1. Introduction
Rivers, lakes, and wetlands are major components of natural water systems (Meng et al., 2020; Liu et al., 2020). They are playing increasing roles in sustaining water not only for human beings but also for ecosystems. At the same time, a well-connected water system can maintain a relatively stable nutritional state in a changing environment (Obolewski, 2011; Kaus et al., 2019). Hydrological connectivity can promote energy and nutrient cycling of the water system and provide important habitats for a variety of birds, fish and benthic organisms, which is important for maintaining biodiversity and promoting community assembly and dynamics (Shao et al., 2019; Lowe 2009; Carrara et al., 2012; Altermatt, Seymour & Martinez 2013; Phillipsen et al., 2014; Seymour, Fronhofer & Altermatt, 2015). Particularly, as having interactive pathways along one temporal dimension (i.e., time scales) and three spatial dimensions (i.e., longitudinal, lateral and vertical ones), the hydrological connectivity patterns of watershed is of significance in formation, development, succession and stability of water ecosystems, as well as promoting the exchange of biological and nutrient substances (Pringle, 2003; Ward, 1989; Pringle, 2001; Chadwick, 2008). However, due to the dual effects of climate change and human disturbances, freshwater storage areas and biological habitats have been destroyed. Hydrological connectivity has been weakened or even broken (Matson et al., 1997; Chapin Iii et al., 2000; Elmore & Kaushal, 2008). Moreover, obstructed hydrological connectivity, on the one hand, could lead to shrinking areas of rivers, lakes and wetlands, in insufficient water resources allocation capacity, frequent drought and poor discharge of floods. On the other hand, it would interfere with the migration and exchange of substances, energy and organisms in water bodies, causing a series of water environmental and ecological problems (Thoms & Reid, 2015; Read et al., 2015; Obolewski, 2011). The above-mentioned problems are getting increasing concerns, attracting international attentions to protect freshwater resources, restore species richness and habitat health as well as improve the ability of flood prevention and resilience.
The concept of hydrological connectivity originated from the “river continuum” proposed by Vannote (1980). It was initially widely used to represent the spatial connectivity for river landscape (Pringle, 2003). Since then, scholars in multiple disciplines have been trying to define the connotation of hydrological connectivity from multiple perspectives (Bracken et al., 2013), such as water cycle (Pringle, 2003), landscape features and spatial patterns, and flow processes (Bracken & Croke, 2007; Tetzlaff et al., 2007; Lane et al., 2004; Hooke 2003; Stieglitz et al., 2003; Croke et al., 2005, Knudby & Ramírez, 2005; Creed & Band 1998, Vidon 2004, Ocampo, Sivapalan, & Oldham, 2006). However, due to the involved multiple research areas and environments, the concept of hydrological connectivity has not been defined uniformly. The widely accepted hydrological connectivity refers to water-mediated transfer of matter, energy and/or organisms within or between elements of the hydrologic cycle (Pringle, 2003; Bracken et al., 2013; Ali & Roy, 2008). Since hydrological connectivity has attracted a lot of attention in recent years, it is necessary to make a comprehensive review and summarize useful conclusions, as well as to find the key advances of hydrological connectivity. The objective of this research is to obtain a comprehensively and systematically scientometrics review of research on hydrological connectivity. Based on the data obtained from Web of Science Core Collection, the analysis in this research will mainly focus on the network of co-citation of authors, journals and references, as well as co-occurrence of keywords and categories, respectively. We attempt to obtain accurate and systematic information on hydrological connectivity research, identify the distribution of subjects and journals, and detect the existing hot topics and emerging trends. Therefore, the knowledge characteristics, intellectual structure, main research frontier issues and future prospects will also be explored.