Satellite and field studies of man’s impact on the surface in arid regions

Abstract

In arid regions with sandy soils, both the steppe protected from excessive anthropogenic pressures and the much brighter areas subjected to such pressures show spectral reflectivities (derived from satellite radiometric imagery) increasing with wavelength (from green to the reflective infrared). The reflectivity dip in the red, characteristic of all plants containing chlorophyll, is absent. In an ecosystem of a steppe protected from man’s activities, the soil is stable and crusted. Dead plants either remain in situ or litter the surface as debris. The surface spectral reflectivities (derived from satellite imagery) can be represented as a weighted average of the soil reflectivities and the plant-debris reflectivities (which were measured in the field by a hand-held radiometer). In areas affected by man, the surface spectral reflectivities (derived from satellite imagery) virtually coincide with those of crumbled bare soil (as determined by the hand-held radiometer). There is little accumulation of organic material. In such regions, a sharp albedo increase results from anthropogenic impact (from 0.28 in the case of the protected Negev to 0.45 in the case of the adjacent, impacted Sinai). This albedo difference cannot be explained as directly due to the removal of live vegetation, but is due mainly to an accumulation of dead plants on the crusted soil in the protected steppe and the absence of such an accumulation in the impacted regions. Models of the nadir reflectivity as a function of the fractional cover by plants or plant debris (either scattered on the surface or forming a simple vertical structure) are presented and are applied to assessing the Negev vs Sinai differences and the sharp recovery (darkening) of the surface in a Sinai exclosure where anthropogenic pressures were stopped in 1974 by fencing-off the area.