and 1989 in all MSS bands except band 7 which shows very little or no change with time. This is due to the increase in biogenic crust and vegetation cover. The Sinai bare sand shows the opposite trend of little to no change in all bands but band 7, which shows an increase in reflectance with time. This is probably due to the effect of further destruction of vegetation in the Sinai after 1982. In the northern part of the area, dunes are low, have a high percentage of fines...

The ecological background, deterioration and reclamation of desert dune sand.

Abstract The physical properties of desert dune sand are reviewed as part of its ecological characteristics. Sandy soils are known for their superiority to other finer soils in deserts. For this reason, sand dunes in arid lands are covered, as a natural process, by vegetation. The drawback of sand is its mobility: erosion of sand, not lack of moisture, is the major limiting factor for vegetation on dunes. The destruction of vegetation by overgrazing and by being collected for firewood and building material is a relatively quick process. This stimulates sand movement and increases deterioration through positive feedback. Whilst deterioration processes are swift, natural reclamation processes are slow. One way of increasing the productivity of dune sand is by the use of sophisticated methods of agriculture. Unlike sand in humid areas, sand in deserts offers many advantages for agriculture: it is the only soil that endures irrigation with brackish water, and its thermal retention forces crops to ripen quickly. The problem of low and unreliable rainfall in arid lands can be turned to advantage by the use of the drip irrigation method, which allows a rational and economic use of fertilizers without the risk of leaching by subsequent rain.

Slope effect on saltation over a climbing sand dune

Abstract Results from a three-year study are presented that integrate field work (in Israel), physical modeling (wind-tunnel testing at UC Davis), and numerical solutions of grain trajectories to model and explain sand transport over a climbing dune. Field grain-size analyses of surface- and saltation-trap materials taken along various positions of the slope suggest that only smaller particles ( u ∗ , equal 30 cm/s) were able to climb a 20 degree slope in an escarpment normal to the prevailing strong wind direction. Numerical solutions of the particle trajectories are in good agreement with field measurements and confirm that particle motion is diminished at the base area of the slope and that the motion of larger-sized particles is completely terminated which causes an accumulation of these particles. The results suggest that the transport of the majority of the larger particles (> 230 μm) is by saltation. The model is numerically extended to a general series of slopes, particle sizes, and friction speeds with similar tends exhibited.

International Borders and Range Ecology: The Case of Bedouin Transborder Grazing

Recent research suggests that, under unconstrained human circumstances, pastoral nomads within arid environments have at their disposal means of evading ecological stress that could impel them to cause damage to their grazing and land resources. The Israeli-Egyptian border has imposed a severe constraint upon the range management strategy of the Bedouin whose traditional territory it bisects. The border forced them to exert an increased pressure upon local resources. Considerable damage was thus caused to the perennial vegetation cover (both macrophytes and microphytes) and to the structure of sand dunes on the Egyptian side of the border, with opposite effects on the Israeli side to which the Bedouin had no access. This case study adds a further dimension to the discussion of range management by pastoral nomads in arid and semi-arid areas.

Wind Tunnel Modeling of Echo and Climbing Dunes

Publisher Summary This chapter describes the wind tunnel modeling of echo and climbing dunes. Eolian sand dunes originate as accretions of sand on existing sand patches, and may develop independent of fixed surface features. It is found that apart from these self-accumulated dunes, sand accumulations are found in front of, or behind, topographic obstacles such as cliffs, shrubs, and boulders. The data concerning the formation of echo and climbing dunes gathered during wind tunnel tests made on several simulated cliffs with different inclinations is presented in the chapter. Three types of measurements were taken, including the modeling of echo and climbing dunes in front of simulated cliffs, tracing of eddies and wind turbulence in front of the simulated cliffs and over echo dune models and through a bubble generator, and measurements of wind velocity (with a hot-wire anemometer) in front of the simulated cliffs and over the echo dune models. It is observed that when echo dune models built of wood were placed in front of the simulated cliff with shape and position similar to those modeled with sand the bubbles showed constant and prominent reverse-flow eddies between the dune and the cliff.