1. Department of Geography; Ghent University; Krijgslaan 281-S8 B-9000 Gent Belgium
2. Department of Land Resource Management and Environmental Protection; Mekelle University; PO Box 231 Mekelle Ethiopia
3. Department of Earth and Environmental Sciences; K.U. Leuven, Celestijnenlaan 200E B-3001 Heverlee Belgium
4. Old Abbey Associates; Tarbet Court, Dirleton East Lothian Scotland UK
5. Jet Propulsion Laboratory; California Institute of Technology; 4800 Oak Grove Drive Pasadena CA 91109 USA