1. Department of Geography, University of Alcalá, Colegios, 2, 28801, Alcalá de Henares, Spain
2. USGS Western Ecological Research Center, Yosemite Field Station, El Portal, CA 95318-0700, USA
3. Remote Sensing Laboratories, Department of Geography, University of Zurich, Winterthurerstr. 190, CH-8057 Zurich, Switzerland