1. Department of Environmental Sciences; University of Virginia; Charlottesville VA 22904 USA
2. Laboratory EFLUM, Faculte' ENAC; Ecole Polytechnique Federale Lausanne; Lausanne Switzerland
3. Department of Geography; University of California; Los Angeles CA 90095 USA
4. USDA-ARS; Jornada Experimental Range; Las Cruces NM 88003 USA