1. Assistant Professor, Dept. of Civil and Environmental System Engineering, Hanyang Univ., Kyeonggi-do 426-791, Korea.
2. Professor and Head, Dept. of Civil Engineering and Engineering Mechanics, and Center for Sustainability of Semi-Arid Hydrology and Riparian Areas (SAHRA), The Univ. of Arizona, Tucson, AZ 85721-0072 (corresponding author).
3. Associate Professor, Dept. of Civil and Environmental Engineering, Korea Univ., Seoul 136-701, Korea.