1. Department of Soil, Water, and Climate, University of Minnesota, St. Paul, Minnesota 55108
2. Soil and Water Management Research Unit, Agricultural Research Service, U.S. Department of Agriculture, 1991 Upper Buford Circle, Room 439, St. Paul, Minnesota 55108
3. Instituto de Recursos Naturales y Agrobiologia de Sevilla, CSIC, P.O. Box 1052, 41080 Sevilla, Spain