1. Introduction
In the process of rapid urban development, social problems (too high
urban population density, and aging urban population), environmental
problems (urban heat island effect), and mental health problems
(depression) gradually occur. Due to the rapid economic development and
the increase in living pressure, these problems gradually become
prominent and inhibit the construction of urban ecological civilization
and the improvement in resident happiness. At the beginning of the 19th
century, western countries began to construct urban spaces with health
as the premise in order to deal with environmental degradation and
public health threats (Corburn, 2004). The exposure to the natural
environment (including water bodies and green spaces) can improve
people’s mental health (Wentworth, 2017) and the natural environment
plays a significant role in promoting urban environmental health (Weng
and Yang, 2004) and social development (van den Berg et al. ,
2015). As an “organic aggregate” composed of water and green spaces,
the blue-green space has gradually attracted the attention of scholars
due to its benefits in ecology, economy, and mental health, and many
related theoretical achievements have been obtained. After the
construction of ”Emerald Necklace” Greenway Park in Boston in the 1910s
(Blackmar et al. , 1997) , the development of green channel
planning (Benedict and Mcmahon, 2006), low-impact development (Grant and
Tsenkova, 2012), water-sensitive city (Wong and Brown, 2009), resilient
city (Bengtsson et al. , 2007), sponge city and other related
concepts has highlighted the key role of blue-green spaces in urban
planning and design.
A blue-green space is a space with a clear water body that is covered by
plants or maintains natural features and can be directly touched or
perceived (Leng et al. , 2021). In other words, blue includes all
natural and artificial water bodies such as rivers, lakes, reservoirs,
and wetlands and green includes mountains, woodlands, farmlands,
grasslands, ecological corridors, green spaces in parks, residential
green spaces, protective green belts, woodlands, nature reserves, and
other areas covered by vegetation. Domestic and foreign scholars found
that blue-green spaces had more significant ecological, economic, and
mental health benefits than single water bodies and green spaces.
Ecologically, the synergy between water and green spaces in the
ecosystem endows the blue-green space with a significant cooling effect,
namely, the Urban Cold Island (UCI) effect, which can effectively
alleviate the Urban Heat Island (UHI) effect (Santamouris et al. ,
2011). Blue-green spaces can provide enough habitat for organisms,
purify the air, and reduce noise and carbon storage, thus greatly
promoting biodiversity and ecosystem stability (Maller et al. ,
2005). Economically, compared to cool materials, urban blue-green spaces
have the advantages of low cost, environmental friendliness and
political acceptability (Hamada and Ohta, 2010; Carvalho et al. ,
2017). In addition, the applications of sustainable drainage systems
(SuDS) and green infrastructure (GI) can better improve urban drainage
efficiency, and promote economic growth (Andrew, 2017). Moreover,
natural exposure has direct or indirect positive effects on mental
health, including perceived recovery (Kaplan, 1995), stress relief
(Ulrich et al. , 1991), and mood improvement (White et al. ,
2017).
In the 1990s, ecosystem health became a new goal in regional
environmental management (Costanza and Norton, 1992). The health
assessment of water and green space ecosystems has been concerned among
many organizations and scholars and has become the core content of the
current comprehensive assessment of ecosystems. As a composite ecosystem
composed of water body and green space, the blue-green space has a more
complex spatial structure and diverse ecological functions. The
traditional single water body or green space evaluation index system
cannot be simply applied in the evaluation of a blue-green space.
Therefore, in this paper, relevant studies on the evaluation system of
blue-green space indices are summarized in order to discuss the
selection of evaluation indices in blue space, green space and
blue-green space. Moreover, different evaluation methods are analyzed in
order to propose an evaluation index system and an evaluation method
suitable for blue-green spaces based on existing studies. The results
provide the basis for the evaluation of the degree of urban blue-green
integration, urban planning, and ecological restoration.