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\begin{document}

\title{Advances in the blue-green space evaluation index system}



\author[1]{Zhao Zhimiao}%
\author[1]{xiangxiang Jiao}%
\author[2]{Wang Zhufang}%
\author[1]{Li Xiao}%
\author[1]{Zhang Yinjiang}%
\affil[1]{Shanghai Ocean University}%
\affil[2]{University of Surrey}%


\vspace{-1em}



  
  \date{May 26, 2024}


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\selectlanguage{english}
\begin{abstract}
Blue-green spaces composed of water and green spaces have certain
ecological, economic, and social benefits for urban development. Many
evaluation index systems have been developed to evaluate the health
status of urban ecosystems. However, in these evaluation index systems,
blue and green spaces are separately evaluated without considering the
synergistic effect between water and green spaces in the ecosystem, thus
affecting the unified planning and construction of cities. Therefore,
based on existing studies, the development process of the blue-green
space evaluation index system is divided into three stages: supply
service evaluation stage, adjustment service evaluation stage, and
cultural service evaluation stage in order to deeply discuss the
development path of the blue-green space evaluation index system. The
single blue and green evaluation index systems are compared from the
perspectives of index selection and evaluation content. Moreover, the
characteristics of conventional evaluation methods are discussed in
order to propose an applicable evaluation index system and evaluation
method of blue-green spaces for evaluating the degree of blue-green
integration of cities. The review provides the basis for urban planning
and ecological restoration.%
\end{abstract}%



\sloppy


\textbf{Advances in the blue-green space evaluation index system}

Jiao Xiangxiang\textsuperscript{1}, Wang Zhufang\textsuperscript{2}, Li
Xiao\textsuperscript{1}, Zhao Zhimiao\textsuperscript{1,3,*}, Zhang
Yinjiang\textsuperscript{1,4}

\emph{\textsuperscript{1}College of Marine Ecology and Environment,
Shanghai Ocean University, Shanghai 201306}

\emph{\textsuperscript{2}Department of Civil and Environmental
Engineering, University of Surrey, Guildford, UK}

\emph{\textsuperscript{3}Hebei Provincial Key Laboratory of Wetland
Ecology and Protection, Hengshui, Hebei 053030}

\emph{\textsuperscript{4}Shanghai Taihe Water Technology Development
Co., Ltd. Shanghai 201306}

*Corresponding author:

Zhao Zhimiao: Lecturer, College of Marine Ecology and Environment,
Shanghai Ocean University; Shanghai 201306. P.R. China.

E-mail: zmzhao@shou.edu.cn.

\textbf{Abstract} : Blue-green spaces composed of water and green spaces
have certain ecological, economic, and social benefits for urban
development. Many evaluation index systems have been developed to
evaluate the health status of urban ecosystems. However, in these
evaluation index systems, blue and green spaces are separately evaluated
without considering the synergistic effect between water and green
spaces in the ecosystem, thus affecting the unified planning and
construction of cities. Therefore, based on existing studies, the
development process of the blue-green space evaluation index system is
divided into three stages: supply service evaluation stage, adjustment
service evaluation stage, and cultural service evaluation stage in order
to deeply discuss the development path of the blue-green space
evaluation index system. The single blue and green evaluation index
systems are compared from the perspectives of index selection and
evaluation content. Moreover, the characteristics of conventional
evaluation methods are discussed in order to propose an applicable
evaluation index system and evaluation method of blue-green spaces for
evaluating the degree of blue-green integration of cities. The review
provides the basis for urban planning and ecological restoration.

\textbf{Key words} : blue-green space; evaluation index system;
ecological service function; water body; green space.

\textbf{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 \emph{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 \emph{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 \emph{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 \emph{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 \emph{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 \emph{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 \emph{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 \emph{et al.} , 1991), and mood improvement (White \emph{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.

\textbf{2. Necessity for establishing a blue-green space evaluation
index system}

\emph{2.1. Multiple benefits of blue-green spaces}

Urban green space and water bodies have multiple functions and can meet
the diverse requirements of urban development, such as human-centered,
organism-centered, and economic construction-centered functions. Among
these functions, anthropocentric functions mainly include health
functions, entertainment functions, etc. Organism-centered functions are
mainly ecological service functions, such as habitat provision, climate
control, and pollution control(Rehachova and Pauditsova, 2004). Economic
construction-centered functions include increasing urban and resident
wealth (Chiesura, 2004). These functions are classified into three
aspects: ecological, mental health, and economic benefits.

\emph{2.1.1. Ecological benefits}

Due to the accelerated urbanization process and dense urban buildings,
the local temperature, humidity, air convection and other factors of the
urban surface have been artificially changed, thus causing the change in
urban microclimate (Xu \emph{et al.} , 2019). Many researchers
investigated the distribution pattern of the UHI effect (Peng \emph{et
al.} , 2016), and the relationships between air temperature (Qiu
\emph{et al.} , 2017), land surface temperature (Yu, 2018), and land
cover (Gao\emph{et al.} , 2019), and their correlation with ecosystem
service values (Anjos and Lopes, 2017) and found that urban green space
and water bodies had a good UCI effect.

A blue space, as a low-temperature corridor, can effectively alleviate
the UHI effect. The shape and contour of water bodies are the main
factors affecting UCI (Dai and Yao, 2021). Due to the unique continuity,
linear waters can effectively divide the aggregation status of heat
islands, block the regional expansion of the UHI effect, and provide
heat dissipation channels for heat exchange between the internal and
external parts of urban heat islands (Cheng \emph{et al.} , 2019).
Through the physical effect of water evaporation, a surface water body
can reduce the temperature of the heat island area well and the closer
it is to the center of the urban heat island, the stronger the cooling
effect is (Yue and Xu, 2013).

As a natural underlying surface for climate regulation, a green space
provides a city with ecosystem services maintaining biodiversity and
regulating the urban climate. Vegetation coverage, planting structure,
and width of green space affect the UCI effect (Anjos and Lopes, 2017;
Yang \emph{et al.} , 2017), and can strengthen the temperature effect in
the horizontal direction of the water body. Compared with densely built
asphalt roads and concrete roads, a green space has a faster heat
absorption rate and a smaller specific heat capacity (Lo \emph{et al.} ,
1997). Good physical shading can reduce the leaf surface temperature and
transpiration can prevent the air temperature from rising (Kobayashi H,
2005), thus alleviating the UHI effect.

Some scholars studied the relationship between green spaces and the
cooling effect of water and found that there was a nonlinear
relationship between green space vegetation coverage and the cooling
effect of water. In addition, the type of vegetation coverage also
affects the cooling effect of water (Table 1). Green spaces and water
bodies have a synergistic effect in reducing the ambient temperature and
their cooling effects on the surrounding environment are mutually
influenced (Robitu \emph{et al.} , 2006; Wu \emph{et al.} , 2018).
Therefore, in a limited urban area, the UCI effect can be maximized by
constructing a blue-green space in order to promote the development of a
city (Montazeri \emph{et al.} , 2017). A blue-green space can provide
the habitat to maintain biodiversity and regulate urban climate,
displaying its ecological service function as a composite ecosystem.

\emph{2.1.2. Mental health benefits}

Blue-green spaces can provide good social benefits and greatly affect
the livability of modern cities and the well-being of urban residents.
The frequency of exposure to the natural environment and vegetation
coverage may be important for improving human well-being, especially
mental health (Wentworth, 2017). The exposure to the natural environment
can increase social cohesion (Kaczynski and Henderson, 2007), reduce
stress, improve attention, relieve fatigue (Grigsby-Toussaint \emph{et
al.} , 2015), improve mood, and decrease the symptoms of major
depressive disorders (Hartig \emph{et al.} , 2014). For example, a good
park environment has the functions of attention recovery and stress
relief effects (Tan and Li, 2009). Blue-green spaces are associated with
children's cognitive development, inhibition of impulsive behavior,
prosociality enhancement, and stimulation of imagination and creativity
(Dadvand \emph{et al.} , 2015; McCormick, 2017). There is a significant
correlation between urban blue-green space and depression in the elderly
(Helbich \emph{et al.} , 2019). In 2021, World Health Organization (WHO)
released a survey report on blue-green spaces and mental health. The
short-term and long-term effects of different types of green spaces and
blue spaces on mental health (Table 2) demonstrated the overall positive
relationship between blue-green spaces and mental health. Especially,
under the background of~COVID-19 pandemic, blue-green spaces can serve
as a sanctuary for interactive leisure.

\emph{2.1.3. Economic benefits}

The economic benefits of blue-green spaces are introduced in two
aspects. Firstly, a blue-green space can provide a good landscape for a
city, increase the attractiveness of the city, promote the development
of tourism, and increase the value of real estate in the city. It
promotes the productivity of citizens and the growth of urban economy.
The beautiful landscape of Singapore and Kuala Lumpur, Malaysia is one
of the factors that attract a lot of foreign investment, enhance the
property value and increase the financial return of land developers by
5\% to 15\% depending on the type of a project (Heidt and Neef, 2002).
Secondly, green buildings or infrastructure in blue-green spaces, mainly
including GI, provides better facilities and entertainment opportunities
for human beings in addition to habitats (Ashley, 2017).With vegetation
features on the ground, such as swamps, cisterns and rain gardens, based
on natural drainage processes SuDS/GI facilities fully integrate the
ecological benefits of water bodies and green spaces, promote ecosystem
services and improve the efficiency of urban drainage systems and the
economic development (van den Berg \emph{et al.} , 2015). In addition,
the blue-green space has the benefit of UHI reduction and can reduce
energy consumption. A Chicago study showed that increasing tree coverage
by 10\% in a city could reduce total energy required for heating and
cooling by 5\% to 10\% (Sorensen \emph{et al.} , 1997). Therefore, the
development of urban blue-green spaces may be a better solution than
cool materials (Carvalho \emph{et al.} , 2017).

\emph{2.2. Lack of practical basis}

A blue-green space is a measure of social environment, economy, and
health and closely related to the happiness and satisfaction of urban
residents. At present, many evaluation index systems of urban water body
and green space have been developed as the basis for evaluating urban
health and urban ecological civilization and have promoted the
construction of blue-green spaces. In existing studies, water bodies and
green spaces were separately evaluated without considering the
coordinated planning and comprehensive urban ecosystems. In addition,
without enough reliable and comprehensive indices, it is impossible to
evaluate the spatial structure and ecological functions of blue-green
spaces. In addition, water conservancy, greening, and landscape
departments have different planning functions, so the separate
construction of water body and green space have resulted in many
problems. such as blue-green division. Natural hydrological conditions,
land-water interlaced zones, or the connectivity of urban water-green
corridors is not considered, thus affecting the integrated construction
of urban blue-green spaces and severely limiting the quality of the
constructed urban ecological environment. Therefore, it is particularly
important to determine the appropriate evaluation indices and evaluation
criteria based on previous studies and establish a scientific index
system and an evaluation method to evaluate the degree of blue-green
integration in urban planning.

\textbf{3. Evaluation index systems of blue-green spaces}

In the process of rapid urban development, some problems such as the
destruction of ecological balance inevitably existed due to imbalanced
planning, thus eventually leading to urban sub-health of a city.
Therefore, the ``symptoms'' and ``causes'' of urban environmental
problems are often explored by constructing an evaluation index system,
in order to obtain suitable countermeasures for urban restoration and
subsequent ecological restoration. Therefore, the construction of cities
has experienced the following transformation stages: the initial stage
focusing on water bodies and green spaces for the purpose of providing
the material basis for human beings, the second stage of providing the
ecological functions of climate regulation, and the third stage focusing
on the cultural and support benefits. In more urban planning, economic
construction has shifted to people-centered construction and the
happiness and satisfaction of urban residents are increasingly
considered. The evaluation index system of blue-green spaces has also
developed (Table 3).

\emph{3.1. Evaluation of supply services}

After World War II, in order to quickly restore urban economy, the
process of urbanization has been accelerated. Humans are eager to obtain
materials from the nature and accumulate resources, thus leading to
serious environmental problems such as sharp decline in biodiversity,
river pollution, and green space destruction. Governments and some
scholars of various countries have begun to pay attention to
environmental quality in order to provide sustainable supply resources
for urban development through restoring water bodies and green spaces.
River health evaluation index systems and green space health evaluation
systems have been successively established.

In 1981, based on the concept that a good water ecosystem must have a
perfect biological community structure, with the biological integrity
index (IBI) was first proposed for evaluating the quality of water
environments. After 16 indices of species composition were quantified,
including the indices of nutritional structure and individual health
status, the water environment quality was divided into 6 levels
according to the characteristics of biological integrity (Karr, 1991).
In 1984, the River Invertebrate Prediction and Classification Project
(RIVP-ACS) was proposed (Wright \emph{et al.} , 1998) to predict the
biomass of macroinvertebrates in a river without any human disturbance
with different characteristics of various areas and then the predicted
biomass value was compared with the actual monitored value of
macroinvertebrates in the river to evaluate the health status of the
river. In 1994, the Australian River Evaluation Program (AUSRIVAS)
(Simpson \emph{et al.} , 1999) was proposed to predict the theoretical
biomass in a river with the hydrological indices (habitat structure,
water flow state, and continuity), physical and chemical parameters,
indices of invertebrates and fish aggregates, water quality indices, and
ecotoxicological indices.

In general, the evaluation based on these indices can obtain intuitive
results. The obtained biomass reflects the available resource from a
water body, and provides the basis for subsequent fishing or protection.
In these evaluation systems, only the changes in biomass in water
bodies, especially invertebrate biomass, are considered, but whether
invertebrates are index species for the habitat is not considered.
Therefore, these systems cannot accurately reflect the stress on river
ecosystems.

In green space evaluation, the symptoms of land diseases can be
evaluated with key indices such as erosion, fertility loss, hydrological
anomalies, infrequent outbreaks or inexplicable local extinctions of
certain species, and degradation of agricultural products and forestry
products (Carmony, 1992). However, due to the economic development, both
greening construction and environmental protection should be considered
equally. Green coverage rate, green space rate, and park area per capita
should be introduced to the evaluation index system so as to improve
evaluation results (Zhang, 2010). Existing green space evaluation index
systems often only focus on the quantity of green spaces and ignore the
quality of green spaces, so they cannot meet the requirements for
building a high-quality ecological city in the new era. Therefore, in
the future subsequent evaluation system, quantitative and qualitative
indices of water bodies and green spaces should be selected and the
interaction between water bodies and green spaces should be
comprehensively considered to reflect the complex structure and
functions of the blue-green fusion space.

\emph{3.2. Evaluation of regulation services}

After a city enters the stable development stage, heat island effect,
environmental pollution, too high urban population density, and poor
social health, threatened safety and other social problems have
gradually emerged. Due to the destroyed urban natural environment, the
influences of the natural environment on human survival, life, and
production are widely concerned. Therefore, the benefits of urban water
bodies and green spaces have been extensively explored in order to
arouse public awareness of nature protection. Subsequently, relevant
evaluation index systems were established to evaluate the environmental
loss and develop a reasonable and targeted restoration plan based on the
key impact indices. Therefore, the targets of evaluation index systems
were gradually changed from ecosystem supply services to regulation
services (Table 3).

The regulation services of an urban ecosystem involve four aspects:
hydrology, ecology, environment, and society. The evaluation indices of
the regulation services of urban rivers and lakes involve the elements
of six aspects: hydrological characteristics, water environment quality,
structure and function of water ecosystems, waterfront spatial
structure, landscape effect, and stress factors (Zhang \emph{et al.} ,
2005). The indices that fully reflect the pressure, state, and response
of river basins, such as population density, annual GDP growth rate,
water resources per capita, annual rainfall, forest coverage, and
industrial wastewater treatment rate, should be selected to evaluate the
health status of ecosystems (Yan \emph{et al.} , 2008). In addition, the
wind environment, light environment, building and surrounding
environment can be selected to comprehensively evaluate the greening
development potential and quantitatively analyze the cooling
characteristics and cooling efficiency of vertical greening (Shi, 2018).
In Wetland Ecosystem Health Evaluation Index System proposed by the
Wetland Conservation and Management Center of the State Forestry
Administration, the health status of wetland ecosystems should be
evaluated in five aspects: water environment, soil, biology, landscape,
and society (Qian \emph{et al.} , 2016).

These evaluation systems comprehensively consider the ecological service
functions of water bodies and green spaces in urban ecosystems for
regulating climate and preventing floods, and can analyze the factors
affecting the stability of urban ecosystems based on specific functional
indices and evaluation results. The comprehensive evaluation based on
water, soil, land, atmosphere reflects the interaction between various
natural elements, so that the environmental problems can be explored and
the health conditions of ecosystems can be judged by evaluation results
for ecosystem restoration. Due to the enhanced awareness of ecological
civilization and natural restoration in urban planning, the
comprehensive evaluation of water bodies and green spaces is
significant. In the comprehensive evaluation, some characteristic
indices are integrated together to reflect the interaction between them
and the characteristics of the urban blue-green space as a composite
ecosystem and improve the evaluation system.

\emph{3.3. Evaluation of cultural services}

The Declaration on the Human Environment issued in 1972 emphasized the
relationship between development and protection from the perspective of
environmental protection. Subsequently, many countries started
large-scale ecological restoration and urban ecosystem restoration in
order to provide a happy living environment for urban residents through
building a healthy ecosystem. Ecosystem health initially meant that the
organization of the ecosystem was not damaged or weakened, and had a
certain restoration ability (Costanza and Norton, 1992). Then, in the
concept of ecosystem health, human health factors were introduced. It
was believed that ecosystem health depended on the judgment by the whole
social system, so human welfare should be considered. Therefore, more
researchers focused on the urban development and residents' perception
of urban blue-green spaces and gradually emphasized cultural services
supplied by blue-green spaces, including aesthetics, landscape, and
mental health benefits (Table 3).

Xia Jihong (Xia, 2005) believed that an ecological riparian zone was a
dynamic open water-land interlaced ecosystem, established a set of
comprehensive evaluation index system for riparian ecosystem, and
expounded the evaluation method of an ecological riparian zone from four
perspectives, including structural stability, landscape suitability,
ecological health, and safety. On this basis of the above system,
ecological characteristic indices such as strong self-healing ability
were introduced to carry out health evaluation of urban lakes and
provide a theoretical basis for ecological restoration, planning, and
construction of urban lakes (Wu \emph{et al.} , 2020). According to the
development characteristics of urban waterfront space, in order to build
a modern economic-ecological-social city, ten principles of waterfront
vitality were proposed and mainly involve ecological indices, public
indices, full-time indices, accessible indices, hydrophilic indices,
permeable indices, economic indices, mixed indices, sign indices, life
indices and other indices (Zhang, 2009). A slow-travel-friendly
waterfront strip park space evaluation system and scoring criteria were
also established based on analytic hierarchy process (AHP), so that the
friendliness of waterfront parks could be evaluated from three aspects:
slow-travel paths, slow-travel facilities, and slow-travel landscapes
(Zhong \emph{et al.} , 2021).

In the history of the blue-green space evaluation index system, the
evaluation indices have been gradually expanded from biological indices
to habitat indices and human-related ecological service indices. The
above history was basically consistent with the stages of urban
development. Urban development has experienced the initial agricultural
society, to industrial society and information society. Therefore, the
demands for the environment has changed from material supply to habitat
provision and cultural services. Therefore, the later blue-green space
evaluation index systems retained the previous key indices that had an
impact on ecological restoration and contained some relevant indices
that were in line with the vitality of modern society, thus emphasizing
the influence of the integrated natural ecosystem in blue-green spaces
on ecological service function and urban development potential. The
later system provided the basis for urban planning. In the future
blue-green space evaluation index system, based on the concept of
economic-society-ecological integration, the benefits of water bodies
and green spaces related to social development and human well-being
should be comprehensively considered. Qualitative and quantitative
indices make the evaluation results more accurate and can better reflect
the quality of a blue-green space (Table 3).

\textbf{4. Evaluation methods}

Many evaluation methods for urban water bodies and green spaces are
available, including AHP, principal component analysis (PCA), fuzzy
comprehensive evaluation method (FCED), cluster analysis (CA), etc. Each
method has its own advantages and disadvantages and is suitable for
different evaluation objectives. According to the complexity of
evaluation calculation and evaluation objects, evaluation methods can be
divided into general evaluation methods and comprehensive evaluation
methods. General evaluation methods include AHP and PCA. Comprehensive
evaluation methods are obtained introducing the membership degree of
fuzzy mathematics and the grayscale concept of gray system theory and
include FCED and CA.

\emph{4.1. General evaluation method}

In AHP, with a system as the research object, the influence of each
factor in each level on the results is quantified. The evaluation method
is characterized by clear logic, simplicity, and practicality and can be
used for the systematic evaluation of unstructured characteristics and
multi-objective, multi-criteria, and multi-period systematic evaluation.
However, the evaluation method requires a large quantity of statistical
data and faces the difficulty in determining the weight. In addition,
the exact calculation methods of eigenvalues and eigenvectors required
in AHP are complicated.

In PCA, original complex variables are transformed into several
comprehensive variables for analysis. Factor analysis requires sample
size and does not fully consider the relative importance of the indices
themselves.

As traditional evaluation methods, both AHP and PCA have the advantages
of clear logic, simplicity, and practicality, and are suitable for
evaluating the systems with simple structures and clear hierarchical
relationships. As a composite ecosystem composed of water and green
spaces, the complex blue-green space may lead to its higher ecological
carrying capacity and stronger vitality. Therefore, a hierarchical
structure model can be established with AHP and the ecological carrying
capacity of blue-green space can be comprehensively evaluated from two
aspects of ecological support and environmental pressure (Zhao \emph{et
al.} , 2020). The main factors affecting the vitality of blue-green
spaces can be analyzed with PCA. Although general evaluation methods
have a clear hierarchy, the weight determination process is subjective
and requires excessive calculation, thus affecting the accuracy of
evaluation results. Therefore, in the future, the basic evaluation
methods can be combined to improve evaluation results.

\emph{4.2. Comprehensive evaluation method}

In FCED, some uncertain problems are determined according to the basic
theory of fuzzy mathematics and the principle of fuzzy relationship. The
evaluation results are more accurate, but the difficulty in weight
determination and the ambiguous statistical interrelationships between
indices also affect the evaluation process.

In CA, according to the principle of ``type-clustering'', similar data
are acquired for the purpose of analyzing the inherent characteristics
and laws of things. The analysis process of CA is simple and intuitive,
but it requires a sample size, which increases the difficulty in
obtaining the clustering conclusion.

Through the improvement in basic evaluation methods, many comprehensive
evaluation methods are gradually obtained. These evaluation methods can
avoid excessive subjective influences and improve the accuracy of
results. The analysis process is relatively simple and applicable to
many problems. In the evaluation process of blue-green spaces, it is
necessary to comprehensively consider its ecological background status,
local social and economic conditions, residents' experiences and
feelings. Index selection and weight determination should be based on
expert consultation and AHP, so the subjective factors affect the
accuracy of evaluation results. Without comparison results, the
rationality of evaluation results cannot be verified. Therefore, the AHP
can be combined with the fuzzy evaluation method or the CA in order to
improve evaluation results. In the future, according to the structures
and functions of evaluation objects, based on the existing evaluation
methods, combined evaluation methods can be used. In addition, more
practical and convenient evaluation methods can be developed.

\textbf{5. Conclusions and Outlook}

Due to rapid urban development, the environmental hotspot has changed
from initial supply services to the later regulation function, and then
to the current cultural service function. Urban development has
gradually shifted from the initial orientation to economic construction
to people-oriented development. The blue-green space system, in which
water bodies and green spaces are intertwined and continuously
integrated, can promote the sustainable development of urban natural
systems, the construction of livable cities, and the improvement in
urban vitality. In the review, the development process of the blue-green
space evaluation index system is summarized because of previous studies.
The relationships between water bodies, green spaces, and blue-green
spaces and the complex structures and functions of blue-green spaces are
analyzed as a composite ecosystem in order to provide the basis for
subsequent blue-green space planning.

In existing studies on blue-green spaces, blue spaces and green spaces
are separately evaluated and a combined evaluation index system for
blue-green spaces is not available. The suggestions are provided as
follows:

1) The indices of water bodies and green spaces should be
comprehensively considered, in order to evaluate the blue-green space.
The blue-green space is a complex system composed of water and green
spaces. The mutual influence between water and green spaces form
ecological service functions, unlike that in a single water body or a
green space. Therefore, in the evaluation of a blue-green space, it is
necessary to select indices considering water and green spaces for
comprehensive evaluation.

2) People-oriented urban construction is significant. Urban blue-green
spaces have environmental landscape functions and ecological service
benefits and provide residents with space for recreational activities,
thereby increasing residents' physical activities, promoting social
interaction and integration, and bringing positive effects to the
public's physical and mental health. In other words, the construction
quality of blue-green spaces is closely related to the happiness and
quality of life of urban residents. Therefore, the selection of indices
should reflect the ecological service function or social function for
human beings.

3) Multi-scale and periodic evaluation are recommended. According to the
size of the range, blue-green spaces can be classified into large
blue-green spaces and micro blue-green spaces. According to different
scales of evaluation objects, a multi-scale evaluation index system with
the hierarchy of ``city-district-community-people'' can be constructed
to cover the macroscopic whole area and an individual. In addition, the
constructed evaluation index system should be based on the overall
planning of urban and rural areas and the ``people-oriented'' planning
idea. It is necessary to improve the supervision mechanism and establish
a periodic evaluation system. Long-term monitoring and evaluation can
provide the basis for improving urban planning quality and subsequent
construction.

\textbf{Declaration of Competing Interest}

The authors declare that they have no known competing financial
interests or personal relationship that could have appeared to influence
the work report in this paper.

\textbf{CRediT Authorship Contribution Statement}

Z-M. Z. and Y-J. Z. conceived the ideas, designed and funded this study;
X-X. J., X L. and Z-F. W. collected the data; X-X. J. led the writing of
the manuscript; Z-M. Z. wrote-review and edited the paper. All authors
contributed critically to the drafts and gave final approval for
publication.

\textbf{Acknowledgements}

The authors greatly appreciated the support from the National Natural
Science Foundation of China (51909157), Open Foundation of Hebei Key
Laboratory of Wetland Ecology and Conservation, and Shanghai River and
Lake Biological Chain Construction and Resource Utilization Engineering
Technology Research Center (20DZ2250700).

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Table 1 Ecological benefits of different types of blue-green spaces.\selectlanguage{english}
\begin{longtable}[]{@{}lll@{}}
\toprule
Types & Research Contents & Research Results\tabularnewline
\midrule
\endhead
Green space & Compared cooling effects of different forms of green
spaces & Among polygonal, linear, single and mixed green spaces,
polygonal and mixed green spaces had the most significant cooling
effects (Park \emph{et al.}, 2017).\tabularnewline
& Purification effect of urban green space on air pollutants & Urban
green spaces could directly reduce air pollutants and noise levels
(Bolund and Hunhammar, 1999).\tabularnewline
Blue space & Influencing factors of water-cooling effect & The cooling
effects of lakes will gradually be decreased as the height of
surrounding buildings increased and larger lakes showed better cooling
effects at their edges and downwind areas (Theeuwes et al.,
2013).\tabularnewline
& Compared cooling effects of different water bodies & Planar waters in
the center of the heat island had more obvious mitigation effects than
the linear waters (Cheng et al., 2019).\tabularnewline
Blue-green space & Microclimate effects of vegetation and water bodies
on urban squares & In the presence of vegetation and water body, the
surface temperature was significantly lower than that in the absence of
vegetation and water (Robitu et al., 2006).\tabularnewline
& Thermal environmental effects of urban reservoirs and forests and
their interactions & There was a nonlinear relationship between the
cooling effects of reservoir water bodies and the vegetation and the
regulating effects of the reservoir and the forest on the environment
are mutually influenced (Wu et al., 2018).\tabularnewline
\bottomrule
\end{longtable}

Table 2 Studies on the impact of blue-green spaces on mental health.\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
Mental health & Mental health & Blue space & Green space\tabularnewline
\midrule
\endhead
Short term & Influence & + & +\tabularnewline
& Vitality & & +\tabularnewline
& Restorative outcomes & + & +\tabularnewline
& Perceived stress & + & +\tabularnewline
& Physiological stress & + & +\tabularnewline
& Problem behavior & + & +\tabularnewline
& Brain activity & & +\tabularnewline
Long term & Overall mental health & + & +\tabularnewline
& Severity of a mental disorder & + & +\tabularnewline
& Prevalence of a mental disorder & + & +\tabularnewline
& Satisfaction with life & + & +\tabularnewline
& Quality of life & + & +\tabularnewline
& Subjective well-being & + & +\tabularnewline
\bottomrule
\end{longtable}

Note: ''+'' represents existing studies.

Table 3 History of the blue-green space index evaluation system\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
Types of Ecological Services & Blue space & Green space & Blue-green
space & References\tabularnewline
\midrule
\endhead
Supply service & [?] & & & \textsuperscript{(Karr, 1991; Wright \emph{et
al.}, 1998; Simpson \emph{et al.}, 1999)}\tabularnewline
& & [?] & & \textsuperscript{(Flader and Clicottal, 1991; Zhang,
2010)}\tabularnewline
Regulating service & [?] & & & \textsuperscript{(Zhang \emph{et al.},
2005)}\tabularnewline
& & [?] & & \textsuperscript{(Yan \emph{et al.}, 2008; Shi,
2018)}\tabularnewline
Cultural services & & & [?] & \textsuperscript{(Xia, 2005; Zhang, 2009; Wu
\emph{et al.}, 2020; Zhong \emph{et al.}, 2021)}\tabularnewline
\bottomrule
\end{longtable}

Note: ''[?]'' represents existing studies.

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