The national government’s decision to invest in the sponge city concept happened relatively quickly, but the groundwork for that transformation leads back to 1997, when Kongjian Yu and his team began focusing their efforts on studying urban water systems (Kongjian Yu，Ye Zheng，1998). They used the concept of a sponge to describe the flood-control capacity of natural systems, pointing out that “the natural wetlands along rivers can function like sponges to retain water during flooding and recharge water during drought” (Kongjian Yu, Dihua Li, 2003). In 2001, Yu and his colleagues proposed "Ten Strategies for Urban Ecological Infrastructure Construction,” which was an early, systematic discussion of ecological stormwater management (Kongjian Yu, Dihua Li, Luomeng Chao, 2001). These strategies emphasize constructing urban ecological infrastructure to safeguard ecosystem services, and two of the strategies were directly related to urban ecological stormwater management, i.e. maintaining and restoring the natural forms of rivers and coasts, and protecting and restoring wetland systems. The article criticized the prevailing phenomenon of river channelization, and the widespread destruction of urban and rural wetlands by urban construction. It emphasized the necessity of providing comprehensive ecosystem services to cities by maintaining and improving river and wetland systems. The principal strategies include clean water provision, drought and flood control, groundwater recharge, diverse habitat preservation, and recreational opportunity provision and aesthetic enrichment for urban residents. In 2004, these ten strategies were incorporated into the Ministry of Construction’s Technical Policy Outline for Construction (Kongjian Yu, Dihua Li, 2004). The ecological infrastructure approach to tackling urban and rural water problems has been applied to the national ecological security pattern planning, the regional ecological planning of Beijing, Taizhou, Weihai, Heze and Dongying, in eastern China, and the ecological planning and design of nearly 200 other cities throughout the country (Kongjian Yu, et al., 2005; Kongjian Yu, Lei Zhang, 2007; Kongjian Yu, Xuesong Xi, Sisi Wang, 2008; Kongjian Yu, Sisi Wang, Dihua Li, et al., 2010; Kongjian Yu, Sisi Wang, Qing Qiao, 2010; Kongjian Yu, Yuan Zhang, Yunqian Liu, 2012; Lin Mo, Kongjian Yu, 2012; Yun Song, Kongjian Yu, 2007). And in the years since, a series of influential urban "sponges" or sponge city demonstration projects have been featured in international publications, as well as winning international awards; Suining（Kongjian Yu，2011）, Qian'an (Kongjian Yu，2010), and Xixian New Area（Kongjian Yu,2013）, among the first batch of pilot cities for sponge city construction, have applied the sponge city construction theory and technology in their planning and river improvement projects.

A series of earlier projects have also contributed to the exploration and refinement of sponge city construction approaches. The first was Beijing Zhongguancun Life Science Park, built in 2000. It introduced a green space system called "earth-life cells," which utilize constructed wetlands to collect rainwater and purify reclaimed water (Kongjian Yu, Dong Zhang, 2001; Kongjian Yu, Dihua Li, Yafan Meng, 2001).

The second was Zhongshan Shipyard Park near Guangzhou, constructed in 2001, which created a resilient waterfront with trestle. (Kongjian Yu, Haibo Hu, Jianhong Li, 2002). During 2002 and 2003, as part of the ecological restoration of the Yongning River in Taizhou, on the eastern coast, concrete embankments were replaced with flood-friendly and flood-adaptive ecological riverbanks and wild grass revetments, which also restored habitats for wild animals and plants (Kongjian Yu, Yujie Liu, Dongyun Liu, 2005； Graham Johnstone, Xiangfeng Kong, 2007). In the design project for the Jianzhu University campus in Shenyang, in northeast China, rice fields were introduced, which can be irrigated by rainwater collected on the campus (Kongjian Yu, Yi Han, Xiaoye Han, 2005; Paisajismo, 2007). Also in 2005, in the phase II design of Qinhuangdao Tanghe Park, a “red ribbon” (500-meter long bench) helped transform the neglected and overgrown floodplain into a popular urban park, using a minimum intervention strategy, rather than traditional concrete embankments. The park contributes to not only maintaining a whole riverfront ecological system, but also creating a modern urban public space (Kongjian Yu, Chen Chen, Jing Niu, 2007；Antje Stokman, Stefanie Ruff, 2008). From 2005 to 2006, a campus reconstruction project at the University of International Relations in Beijing emphasized ecological stormwater management theory and techniques, and permeable ecological design was employed around buildings, along road sides, and in the plaza and parking lots of the campus (Kongjian Yu, 2005). In the same period, the eastern park of the Central Party School of the Chinese Communist Party in Beijing was redesigned following green sponge and ecological stormwater management principles. Existing concrete banks were replaced with waterfront habitats and a large area of permeable activity space. Another example is the Qiaoyuan Park in Tianjin City, built in 2007. Simple cut and fill help create a bubble-shaped ecological sponge system that can collect stormwater, thereby mitigating urban flooding and simultaneously remediating contaminated soil (Kongjian Yu, Chun Shi, Hangjian Wen, 2006; Yu, 2010; Kongjian Yu,2010). Similar sponge city projects include coastal ecological restoration in Qinhuangdao City and Harbin Qunli National Wetland Park (Kongjian Yu, Shihong Ling, Xiangjun Liu, 2009; Yu, 2011; Kongjian Yu, 2011; Kongjian Yu, Topos, 2011; https://www.asla.org/2012awards/026.html). From 2015-2017, Yu and his team conducted large-scale sponge city construction and "ecological restoration and urban renovation" projects in Sanya and Haikou cities, on Hainan, presenting an integrated application of the team’s knowledge and experience accumulated over the preceding 20 years（https://www.asla.org/2020awards/178.html; Yu, K., Urban Solutions, 2020）. The lessons gained from these projects were widely disseminated by MOHURD throughout the country. In 2018, the completion of an ecological restoration project at the Kaban Lakes in Kazan, Russia, added to the growing visibility of Yu and his team's design philosophy in the wider world（https://worldarchitecture.org）. Turenscape’s projects have also been recognized with a dozen awards from the American Society of Landscape Architects, as well as five World Architecture Festival world landscape awards.

With the increasing frequency of urban floods, the sponge city concept and related planning concepts and approaches have become more widely recognized. In many important conferences and media interviews, Yu has called calls for "making the whole country a 'green sponge system,' so that rainwater can be stored and utilized locally, forming a water ecological infrastructure for natural protection with parks and wetlands in the city." He also made planning and construction suggestions for cities such as Beijing, Xiamen, and Chongqing (Wei Liu, Outlook Newsweek, 2012; Jingwen Liao, Guangzhou Daily; Yue Lu, xiamen.com, 2013; Lujie Tan, Jing Yi, 2013; Lijuan Wang, 2013), and suggestions for leaders at various levels (Kongjian Yu, 2012).

In recent years, the concepts of ecological runoff control and stormwater management have gradually gained recognition within academia and industry, and sponge city theories and methods have been increasingly applied to planning and design practice (Su Yijing, Wang Sisi, Che Wu, et al., 2014; Wang Yuncai, Cui Ying, Peng Zhenwei, 2013). For example, Dong Shuqiu et al. proposed adopting an "ecological sponge city" planning concept, including "ecological drainage and pipe network drainage" strategies, in the reconstruction plan for Shougang Industrial Zone in Beijing, with an eye to solving rainwater utilization problems (Dong Shuqiu, Han Zhigang, 2011). The Taiwan water resources administration also proposed building a sponge city based on Low-Impact Development (LID) technology in its recent comprehensive watershed management plan.

∆ Beijing Zhongguancun Life Science Park’s green sponge system, designed in 2000, introduces the concept of “living cells,” green spaces that can collect and purify stormwater and reclaimed water (Kongjian Yu, Dong Zhang, et al., 2001)

∆ Built in 2007, Tianjin’s Qiaoyuan Park is an urban sponge system designed to collect rainwater for ecological restoration of brownfield (Yu, 2010)

∆ Coastal ecological sponge system built in 2007: Qinhuangdao coastal ecological restoration (Kongjian Yu, Shihong Ling, Xiangjun Liu, 2009)

A catastrophic rainstorm that hit Beijing in 2012 killed 79 people. Media coverage and public outcry led decision makers to take the sponge city concept much more seriously—and ultimately make it a matter of national policy. The emphasis on the sponge city in official policy represents a recognition of its real-world effectiveness, and is opening important new opportunities to tackle water problems and other ecological and environmental challenges in Chinese cities of all sizes. It provides a systematic approach to stormwater management, ecological flood control, water purification, groundwater recharge, brownfield restoration, biological habitat construction, green space construction and urban microclimate regulation.

This special topic will trace the development and evolution of the sponge city concept in China in more detail. While focusing on the research, design work and on-the-ground implementation by Kongjian Yu and his team at Turenscape, it is intended to spur discussion about similarly inspired efforts elsewhere—and about the sponge city concept more broadly.