1. a University of Botswana , P/Bag UB00704, Gaborone , Botswana
2. b Department of Physical Geography and Quaternary Geology , Stockholm University , SE—106 91 , Stockholm , Sweden
3. c Department of Physical Geography and Quaternary Geology , Stockholm University , SE—106 91 , Stockholm , Sweden
4. d Harry Oppenheimer Okavango Research Centre , P/Bag 285, Maun , Botswana