1. Department of Rural Systems Engineering, College of Agriculture and Life Sciences, Seoul National University, San 56-1, Sillim-dang, Gwanak-gu, Seoul, 151-742, Korea, E-mail: iamchoi@snu.ac.kr
2. Department of Agricultural and Biological Engineering, Purdue University, 225 S. University Street, West Lafayette, IN, 47907-2093, USA, E-mail: engelb@purdue.edu
3. Department of Forestry and Natural Resources, Purdue University, 195 Marsteller Street, West Lafayette, IN, 47907-2033, USA, E-mail: rlfarnsw@purdue.edu