A review of urban rainwater harvesting in China

Over the past 30 years, Chinese cities have experienced a period of rapid development and construction. Accordingly, China's urbanization rate has increased from 26.41% in 1990 to 63.89% in 2020. However, the urbanization process has been accompanied by increasing urban water problems, and in some cities, water scarcity has severely affected and restricted development (Yang et al. 2020; Cai et al. 2021; Wang et al. 2021a, 2021b, 2021c; Zheng et al. 2022). At the Urban High-level Development Forum in 2010, scholars classified China's urban water problems into four categories: water scarcity, urban flooding, water pollution, and damage to water ecology (Liu et al. 2017; Zhou et al. 2019; Ahmad et al. 2020). Stormwater management in urban areas has become a hot topic in urban hydrology research and also plays a key role in solving urban water problems (Jin et al. 2014; Moravej et al. 2021).

Urban rainwater harvesting (URWH) is an integrated rainwater management measure that collects, stores, treats, uses and discharges rainwater in urban areas (Campisano et al. 2017; Zheng et al. 2021). Rainwater can be used as a resource to supplement urban water supply and alleviate urban water scarcity (Mankad et al. 2013; Zhang et al. 2019a, 2019b). Rainwater harvesting and storage can effectively reduce urban surface runoff and alleviate urban flooding (Zhang et al. 2012) URWH can increase the retention and penetration of rainwater, reduce the risk of urban flooding, decrease river and lake pollution and improve the environmental and ecological quality of watersheds in urban areas (Campisano et al. 2014; Norman et al. 2019). URWH is an effective concept and approach to solve comprehensive urban water problems (Zhang et al. 2012; Kaposztasova et al. 2014; Campisano et al. 2017; Hu et al. 2019). For China, the utilization of urban rainwater resources has become very important in the rapid urbanization process.

Urban rainwater utilization in China has a long history, with the construction of ponds to store domestic rainwater recorded as early as the Qin and Han dynasties, and the construction of water cellars in northwest China for several hundred years. However, the research and application of urban rainwater utilization in the real sense began in the 1980s and developed in the 1990s, later than in the United Kingdom, the United States, Australia, Germany and other developed countries (Dai et al. 2019). However, the research on rainwater utilization has developed quickly during rapid urbanization (Zhang et al. 2020; Wang et al. 2021a, 2021b, 2021c). The initial driving factor for URWH development was water scarity in urban areas of China (Song et al. 2010). In the 1990s, several cities with severe water shortages began to explore rainwater harvesting and utilization (Sun et al. 2001; Zhang et al. 2004; Ding et al. 2006; Kong 2007). With the organization of the International Conference on Rainwater Harvesting and Utilization in China and the China Urban Rainwater Harvesting and Utilization Conference, the Chinese government began to pay attention to urban rainwater sanitation. The central and local governments issued a series of policies related to urban rainwater utilization (Xu et al. 2017), research on URWH methods and technologies has helped to formulate a series of national and local standards to guide the standardized construction and development of urban rainwater management and utilization in China (Cai et al. 2021; Wang et al. 2021a, 2021b, 2021c). The concept of sponge cities has been proposed in China, and the construction of sponge city pilot projects has been carried out in several cities (Song et al. 2018; Dai et al. 2019; Liu et al. 2020). As an important sponge city measure, URWH has been widely promoted and applied in the ‘sponge city’ pilot cities (Wei & Chen 2018; Liu et al. 2019; Ma et al. 2019).

Extensive references cover URWH in China, including many research topics: rainwater utilization and reuse policies, methods and technologies, construction implementation and benefit evaluation. Existing experiences or issues related to these topics will influence URWH development and urban water issues in Chinese cities. Therefore, there is a growing demand for information and guidelines on URWH developments, issues and recommendations in urban areas.

This review summarizes the URWH policy evolution in China, analyzes the methods, technologies, construction and benefits of URWH and puts forward recommendations for the further development of URWH. This review provides a bibliometric analysis of topics related to rainwater harvesting in the relevant literature, covering the timelines of URWH policy development; methods, technologies and construction; and the evaluation of benefit analysis methods.

Bibliometric analysis is a scientific and effective approach to literature analysis: a quantitative method to obtain the qualitative characteristics of scientific research by examining science as a knowledge generation system (Wang et al. 2021a, 2021b, 2021c). VOSviewer is a software tool for analyzing bibliometric networks, developed by Nees Jan van Eck and Ludo Waltman at Leiden University's Centre for Science and Technology Studies (CWTS). VOSviewer can be used to construct networks of scientific publications, scientific journals, researchers, research organizations, countries, keywords or terms (van Eck & Waltman 2010). Here, we selected keywords and countries to construct an analysis network through VOSviewer (version 1.6.17.0) using the data source Web of Science.

Based on the analysis of high-frequency buzzwords of URWH in the Chinese literature, we selected the following: ‘urban rainwater harvesting’ OR ‘urban rainwater utilization’ OR ‘urban rainwater management’ OR ‘urban rainwater harvesting and utilization’ as the ‘topic’. The ‘address’, including China, to search the literature on the Web of Science. A total of 1,649 related articles were retrieved from Web of Science. We exported all relevant data from these articles as basic data and then performed bibliometric analysis using VOSviewer software.

In Figure 2, rainwater recycling at the national level in China includes relevant international conferences, national conferences, technical specifications, industry standards, government policy documents and typical application cases. In the past 30 years, the development of Chinese urban rainwater resources under its own demand traction, with domestic and international academic conferences as a starting point, under the lead of national policies and government documents, development of relevant technical specifications, to carry out the construction of rainwater utilization practices, especially in Beijing 2008 Olympic Games as an opportunity to China urban rainwater utilization technology to get fast development. Since 2015, with the implementation of sponge city construction in China, major cities have started the construction practice of rainwater resource utilization measures.

From the perspective of policy evolution and development for specific cities, many cities have successfully carried out research on URWH since the late 1990s. Beijing was the most prominent representative city in the development and utilization of URWH (Kong 2007). In Beijing, URWH policies cover several aspects, including legal, economy, administration, technology, publicity, and education. An initial development framework has been formed (Wang et al. 2014). The economic measures in Beijing include subsidies, fines and relief systems (Zuo et al. 2009). We take Beijing as a case study to summarize and analyze the development process of URWH in a specific Chinese city.

The development of rainwater resources in Beijing is mainly marked by policy issuance and standard formulation. In the past 20 years, the Beijing Municipal government has promulgated a number of policies related to URWH and issued a number of local standards related to urban rainwater control and utilization. Beijing is the most representative city in the utilization of urban rainwater resources in China, and its development process of urban rainwater resources is a model that other Chinese cities should learn from.

Roof RWH can be divided into hardened roof RWH and green roof RWH (Hu et al. 2012; Chubaka et al. 2018). The term ‘hardened roof’ refers to the traditional impermeable roof, which can intercept rainfall that will be collected in a rainwater tank, rainwater barrel or rainwater pound through a draft tube (Sánchez et al. 2015; Dai et al. 2019). The rainwater storage container can be installed on the roof, wall or other high places, or placed on the surface or underground (Arthur & Wright 2005; Kim et al. 2015). Green roofs are also referred to as planted roofs and roof greening (Van Mechelen et al. 2015; Abu-Zreig et al. 2021). This refers not only to roof planting but also to all kinds of terraces, roofs, balconies, walls, tops of underground garages and overpasses that are above the ground and not connected to the natural soil (Zhang et al. 2014a, 2014b; Van Mechelen et al. 2015). Green roof RWH absorbs rainwater through vegetation and the root layer, which can then form a micro-water cycle of vegetation–soil–atmosphere, effectively improving the local microclimate (Konopka et al. 2021; Zhang et al. 2021). Meanwhile, the urban water-storage green roof has a significant effect on the removal of pollutants such as SS, COD, TN and TP in rainwater runoff. After rainfall, rainwater purified by the vegetation rhizosphere is discharged into rainwater harvesting containers through soakaways and rainwater pipes for irrigation of green areas or toilet flushing (Aloisio et al. 2016). The rainwater collected from the roof has good water quality, so this rainwater can be used for a variety of purposes (Ahmed et al. 2017; Sepehri et al. 2018), not only for outdoor uses such as watering and washing cars but also for indoor flushing, washing clothes and cleaning (Norman et al. 2019; Zabidi et al. 2020; Konopka et al. 2021). In Chinese cities, the acceptance of roof rainwater as indoor water is relatively low and is mainly used for flushing. This water is more often used for outdoor vegetation irrigation and car washing (Zhang et al. 2014a, 2014b; Hai et al. 2019).

• 2.

  Hardened ground rainwater harvesting

Hardened ground RWH is divided into permeable pavement hardened ground RWH and impervious hardened ground RWH (Hou & Zhang 2014; Zhou et al. 2021). For permeable hardened pavements, rainwater passes through the permeable hardened layer mainly through infiltration and enters the rainwater collection device under the pavement, and this rainwater is used after purification (Qin et al. 2019; Zhou et al. 2021). The impervious ground intercepts rainfall and collects rainwater through scattered perforated strainers (Roy & Shuster 2009; Dams et al. 2013). The pavement can be all types of urban ground such as squares, parking lots, sidewalks and motor vehicle lanes (Liu et al. 2015; Elqattan & Elryies 2021). The main storage containers for such rainwater include rainwater tanks, rainwater ponds and other rainwater storage modules (Mahmoud et al. 2014). Usually, these rainwater storage containers are placed below the ground. Rainwater collected from urban ground is mainly used for municipal environmental water and irrigation of greenbelts (Chakraborti et al. 2017; Gado & El-Agha 2020). In Chinese cities, rainwater collected from the ground is mainly used for road sprinkling, vegetation irrigation and ecological water replenishment for rivers and lakes (Luo & Wang 2018; Zhang et al. 2019a, 2019b).

• 3.

  Green land rainwater harvesting

Green land RWH includes rainwater gardens, rainstorm gardens, sunken green spaces, constructed wetlands and other green and low-impact development infrastructure (Zhang 2005; Kong 2007; Wang et al. 2019). Greenland rainwater collection and utilization methods intercept rainwater mainly through vegetation, absorb and purify it through vegetation rhizosphere, and surface runoff flows into water bodies such as ponds and pools or human-constructed wetlands (Song et al. 2018; Wei & Chen 2018; Wang et al. 2019; Liu et al. 2020). Rainwater collected from urban green space is mainly used as urban ecological and environmental water (Weichselbraun et al. 2014; Wiesner et al. 2016). In Chinese cities, rainwater is mainly used for vegetation irrigation, groundwater recharge and ecological recharge of rivers and lakes (Yu et al. 2011; Zhang & Chui 2019).

Table 1 shows the implementation cases of various URWH methods or measures in Chinese cities, taking Beijing as an example. The selected cases are representative of Beijing's rainwater harvesting and utilization measures. The scale of implementation, the volume of rainwater available, the rainwater utilization method and other parameters are shown in Table 1.

The average annual rainfall in Beijing is 585 mm, which is unevenly distributed throughout the year; the majority of rainfall occurs from June to August (Zhou et al. 2020). Beijing's per capita water resources are below 150 m³/year, substantially less than the standard value (1000 m³/year) marking severe water shortage in cities according to the United Nations. Therefore, Beijing is a city with severe water shortages (Jia et al. 2017). Beijing has been exploring and implementing urban rainwater resource policies since the 1990s (Zuo et al. 2009; Zhang et al. 2011). Presently, URWH implementation in Beijing is at the forefront of Chinese cities (Xu et al. 2017). Therefore, we choose Beijing as an exemplary city to summarize and analyze the construction and implementation of URWH in Chinese cities.

In 2000, the Beijing Municipal Water Resources Bureau and the University of Essen of Germany jointly initiated a rainwater utilization project, marking the initiation of URWH construction in Beijing (Zhang et al. 2011). In 2004, the number of rainwater utilization projects in Beijing stood at 38, with a catchment area of 7.45 km² and a rainwater utilization volume of 920,000 m³. By the end of 2006, Beijing had completed 60 rainwater utilization projects within the urban area, distributed in schools, parks, government agencies and residential communities (Li et al. 2008). The utilization volume of rainwater then exceeded 20 million m³. In 2007, more than 300 rainwater utilization projects were newly built in Beijing. By the end of 2010, there were nearly 700 rainwater utilization projects in Beijing, with a rainwater utilization volume of more than 40 million m³ (Li et al. 2015).

From the statistical data, it can be seen that the project number, comprehensive utilization capacity and comprehensive utilization of rainwater resources in Beijing have not changed significantly since 2017. In the past five years, the annual rainfall in Beijing has changed significantly, but the utilization of rainwater has not changed significantly. The main reason may be that the construction of rainwater resources in Beijing has not made significant progress and changes in recent years, and the utilization demand of rainwater in Beijing has not changed significantly.

The data investigation and data collection show that no cities in southern China have carried out systematic and large-scale construction of rainwater resources, and no official reports on rainwater collection and utilization have been found like Beijing. In order to understand the construction and development of rainwater resources in other cities in China, the literature analysis was carried out as follows: The construction of the 2008 Beijing Olympic Games venues was a typical RWH case in Chinese cities, and the 2012 Shanghai World Expo also adopted a large number of rainwater recycling measures (Li et al. 2021). After 2015, with the implementation of sponge city construction in China, urban rainwater harvesting measures, as comprehensive sponge measures, have been widely applied to the construction of sponge communities, sponge parks or sponge roads (Zhang et al. 2017; Li et al. 2022). The practice of rainwater utilization in Nanjing City shows that 55% of the roof with rainwater collection boxes can reduce 15–58% of the surface runoff under different rainfall conditions (Zhang et al. 2012). The case study of rainwater collection and utilization in residential communities in Shanghai shows that when the life cycle of rainwater recycling system is 15 years, the return on investment time is 8.26 years, furthermore, rainwater utilization can improve the sustainability of cities. Guangzhou, Wuhan, Beijing and Harbin were selected as the representatives of four cities with different climate regions to evaluate the water-saving potential and economy of the rainwater resource system (Chen et al. 2022). The results show that Guangzhou has the largest water-saving potential and the highest return on investment (Chen & Zhang 2021).

URWH brings economic, social, ecological and environmental benefits (Christian Amos et al. 2016; Zhang et al. 2020). From the perspective of economic benefits, URWH can save on the costs of alternative water resource treatment and transportation, and it can also save the costs of rainwater discharge and treatment (Dallman et al. 2021). In the long run, it can also save on the costs of urban water transportation projects and inter-basin water transfer projects, along with reducing energy consumption caused by water supply and drainage (Zuo et al. 2010). The indirect method of URWH economic benefits evaluation is mainly based on the amount of rainwater that can be collected and used (Christian Amos et al. 2016). The methods for evaluating the amount of rainwater available in cities are the empirical method and the runoff coefficient method (Zuo et al. 2009, 2010). The direct evaluation method for economic benefits adopts the cost–benefit analysis method for Chinese cities (Fan & Matsumoto 2019).

Chinese cities have promoted URWH social benefits to urban residents in terms of using rainwater resources, along with enhancing awareness of water conservation (Chai et al. 2019). Construction of China's sponge cities focuses on policy publicity and citizen participation (Zhang et al. 2018). In the process of participation, citizens gradually realize the severe situation of water scarcity in Chinese cities, which promotes social recognition and the development of URWH (Hai et al. 2017). URWH construction in Chinese sponge cities can effectively reduce surface rainfall-runoff peak and volume of rainfall in stormwater management areas, effectively reduce and mitigate the risk of urban flooding and protect the lives and properties of urban residents, with yielding significant social benefits (Hai et al. 2019).

In terms of ecological and environmental benefits, URWH projects, such as green roofs, rainwater gardens, permeable pavements and constructed wetlands, not only utilize rainwater but also infiltrate it into the soil (Jóźwik 2020), which contributes to a vegetation–soil–atmosphere water cycle model that can effectively improve the microclimate and mitigate the heat island effect in urban areas (Novo et al. 2013). Therefore, URWH can effectively improve the water environment and water ecological quality in urban areas in China.

Table 2 shows several calculation methods for rainwater collection/utilization volume and benefits analysis of URWH projects in China. These methods are applicable mainly to URWH projects based on underlying surface types in Chinese cities (Sun et al. 2001; Li et al. 2011). The estimation method of rainwater collection volume is the runoff coefficient method. The main methods of economic benefit evaluation are cost–benefit analysis and rainwater utilization benefit index analysis. There are also comprehensive analysis methods of economic, social and environmental/ecological benefits based on the vague set and AHP (Analytic Hierarchy Process) model (Zuo et al. 2009). These methods are suitable for the estimation of rainwater amount, analysis of economic benefits and analysis and evaluation of comprehensive benefits for URWH (Chao et al. 2011; Zhu et al. 2017). Furthermore, these methods are applicable urban and regional scales and usually monthly or annual time scales.

Table 2

Calculation methods for rainwater utilization estimation and benefit analysis for URWH in China

Order .	Evaluation target .	Calculation method .	Spatial scale .	Temporal scale .	Remarks .
①	Rainwater volume (Huang et al. 2007)		City/region	Year/month/Specific rainfall	is regional rainwater utilization volume, F is the regional area, P is precipitation, r is combined runoff coefficient
②	Rainwater volume (Ma et al. 2019)		Plot/ underlying surface type	Year/month/Specific rainfall	is rainwater utilization volume, A is plot/surface area, H is precipitation, is combined runoff coefficient, is abandon flow coefficient, is seasonal reduction factor
③	Rainwater volume (Yu et al. 2007)		City/region	Year/month	is potential urban rainwater harvesting, is the amount of rainwater generated by impervious underlying surface, is the amount of rainwater resources generated by green space, is the amount of rainwater in the water area
④	Rainwater use efficiency (Zhu et al. 2017)		City/country	Year	is urban rainwater utilization efficiency index, is rainwater utilization potential index, is the weight of water scarcity of city
⑤	Cost–Benefit Analysis (Zuo et al. 2009; Chao et al. 2011)		City/region	Year	is the benefit–cost ratio, E is total benefits value of RWH systems, P is cost value of RWH projects, B is total benefit of URWH projects in one year, C is annual operating costs of stormwater utilization projects, I is fixed gross investment of URWH, n is the design life of URWH
⑥	Comprehensive benefit evaluation of based on Vague set (Jiang et al. 2014)		Region	Year	is an evaluation matrix of a vague set of economic, social and ecological benefits, which is determined by the AHP, is the operation symbol of the vague set, is weight vector corresponding to indexes.
⑦	Comprehensive benefits based on the AHP model (Sun et al. 2001; Li et al. 2017a, 2017b)		City/region	Year	is economic benefits, is social benefits, is environment benefits

Order .	Evaluation target .	Calculation method .	Spatial scale .	Temporal scale .	Remarks .
①	Rainwater volume (Huang et al. 2007)		City/region	Year/month/Specific rainfall	is regional rainwater utilization volume, F is the regional area, P is precipitation, r is combined runoff coefficient
②	Rainwater volume (Ma et al. 2019)		Plot/ underlying surface type	Year/month/Specific rainfall	is rainwater utilization volume, A is plot/surface area, H is precipitation, is combined runoff coefficient, is abandon flow coefficient, is seasonal reduction factor
③	Rainwater volume (Yu et al. 2007)		City/region	Year/month	is potential urban rainwater harvesting, is the amount of rainwater generated by impervious underlying surface, is the amount of rainwater resources generated by green space, is the amount of rainwater in the water area
④	Rainwater use efficiency (Zhu et al. 2017)		City/country	Year	is urban rainwater utilization efficiency index, is rainwater utilization potential index, is the weight of water scarcity of city
⑤	Cost–Benefit Analysis (Zuo et al. 2009; Chao et al. 2011)		City/region	Year	is the benefit–cost ratio, E is total benefits value of RWH systems, P is cost value of RWH projects, B is total benefit of URWH projects in one year, C is annual operating costs of stormwater utilization projects, I is fixed gross investment of URWH, n is the design life of URWH
⑥	Comprehensive benefit evaluation of based on Vague set (Jiang et al. 2014)		Region	Year	is an evaluation matrix of a vague set of economic, social and ecological benefits, which is determined by the AHP, is the operation symbol of the vague set, is weight vector corresponding to indexes.
⑦	Comprehensive benefits based on the AHP model (Sun et al. 2001; Li et al. 2017a, 2017b)		City/region	Year	is economic benefits, is social benefits, is environment benefits

View Large

Method ① is mainly used to estimate the rainwater availability of a city or region. It can be assessed for specific rainfall events, or for monthly or annual rainwater availability. In method ②, the discard coefficient and seasonality coefficient are added on the basis of ①, and the use condition and range are the same as ①. Method 3 is to divide the different land use types to evaluate and calculate the rainwater available use, which is applicable to urban or regional scale, mainly calculating the rainwater available use at the monthly or annual scale. Method ④ is used to calculate the efficiency of rainwater use in a city, which is usually calculated in terms of years. Method ⑤ is used for the cost–benefit analysis of rainwater collection and utilization in urban units, which is calculated in the time unit of year. Method ⑥ is the comprehensive benefit analysis of rainwater resource utilization, mainly the fuzzy logic method. Method ⑦ uses the AHP to calculate the comprehensive benefit.

Most URWH measures in Chinese cities are government investment activities, primarily through low-impact development infrastructures such as permeable paving, sunken green space and shallow grass ditch in public areas such as urban parks, green spaces, municipal roads and squares (Chai et al. 2019). From the literature review results, the benefits of such URWH projects are mainly reflected in social benefits and ecological benefits (Fan & Matsumoto 2019). Especially since the construction of sponge cities in China, URWH measures have been implemented as important sponge city measures (Hai et al. 2017; Hu et al. 2019). Considering the economic, social and environmental benefits, the comprehensive benefits of rainwater harvesting measures are even greater (Hai et al. 2017). In some areas, rainwater conversion measures have effectively alleviated the flooding problem and avoided the economic impact caused by urban floods and can effectively protect people's lives and property.

• 1.

  URWH research topic needs to be expanded

URWH research topics in China are mainly focused on rainwater harvesting, rainwater management and urban runoff management (Feng et al. 2022). The shortcomings and deficiencies of URWH research include benefit evaluation, relevant policies and laws, professional technical standards and rainwater quality (Jing et al. 2018). The main reason for this may be that rapid urban development in China has brought about the demand for the rapid development of URWH. In the process of the rapid development of URWH, attention has been paid mainly to rainwater utilization methods and technology (Mi et al. 2018). The development of supporting laws and regulations, special policies, benefit evaluation and other aspects is relatively lagging.

Attention should be paid to rainwater treatment, water quality, economic benefit evaluation, laws and regulations regarding construction in future research on URWH in China. We hope to further enrich and expand related research topics, which would be helpful to solve the problem of URWH promotion and construction in Chinese cities.

• 2.

  URWH policies lack mandatory requirements and legal protection

Beijing, Dalian, Shenzhen and other cities have formulated rainwater utilization planning or technical regulations, but most of the other regions in China have not carried out relevant research and applications. Such areas merely refer to the relevant standards and documents of the national files (Yang 2018). From the state to local governments, China has not yet issued laws related to urban rainwater harvesting (Lei et al. 2018), this may be one reason why URWH has not become an important means of sustainable development in China, unlike Germany, the United States, the United Kingdom and Australia. These developed countries have issued laws or regulations related to rainwater harvesting and they have mandatory requirements, which may be one reason for the large-scale promotion of rainwater harvesting in these countries. In addition, China's urban rainwater resources are mainly government investment, lacking the participation of social capital and the financing of financial markets. If the legal construction of rainwater collection and utilization is not strengthened, it is difficult to ensure the promotion and implementation of rainwater resources in Chinese cities.

It is necessary for the Chinese government to strengthen the construction of laws and regulations related to urban rainwater resources and increase mandatory government documents. Particularly in cities with abundant rainwater resources but few urban water resources, laws on rainwater resource utilization could help the government to supervise and promote the construction of urban rainwater resource utilization and improve utilization of urban rainwater resources.

• 3.

  Construction scope and implementation scale of URWH are small

URWH methods are relatively comprehensive in China, including roof RWH, hardened ground RWH and green land RWH (Chiu 2012). However, the scale of URWH construction and rainwater use volume in Chinese cities is still quite small. For example, for Beijing, which is the leader in URWH development in China, according to preliminary estimates, the rainwater collection area of URWH was less than 2% of the total area in 2020. The rainwater collection capacity of Beijing in 2020 was 70,267,400 m³, which accounts for 0.78% of total annual rainfall. China has carried out sponge city pilot construction since 2015 (Hai et al. 2017; Zhou et al. 2018). The construction of sponge cities has promoted the development of rainwater resource utilization (Ma et al. 2019; Liu et al. 2020). As a comprehensive measure capable of intercepting, collecting and utilizing rainwater resources, URWH has been widely used in sponge city construction (Zhang et al. 2018; Feng et al. 2022). However, the number of sponge cities in China is not large at present, and each pilot city selects only a small area for construction (Dai et al. 2019; Sun et al. 2020). Therefore, the current URWH construction scale of sponge cities in China is not large.

Based on expanding the research theme and strengthening the formulation of laws and regulations, it is necessary to strengthen the standardization of construction of URWH projects, promote the construction scale of URWH in Chinese cities and expand project coverage. In particular, it is necessary to formulate engineering standards and technical requirements for rainwater harvesting and treatment measures to ensure the replicability of URWH projects and engineering.

• 4.

  Direct economic benefits from URWH projects are poor

From the viewpoint of benefit evaluation, China's URWH pays more attention to social and ecological benefits, which may be because most URWH construction occurs in public spaces or regions (Zhang et al. 2020). URWH projects are mainly municipal projects or environmental projects paid for by the government. In particular, since the implementation of sponge city construction, China has invested heavily in URWH construction in pilot cities (Fan & Matsumoto 2019; Leong et al. 2019). According to our literature analysis, URWH is a comprehensive measure with high investment cost, and its construction cost per unit area in sponge city construction, is 230 million yuan/km² which is twice the cost of non-URWH measures (Fan & Matsumoto 2019). Although China attaches importance to the economic benefit analysis of URWH, such analysis mostly consists of qualitative analysis of the costs and benefits of rainwater use, while quantitative analysis is lacking (Wang et al. 2021a, 2021b, 2021c), which may be due to the small scale of urban rainwater utilization in China, the low degree of quantitative construction or the imperfect evaluation system. Some scholars have used cost–benefit analysis methods to analyze urban rainwater resource utilization (Zuo et al. 2010; Jing et al. 2017), but this needs further refinement regarding direct economic benefits.

China should pay more attention to the evaluation of the economic benefits of URWH construction, especially to strengthen the analysis of direct economic benefits, reduce the investment cost of construction, improve the utilization efficiency of rainwater resources and attract more social capital to participate in URWH construction.

• 5.

  Difficulties and problems of URWH construction in China

With the development of more than 30 years, China has achieved good technical development and built a number of international typical sample projects of rainwater resource utilization (Yang 2020; Li et al. 2021). Since the further implementation of sponge city construction, pilot cities have adopted rainwater harvesting measures (Liu et al. 2022). However, the current construction process of scale and standardization of urban rainwater harvesting in China is slow, and there are some major difficulties and problems. The funding source of the construction is relatively single, dominated by government investment, and lacks the introduction of social capital (Liu et al. 2022). In China, rainwater recycling seems to be a public welfare cause, which lacks public participation and the attention and support of the whole society (Zhong et al. 2022). Rainwater harvesting project has low economic benefits, low return on investment, and lack of effective and sustainable project operation management (Meng et al. 2023). RWH lacks the mandatory and legal protection of laws, and the lack of incentive policies, and the enthusiasm of enterprises and the public to participate in the RWH is poor. Rainwater harvesting in Chinese cities is an important way to increase urban alternative water sources, reduce the risk of urban flood disaster and improve urban water environment and water ecology.

In this study, the bibliometric analysis results showed that URWH development in China is focused on rainwater harvesting, rainwater management, rainwater utilization and other technical aspects; there is insufficient research on standardization of construction, benefit evaluation and legal development. Compared to URWH research in developed countries, the research topics related to URWH in China should focus on economic benefit analysis, rainwater treatment and legal development.

URWH in China has been developing since the 1990s and has experienced three stages: initial development; rapid construction and sponge city construction. During these stages, China's URWH policy has moved from single to comprehensive, and technical standards have changed from rainwater harvesting and utilization to sponge city. Since the implementation of sponge city construction in 2015, China's URWH development direction is green, low-carbon and sustainable. And China's URWH policy has been developing in terms of professional technical standards and economic and efficient rainwater resource utilization technologies. The construction and development of laws and regulations should be emphasized in China in the future.

The construction of URWH projects in China is uneven. Beijing is in a leading position in terms of policies, standards, technology and other aspects, with several typical URWH projects that are examples and models for Chinese cities. However, in terms of the construction and development scale of Beijing, the coverage of rainwater recycling projects remains small, and the penetration of URWH is insufficient. There are significant differences in URWH construction among Chinese cities. Cities that have implemented sponge city construction have had a better implementation effect from URWH projects, while other cities have lagged relatively behind in URWH construction. URWH projects in Chinese cities need to improve the utilization rate of rainwater harvesting. For large-scale construction, further consideration needs to be given to rainwater recycling in cities with different climatic regions and different levels of economic development.

China's URWH construction constitutes government-led investment projects embodying positive social and ecological benefits. The investment cost of Chinese URWH implementation is generally higher, especially in the context of sponge city construction. The economic benefits of URWH have been low, even representing a deficit investment. Therefore, it is necessary to optimize the design of URWH projects, reducing the cost of URWH engineering and attracting more social capital to invest in future URWH projects.

This research was funded by the Beijing Municipal Natural Science Foundation, grant number 8214046; supported by major projects of the natural science foundation of China, grant number 52192671; supported by Beijing Postdoctoral Research Foundation, grant number 2021-PC-005; supported by the Open Research Fund of State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China Institute of Water Resources and Hydropower Research, Grant number IWHR-SKL-202105; and funded by the China Postdoctoral Science Foundation Project, grant number 2019M660380.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.