Desert scrub optical density and spectral-albedo ratios of impacted-to-protected areas by model inversion
Bidirectional surface reflectances measured from NOAA AVHRR over the Negev (southern Israel) and the Sinai are analysed to assess the impact on the surface characteristics of anthropogenic pressures of overgrazing. The impacted Sinai is assumed bare, while the Negev is vegetated by desert scrub. The Negev plants are known to be much darker than the underlying soil, and thus assumed to be absorbing (black). The leaf area distribution as a function of the zenith angle is modelled initially as that of small spheres, which specifies a pronouncedly vertical architecture. We infer from the Negev-to-Sinai reflectance ratios the optical thickness b of the plants (spheres) in the range 0.12 to 0.20 for channel 1 (band centre at 0.63 m), with only weak seasonal variability. Evaluated from average values of b, the Negev-to-Sinai ratios of the spectral albedos (hemispheric reflectances) are 0.63 and 0.55 in channel 1 and 0.67 and 0.60 in channel 2, at solar zenith angles of 30° and 60°, respectively. These ratios indicate the severe climatic impact of overgrazing in the Sinai, inasmuch as a high albedo means reduced shortwave heat absorption (which is detrimental to rainfall-inducing convection). We subsequently proceed to invert the Negev-to-Sinai reflectance ratios assuming a plant-element distribution tending even more to the vertical. The values of b are reduced when derived for a greater tendency to vertical architecture. The Negev-to-Sinai ratios of the spectral albedos are also significantly lower in these cases, which means that the assessed impact of over-grazing in the Sinai is indeed extremely severe. We conclude that plant architecture (which controls the reflection anisotropy) should be considered when evaluating the albedos of vegetated versus bare (impacted) surfaces from satellite-measured bidirectional reflectances. Uncertainty in the zenith angle distribution of the leaf area produces significant uncertainty in the albedo assessment. Multidirectional reflectance measurements made near the ground would greatly reduce uncertainties about the surface-reflection anisotropy, and thus enhance the value of satellite measurements.