1. Assistant Professor, Dept. of Soils, Water, and Agricultural Engineering, College of Agricultural and Marine Sciences, Sultan Qaboos Univ., P.O. Box 34, al Khoud 123, Muscat, Sultanate of Oman (corresponding author).
2. Visiting Research Professor, Dept. of Environment, Earth and Ecosystems, Open Univ., Walton Hall, Milton Keynes MK7 6AA, U.K.