Detecting vegetation change in the southern Kalahari using Landsat TM data

Using Landsat TM data we explored the magnitude and direction of change in reflectance from two study areas in the southern Kalahari Desert from 1989 to 1994. The study areas comprise three land use types: nature conservation, commercial game ranching and commercial pastoralism. The presence of these land use types provides the opportunity to explore the impacts of land management policies on vegetation structure, cover, species composition and the radiometric signal. In other semi-arid savannas provision of surface water and the associated increase in herbivory by domestic stock have resulted in a shift from grassy arid savanna to a shrubland with low perennial grass cover and high annual grass cover. We wished to ascertain whether these changes around water points and across fence lines could be detected using satellite-derived imagery. Using change vector analysis (CVA), we explored three bands (visible, red, near-infrared) of Landsat TM data recorded in April 1994. Change vector analysis shows the direction and magnitude of change between two anniversary data images. This study found that near-infrared activity has increased on active dunes near water points on commercial farms since 1989. We attribute this increase to two major changes to the ecosystem: (a) an increase in the exposed soil surface area; and (b) an increase in active greenness of woody shrubs. A previously identified (before 1989) site of invading Rhigozum trichotomum in the dune swales did not significantly change between the 2 years. Reflectances in the conservation area did not significantly change in magnitude or direction between 1898 and 1994. (C)1998 Academic Press Limited.