1. Researcher, Dept. of Agricultural Engineering, Graduate School, Chungnam National Univ., 220 Goong-dong, Yooseong-gu, Daejeon 305-764, Republic of Korea
2. Professor, Dept. of Rural Infrastructure Engineering, College of Agriculture and Life Sciences, Chungnam National Univ., 220 Goong-dong, Yooseong-gu, Daejeon 305-764, Republic of Korea (corresponding author).