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.