Abstract

Shuttle imaging radar (SIR-A) coverage across Bolivia shows the major physiographic provinces, including the Andean Altiplano. The Altiplano contains a variety of eolian features, many of which are visible as radar-dark, radar-mottled, and radar-bright streaks aligned parallel to the prevailing winds. The streaks form downwind from hills and are as much as 15 km long and 800 m wide. Dark streaks originate between closely spaced hills, whereas bright streaks form in the immediate lee of hills. The radar brightness of the streaks is modulated by the proportion of sand cover, vegetation, and exposed substrate, all of which relate to the local wind regime. In the radar-dark streaks, vegetation is sparse and the sand is organized into dune forms, some of which can be recognized as barchan dunes on the radar image by outlining of the dark dune by bright substrate. Radar-mottled zones are covered by numerous 0.5-2-m-high coppice dunes, and in radarbright streaks, the sand forms flat sheets but also includes sand mounds and abundant vegetation. Measurements indicate that winds are strongest and most turbulent in the region of active sand sheets and dunes, inhibiting the growth of vegetation and the formation of mounds and resulting in a radar-dark zone. These results are consistent with previous studies that have demonstrated the existence of turbulent eddies in the wake of topographic obstacles that produce zones of increased surface wind shear and enhanced sand mobility. The relation between the radar signature and the wind regime allows inferences to be drawn for the wind regime from image interpretation.

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