1. a Environmental Systems Analysis Group , Wageningen University , PO Box 47, 6700 AA , Wageningen , Netherlands
2. b Department of Geography , Federal University of Technology, Yola , P.M.B 2076 , Yola , Adamawa State , Nigeria
3. c International Water Management Institute, Southern Africa Office , Private Bag X813, Silverton, 0127 , Pretoria , South Africa
4. d Irstea – UMR G-EAU , 361 rue Jean-François Breton, BP 5095, 34196 , Montpellier Cedex 5 , France