1. State Key Lab. of Earth Surface Processes and Resource Ecology; School of Geography, Beijing Normal Univ.; Beijing 100875 PR China
2. Dep. of Hydrology; Geological Survey of Denmark and Greenland (GEUS); Ø. Voldgade 10 DK-1350 Copenhagen
3. Dep. of Geography and Geology; Univ. of Copenhagen; Øster Voldgade 10 DK-1350 Copenhagen