Urban water dissipation is a significant part of the urban hydrologic cycle and has a typical natural–social dualistic attribute. Besides natural evaporation, the water dissipation in people's daily life and production process cannot be ignored. This study developed an urban water dissipation model based on different land uses and applied it in urban-built areas in Beijing. The results showed that the water dissipation of buildings and green spaces occupied the dominant position, and the water dissipation intensity of each district exceeded 500 mm, among which the six core districts were 700–1,100 mm. Comparing the water dissipation contribution rate and area rate of each underlying surface, it showed that the water dissipation intensity from strong to weak was building, water surface, green spaces, and hardened ground. According to the dualistic analysis of urban water dissipation, the contribution rates of social water dissipation in the six core districts were 45.3–69.1%, which was higher than the 17.8–36.1% of other suburbs obviously. This study reflected that the higher the degree of regional urbanization, the greater the water dissipation intensity, and artificial water dissipation was the main influencing factor.
Developed a city-scale water dissipation calculation model based on urban land types.
Urban water dissipation includes traditional natural evapotranspiration and artificial water dissipation.
Buildings and green spaces are the major part of water dissipation in urban areas.
Regions with a high degree of urbanization in urban areas have greater water dissipation intensity.
Urban region is an area where human activities and the natural water cycle are coupled, and here, the water cycle presents an obvious ‘natural–social’ dualistic attribute (Wang et al. 2013; Zhou et al. 2019a). The water cycle, serving as a vital conduit for energy transfer, exerts a profound impact on urban ecosystems. As the process of urbanization continues to deepen, the population are steadily congregating in urban areas (Molina-Gómez et al. 2022). According to the 2020 report by UN-Habitat, the world will be further urbanized in the next 10 years, with the proportion of the global population residing in urban areas expected to rise from the current 56.2 to 60.4% by 2030. China, among the three major countries, is set to experience substantial growth in its urban population from 2018 to 2050 (United Nations Habitat 2020). In the process of urbanization, humans are progressively altering the Earth's surface, and such activities profoundly impact the near-surface climate by changing the energy and water balance of urban ecosystems (Chelu et al. 2022; Das et al. 2022; Tong et al. 2022). The natural underlying surfaces such as cultivated land and vegetation are gradually transformed into impervious surfaces such as buildings and roads (Sterling et al. 2013; Gao et al. 2020; Wang et al. 2021). The heterogeneity of the underlying surface coupled with the diversity in the processes of social water use makes the urban hydrological process more complex (Hao et al. 2015; Wang & Jia 2016; Wang et al. 2016; Fidal & Kjeldsen 2020). As a necessary part of the urban hydrological process, urban water dissipation refers to the myriad forms of water dissipation occurring in urban areas, serving as a pivotal source of urban water vapor and exerting a profound impact on urban dry and wet island effect (Wang et al. 2016; Zhou et al. 2019a; Luo et al. 2021a). Just like the urban water cycle, urban water dissipation is also divided into two components: one is the evapotranspiration that transpires on the natural underlying surface, including water surface evaporation, vegetation transpiration, and soil evaporation, and the other is the water dissipation engendered through the diverse domestic and production-related water utilization by human activities (Zhou et al. 2019a; Luo et al. 2021b).
Most previous studies believed that urban evapotranspiration predominantly arises from vegetation transpiration in green spaces (Loridan & Grimmond 2012; Jacobs et al. 2015). Many researchers have predicted evapotranspiration in urban areas based on energy balance equations and aerodynamics (Bowen 1926; Thornthwaite & Holzman 1939). The corresponding hydrological models were widely used in the estimation of urban evapotranspiration, including the SIMGRO model, the Urban Forest Effects-Hydrology, the SEBS model, the SPAC model, etc. (Noilhan & Lacarrere 1995; Su 2002; Wang et al. 2008; van Walsum & Veldhuizen 2011). Additionally, experiments have been employed as an important method for vegetation evaporation research. Some scholars have studied the transpiration and water consumption characteristics of urban plants through experimentation (von Allmen et al. 2015; Livesley et al. 2016). As urbanization causes natural vegetation, soil, and water surfaces to be replaced by artificial concrete underlying surfaces, some studies suggested that urban development would significantly reduce urban evapotranspiration (Dow & DeWalle 2000; Hao et al. 2015; Zheng et al. 2020). Nevertheless, it is crucial to recognize that evapotranspiration from natural underlying surfaces represents only a part of the urban water dissipation. Impervious ground and building roofs, as primary urban underlying surface, also contribute to evaporation following rainfall and artificial watering (Ramamurthy & Bou-Zeid 2014; Zhou et al. 2021). In addition to outdoor evapotranspiration, urban water dissipation includes the dissipation generated by various indoor water use activities (Zhou et al. 2018, 2019b), which constitutes a substantial portion of the social side of urban water dissipation (Zhou et al. 2019a). Given the dense concentration of buildings and the high intensity of human activities in urban areas, some researchers proposed the concept of building water dissipation, which believed that the water dissipation inside the building stems from a series of activities such as steam cooking, drying of clothes, water vapor bathing, floor wetting, and so on (Zhou et al. 2018, 2019a, 2019b). After considering the artificial activities, Zhou et al. (2020) calculated the water dissipation in Beijing in 2015, revealing that water dissipation in urban areas was greater than natural evapotranspiration, with water dissipation on the social side accounting for more than 40% in the central urban area. This underscores the pivotal role of artificial water dissipation in total urban water vapor, particularly in highly urbanized regions. Under the background of global urbanization, the connection between human and water use is closer, leading to more intense artificial water dissipation activities, which accelerates the transformation of water from liquid to gas and becoming an integral part of the urban hydrological cycle, directly impacting the urban water balance. However, most of the existing studies focus on evapotranspiration from natural underlying surfaces in urban areas, with limited attention paid to domestic and industrial production water dissipation (Zhou et al. 2017). Thus, it is necessary to quantitatively study the dualistic attribute of urban water dissipation, especially concerning the structure of artificial water dissipation inside buildings.
This study took Beijing as the research area, and analyzed the ‘natural–social’ dualistic attribute of urban water dissipation. It employed an urban water dissipation model to calculate water dissipation for different types at the district scale, comparing the water dissipation contribution rate and area rate of each land use type, and analyzed the water dissipation structure inside buildings. The research results further enrich the understanding of the urban hydrologic cycle and provide a reference for urban water flux calculation.
Urban water dissipation analysis framework
Urban water dissipation model
Beijing has experienced rapid urban development and rapid population growth over the past few decades, solidifying its status as a quintessential rapidly urbanizing city in China. By the end of 2020, Beijing was comprised of 16 districts with a permanent population of 21,893 million. The division and population distribution of Beijing are shown in Figure 2(c) (The population density data were obtained from the Worldpop database with a resolution of 100 × 100 m, https://www.worldpop.org/). The population density, urbanization rate, underlying surface characteristics, and development degree vary greatly among districts. The six core districts of Beijing are Dongcheng, Xicheng, Chaoyang, Haidian, Shijingshan, and Fengtai, which have a much higher urbanization degree than the other 10 districts, and most of the urban-built area is concentrated here. The human activities are intense in the six core districts and have a great impact on the hydrological cycle process.
The data used in water dissipation calculation in this research mainly include meteorological data, land use data, population data, domestic water dissipation data, and industrial water use data. Meteorological data encompasses key parameters such as rainfall, temperature, humidity, wind speed, and so on, which were derived from daily scale monitoring data of the National Meteorological Station of Beijing Observatory, China (Station No. 54511). Land use data were from the Beijing Statistical Yearbook and Beijing Regional Statistical Yearbook. The population data were the permanent resident population of Beijing, which was sourced from the Beijing Statistical Yearbook. Industrial water use data were acquired from the Beijing Municipal Bureau Statistics and Beijing Municipal Ecology and Environment Bureau. The evaporative water dissipation of green spaces and the water surface were calculated by meteorological data based on the Penman model. The daily water dissipation in buildings was based on the experimental test and questionnaire results of Zhou et al. in Beijing (Zhou et al. 2017, 2019a). Experimental measurements were conducted on various daily water dissipation activities by the weighing method, and the average water dissipation of each daily activity was obtained. Combined with the questionnaire, the frequency of daily water dissipation was calculated, and the per capita daily water dissipation quota was established comprehensively. Based on population data, the total amount of daily water dissipation was further ascertained. For the calculation of evaporation water dissipation on the hardened ground, the selection of runoff coefficient was based on relevant Chinese specifications such as the Code for urban wastewater and stormwater engineering planning and standard for the design of outdoor wastewater engineering, combined with relevant research results (Zhou et al. 2017).
RESULTS AND DISCUSSION
Water dissipation of different land types in urban-built areas of each district
|Land type .||Calculated value (m³) .|
|Green space||1.08 billion|
|Water surface||97 million|
|Land type .||Calculated value (m³) .|
|Green space||1.08 billion|
|Water surface||97 million|
Comparison of water dissipation intensity among each land use type
Analysis on dualistic attribute of urban water dissipation
Combining the water dissipation intensity of each district calculated in this study and summing it up to the whole of Beijing, it should be noted that the water dissipation intensity of the urban-built area in Beijing was about 705 mm in 2020. Cong et al. (2017) simulated the evapotranspiration in Beijing from 2003 to 2012 using the traditional SEBS model and found that the average annual evaporation was 348 mm. The calculation result of this study was nearly twice that of Cong et al. (2017), mainly because the traditional remote sensing model ignored the influence of anthropogenic heat (Cong et al. 2017). In megacities like Beijing human activities are highly intense, and human activities have an important impact on surface energy (McCarthy et al. 2010; Allen et al. 2011; Cong et al. 2017). Cong et al. (2017) improved the SEBS model after considering anthropogenic heat and other factors, and the simulation results showed that the evapotranspiration in urban areas was significantly higher than that in suburban areas. This study also supported the conclusion that water dissipation in the core districts was higher than that in the suburbs, driven by their higher level of urbanization and more intense artificial water dissipation activities. Domestic water dissipation inside buildings emerged as the main influencing factor of urban evapotranspiration (Cong et al. 2017). It further reflected the high water dissipation in central urban areas.
Indeed, various factors play a significant role in urban water dissipation. The water dissipation on the natural side is mainly affected by climate and other related factors. The water dissipation on the social side is mainly affected by human production and daily activities. In this study, the meteorological data used monitoring data from a Chinese national meteorological station. However, to achieve more accurate water dissipation calculations, it would be beneficial to incorporate data from weather stations within each district, as this would capture the impact of local microclimates on water dissipation more effectively. In addition, the domestic water dissipation inside the buildings was calculated by the water dissipation quota method, and the relevant water dissipation parameters were obtained from the experiments and survey statistics of Zhou et al. (2017, 2019a). It is important to consider that different regions and cities may exhibit varying water consumption habits and seasonal variations. When calculating water dissipation in other cities in the future, it is essential to fully account for the influence of these factors to improve the accuracy of the estimations.
In this study, an urban water dissipation model was developed based on different water dissipation types. Taking Beijing as the research area, the water dissipation of urban-built areas in 2020 was calculated, and the water dissipation contribution rates of various underlying surfaces were analyzed. The conclusions are as follows: (1) The water dissipation in urban areas was higher than that in the suburbs. In 2020, the water dissipation intensity of urban-built areas in the six core districts of Beijing ranged from 700 to 1,100 mm, while other suburbs ranged from 500 to 700 mm. (2) Building and green spaces were the main contributors to water dissipation. Comparing the water dissipation contribution rate and the area rate, it was observed that the water dissipation intensity from strong to weak was building, water surface, green spaces, and hardened ground. (3) As the high water dissipation underlying the surface, building water dissipation was composed of the rainwater interception evaporation outside and water dissipation inside. The water dissipation inside accounted for more than 90% and it was the main source of artificial water dissipation. (4) Water dissipation on the social side refers to artificial water dissipation and it is a significant component of urban water dissipation, which is positively correlated with the degree of regional urbanization. The contribution rate of water dissipation on the social side in the six core districts of Beijing was 45.3–69.1% in 2020, which was higher than 17.8–36.1% in other districts obviously. Under rapid urbanization, artificial activities in urban areas will be more intense and urban water dissipation will continue to increase in the future. Our study advances our understanding of urban water dissipation and could be a reference for water flux calculation in urban ecosystems.
This research was supported by the National Natural Science Foundation of China (Nos 51739011 and 51979285), the Chinese National Key Research and Development Program (No. 2021YFC3001400), and the Open Research Fund of Key Laboratory of River Basin Digital Twinning of the Ministry of Water Resources.
J.L. conceptualized the study; C.L. and J.L. prepared the methodology; C.L. and W.S. did formal analysis and investigated the study; C.L. wrote and prepared the original draft; C.L., X.D., X.S. wrote, reviewed, and edited the article; J.L. and X.D. supervised the study. All authors reviewed the manuscript.
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DATA AVAILABILITY STATEMENT
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CONFLICT OF INTEREST
The authors declare there is no conflict.