1. Dep. of Agroecology, Faculty of Science and Technology; Aarhus University; Blichers Allé 20, P.O. Box 50 DK-8830 Tjele Denmark
2. Dep. of Soil, Water and Environmental Science; The University of Arizona; 1177 E. 4th Street Tucson AZ 85721-0038 USA
3. Dep. of Civil Engineering; Aalborg University; Sohngaardsholmsvej 57 DK-9000 Aalborg Denmark