1. a School of Water Resources and Environment, China University of Geosciences , Beijing, 100083, PR China
2. c Wageningen University, Centre for Geo-Information , Wageningen, The Netherlands
3. b Department of Geoscience , University of Iowa , Iowa City, IA, 52242, USA
4. d International Institute for Geo-Information Science and Earth Observation (ITC) , Enschede, The Netherlands