3.2 Subject category distribution
The co-occurrence analysis of subject categories was conducted. As the co-occurrence network from 1998 to 2018 shown in Fig. 2, Environment Science and Ecology is the most popular research area (frequency, 1,413) above all. The others of top 3 are Water Resources (1,135) and Environmental Sciences (1,029). In addition, Fig. 2 displays some nodes have high centrality as there are purple outer cycles surrounding them, the details are shown in Table 1. Among the top 15 subject categories, Environmental Sciences has the highest centrality in hydrological connectivity. The second is Biodiversity and Conservation, followed by Geosciences, Multidisciplinary, Ecology, physical Geography. Therefore, the study of hydrological connectivity is carried out within the scope of a multidisciplinary diversity, from Environmental Sciences to Biodiversity, Water Resources, Ecology. Particularly, the Environment Science and Ecology and Water Resources played predominant roles.
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Please Place Figure 2 and Table 1 here
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