3.4 Keyword co-occurrence analysis
Research topics and hot spots in a period can be obtained through
co-occurrence analysis of keywords, and research frontier of three
periods can be analysis accordingly (Ouyang et al., 2018; Yu et al.,
2017). As shown in Fig. 4, 5 and 6, with time going on, the nodes of
keyword number are increasing and diversified, and the density of the
keyword co-occurrence is also getting stronger. In addition, the first
15 keywords related to hydrological connectivity in the three stages are
also listed in Table 3. The analysis of keywords is discussed based on
frequency and centrality according to Fig. 4 to 7 and Table 3.
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Please Place Table 3 and Figures 4 to 7 here
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Fig.4 shows the co-occurrence network of keywords during 1998-2004. It
can be seen that the keyword “ecosystem” is the main research content
in this period and has high co-occurrence with connectivity, wetland,
floodplain and biodiversity. During this period, land use change and
river reconstruction in European countries lead to a serious decline in
the hydrological connectivity of floodplain wetland which further
affects the ecosystem and ecological functions (Gumiero et al., 2013).
As a sequence, with the decrease of hydrological connectivity of
floodplain, the richness of species and diversity of ecosystem gradually
decrease. According to the Flood Pulse Concept (FPC), the exchanges of
nutrients and sediments between river and its floodplain is a more
significant ecosystem function than those of longitudinal processes
(Aarts, Van den Brink, and Nienhuis, 2003). Hence, scholars in this
period have done a lot of research on the effects of floodplain on
ecosystem and biodiversity. Chovanec and Waringer (2001) believe that
horizontal connectivity is an important feature of ecological integrity
in floodplains, which is coincided with the main idea of FPC. By
analyzing the breeding behavior of dragonflies, he provided a new method
for the ecological integrity evaluation of the river-floodplain system.
In addition, although hydrological connectivity is the theme of this
paper, it did not appear in the top 15 of the keyword co-occurrence
networks at this period, which indicates that hydrological connectivity
had not been the major research focus at this period.
With the passage of time, the number of nodes in the keyword
co-occurrence network is increasing, and the connection density between
each node is significantly enhanced, which means that hydrological
connectivity attracts increasing attention in this period (Fig. 5). As
shown in Table 3, during the period from 2005 to 2011, the ecosystem
maintained a high co-occurrence frequency and centrality, which was
inseparable from the hydrological connectivity and the close connection
between the ecosystem and biodiversity. The word “hydrological
connectivity” appeared and rank second in the top 15, indicating that
the research topic of hydrological connectivity developed rapidly at
this period. Furthermore, the “water” and “hydrology” rose up to the
top five, while “wetland” “floodplain” and “biodiversity” have
heated down or even drop out of the top 15, this might because that the
researchers in the previous period were mainly from the field of
ecology, and more researchers doing hydrology and water resources were
involved in 2005-2011. In addition, the frequency of node “dynamics”
has increased significantly, and its centrality suggests it has
attracted more attention in this period.
The
hydrological connectivity of water system has a decisive influence on
the hydrological dynamic characteristics. With the intensification of
human activities at this period and the improvement of urbanization, the
utilization measures of rivers, such as the construction of dykes and
dams, changed the hydrological dynamic conditions of rivers, thus
blocking the hydrological connectivity of rivers.
The keyword co-occurrence network from 2012 to 2018 is shown in Fig. 6.
It can be seen, in recent years, the study of hydrological connectivity
has made great progress. “Ecosystem” still occupies a major position
as a key point of hydrological connectivity. In addition, “climate
change” first appeared in top 5, which means the impact of climate
change on hydrological connectivity has gradually entered the research
focus of many scholars. In the past half century, as the climate warms,
the total amount of global river runoff tends to decrease, the frequency
and intensity of extreme hydrological events increase, and the
hydrological pattern of wetlands has undergone profound evolution and
developed in the direction of severe decrease of hydrological
connectivity. Future climate projections for the Amazon indicated a rise
of temperature (Ipcc et al., 2013), total rainfall is likely to decrease
(Malhi et al., 2008). Such dry-warm conditions would likely reduce the
capacity of ecosystems to store water and buffer emissions and increase
the frequency of low-water events in large rivers (Costa, Botta &
Cardille, 2002), lead to the interruption of hydrological connectivity,
which adversely affects the local ecosystem, hydrological dynamic
processes and water quality accordingly.
Though “management” does not have a high frequency as “climate
change”, the number of nodes co-occurrence with “management” are more
than “climate change”, and the centrality is higher, too. Facing the
problems of water resources, environment and ecology brought by climate
change and human activities, it is a good management method to build
hydrological connectivity projects of rivers, lakes and marshes to face
the needs of water supply, flood control and drought prevention, as well
as water quality and ecological security. Hence, in recent years,
scholars have conducted a large number of studies on the
characteristics, classification systems and influencing factors of river
and lake system connectivity, aiming to provide technical support for
the construction planning and operation management of hydrological
connectivity projects. Therefore, the topic related to hydrological
connectivity at this period mainly focus on the comprehensive impacts of
climate change and human activities, including but not limited to water
quality, hydrological hydrodynamics and water ecosystem.
3.5 Author co-cited
analysis
The network of co-cited authors is shown in Fig. 7. Compared with the
total number of articles published by the author, co-citation analysis
is more representative of the value of published articles. It can be
seen in Fig. 7 that Ward has the
most co-cited frequency, account for 461, followed by
Junk (409) and
Tockner (396), and the top three
co-cited authors also have strong centrality, which means their
publications were accepted by more scholars.
A citation burst means that a
reference has a sharp increase in citations in one year or multiple
years, which can reflect whether a publication is hot or not for the
research area (Yu, 2015). Table 4 lists the top 15 co-cited authors with
the Strongest Citation Bursts. The
top ranked is Bracken L J with
bursts strength of 30.3936, second one is Tetzlaff D, 21.2137, followed
by Schiemer F (17.6171). In addition, we found that the burst detection
red line of some co-cited authors expired several years ago (Schiemer
F), which means that he had a strong burst period and was the hot topic
at that time, but now the heat has decreased. While some authors’ (e.g.,
Bracken and Tetzlaff) burst detection red lines extend to 2018,
indicating that the author’s articles still have high hot spots in
recent years which may continue in the future.