Acknowledgements
We would like to express our best gratitude for Prof Joseph Holden (School of Geography, University of Leeds) for reviewing and providing suggestions to improve the manuscript. This work is supported by the National Natural Science Foundation of China (grant number NSFC 41850410497; NSFC 41807063), the Ningbo Municipal Bureau of Science and Technology (Grant number 201401C5008005) and Grant of Shaanxi Provincial Department of Education (19JK0522).