1. a Institute of Hydrology, University of Freiburg , Fahnenbergplatz , D-79098 , Freiburg , Germany
2. b Department of Water Engineering , UNESCO-IHE , PO Box 3015, 2601 DA , Delft , The Netherlands E-mail:
3. c School of Bioresources Engineering and Environmental Hydrology, University of KwaZulu-Natal , Pietermaritzburg , South Africa