How many urban lakes are observed in China?

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Lakes comprise a critical component of global hydrological and biogeochemical cycles and provide essential resources for human and ecosystem well-being. Urban lakes, the majority of which are shallow, relatively small, highly artificial, and often hypertrophic, provide essential ecological services to local communities, such as flood mitigation, phosphorus, and nitrogenous retention, biodiversity, maintaining species population and habitats, regulation of the urban microclimate, as well as educational and recreational services. Due to these fundamental functions, urban lakes are closely related to the environmental quality of life in urban areas. Of small to medium size in general, urban lakes differ from large lakes regarding physical and biogeochemical conditions and processes. For example, they tend to be high in dissolved organic matter, CO2, and nitrogen concentration, and therefore more significantly impacting methane production and CO2 efflux.

Over the past several decades, urbanization has become a major way of the Earth’s surface alteration. Globally, 55% of the world's population lived in urban areas in 2018, and this proportion is projected to be 68% by 2050. Given the direct impacts of human activities, urban lakes are considered one of the most vulnerable freshwater ecosystems in the world. Rapid urbanization has been posing increasingly significant threats to the urban scenery waterbodies and has resulted in various environmental and ecological problems. China has experienced an accelerated expansion of cities since the 1978 implementation of economic reforms, which raises growing demands on services provided by nearby and distant aquatic ecosystems. With rapid urban sprawl and population growth, intensive land-use changes have seriously influenced Chinese urban lakes, which calls for urgent attention to systemic lake management and environmental protection. To achieve this, mapping a comprehensive inventory of Chinese urban lakes is urgently needed for further investigations and management of urban lakes.

This study provides a high-resolution map of urban lakes (surface water area ≥0.001 km2) in China around 2020. In this study, published last week in Nature’s Scientific Data (https://doi.org/10.1038/s41597-022-01874-6), we define three kinds of water bodies as urban lakes according to the spatial relationships with urban boundaries by referring to a set of published global urban boundary (GUB) data (see Fig. 1 below), including the lakes that fall entirely in the extent of GUB (Type-1) or locate on the periphery of the city or town (Type-2). The third category (Type-3), which is adjacent to principal cities but does not intersect with GUB, has large sizes and is thus well known to the public. To produce the national-scale lake dataset, we used a Two-Step Urban Water Index method to detect urban waterbody, which was designed for water extraction in urban areas based on 10-m Sentinel-2 imagery acquired in 2019-2021. Based on the preliminary waterbody extraction data, we conducted a strict quality-control procedure and manually screened the automate-mapping results according to the definition of urban lakes by overlapping with the World Image Wayback imagery at high spatial resolution.

The conceptual diagram of three types of urban lakes where the brown dashed line represents urban boundaries

Figure 1. The conceptual diagram of three types of urban lakes where the brown dashed line represents urban boundaries.

Figure 2 shows the spatial distribution map of urban lakes in China. China has 110,698 urban lakes (over 0.001 km2) with a total lake water area exceeding 2.13×103 km2 (excluding 14 Type-3 lakes). Overall, urban lakes in China show strong spatial heterogeneity, more concentrated in the east and south and less in the west and north. The most prominent concentrations can be observed in China’s top three urban agglomerations, the Guangdong-Hong Kong-Macao Greater Bay Area, the Yangtze River Delta area, and the Beijing-Tianjin-Hebei area. Along latitudinal and longitudinal profiles, regions within 110°E to 125°E and 20°N to 35°N account for the majority of urban lakes. In contrast, there are only a few urban lakes in the broad regions extending from 100°E to the west, which is consistent with the arid climate and low economic level in the west.

This study is among the few attempts to produce a national-scale dataset focusing on urban lakes in China. Our dataset provides fine spatial details of urban lake distributions, which differ from previous datasets about various lakes in China that tend to ignore the tiny and variable urban lakes. In the context of rapid urbanization, this dataset provides a basis for the ecological and environmental management of urban lakes and sustainable development planning in China.

The distribution map of urban lakes in China and photos for three famous Chinese urban lakes (West Lake in Hangzhou, photo by Mr. Dengxing CHEN and Mr. Yuanzheng CUI, Xuanwu Lake in Nanjing, photo by Ms. Linghong KE, and Kunming Lake in Beijing, photo by Mr. Chunqiao SONG)

Figure 2. The distribution map of urban lakes in China and photos for three famous Chinese urban lakes (West Lake in Hangzhou, photo by Mr. Dengxing CHEN and Mr. Yuanzheng CUI, Xuanwu Lake in Nanjing, photo by Ms. Linghong KE, and Kunming Lake in Beijing, photo by Mr. Chunqiao SONG).

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