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.