Remote monitoring of shifting sands and vegetation cover in arid regions

A significant factor in the degradation of arid and semi-arid lands is the removal of sand and soil through wind action. Mobilized sand can destroy downwind vegetation through sand blasting and burial. Since vegetation cover is an important factor in determining the erodibility of an area, there is a potential positive feedback that can lead ultimately to the mobilization of larger areas of sand. Mobilized areas of sand in irrigated arid regions can be detected in the visible and near-infrared because of increased albedo and the decreased plant cover, in the thermal infrared due to emissivity differences between the very loose sheet of active material and the relatively consolidated stable soils, and in the radar wavelengths because of the unusual, but theoretically predicted, polarization effects caused in radar reflection from wind-rippled surfaces. This paper focuses on the application of these techniques to the Manix Basin area of the Mojave Desert in Southern California. The Manix Basin area has been the site of extensive cultivation using center-pivot irrigation systems. Progressive abandonment of many fields since 1972 has mobilized some areas of sand. The authors have used sizable body of remote sensing data from airborne and satellite platforms to study this area along with in-situ investigations. These investigations have shown that even though arid region vegetation presents special problems for the quantitative use of remote sensing instruments, indices such as IPVI can be used to make qualitative determinations of arid region plant cover. Lower levels of Infrared Percentage Vegetation Index indicate either lower plant cover, or a shift from the stable climax community vegetation (creosote bush) to communities more typical of disturbed areas. A temporal study of this area has revealed that the sand mobilization is episodic and tending to increase in areal extent with each episode.