USDA-ARS,National Soil Erosion Research Laboratory(NSERL),Purdue University,West Lafayette,Indiana,USA,E-mail:norton@purdue.edu;
British Petroleum-Global Environmental Management,El Torrence,California,USA;
USDA-ARS,National Soil Erosion Research Laboratory(NSERL),Purdue University,West Lafayette,Indiana,USA,E-mail:norton@purdue.edu;