关键词: 生态风险, 胁迫, 时空变化, Marxan模型, 情景, 管控成本

Abstract:

Socioeconomic activities and population have become highly concentrated in urban areas in China because of the rapid urbanization process since the reform and opening up. However, intensified human activities and climate change have led to increased risks of urban flooding and high summer temperatures, which pose enormous pressure on urban ecosystems. This necessitates the urgent assessment and control of these two ecological risks in urban areas. Urban areas are complex natural socioeconomic ecosystems with numerous influencing factors, sources of ecological risk stressors, and diverse risk receptors. Current regional ecological risk assessment methods inadequately address multiple risk stressors and receptors, making them difficult to apply to urban flooding and summer high-temperature risk assessments. In addition, intensive socioeconomic activities in urban areas significantly constrain the implementation of ecological risk-control measures. Therefore, establishing urban flooding and summer high-temperature risk assessment methods that reflect multiple risk stressors and receptors, and developing risk control zone identification methods that consider socioeconomic factors are crucial for urban ecological risk assessment and control. The area within the Fifth Ring of Beijing (AWFRRB) is located on the North China Plain, and features flat terrain and a typical temperate semi-humid continental monsoon climate. This area encompasses Dongcheng, Xicheng, Chaoyang, Fengtai, Haidian, Shijingshan, and Daxing Districts, including 109 sub-districts and townships, with a total area of 667.28 km². The main land cover type in this area is impervious surface, accounting for 67.88%, followed by green space, accounting for 27.21%. The AWFRRB has the highest intensity of socioeconomic activities in Beijing because of its high population density; well-developed infrastructure; developed commerce; prosperous digital economy; and frequent cultural, scientific, and international exchange activities. Although the quality of the ecological environment in this area has improved significantly in recent years, ecological risks, such as urban flooding, extreme summer temperatures, soil heavy metal pollution, and air pollution still exist, affecting residents’ health and socioeconomic activities to varying degrees. Taking AWFRRB as an example, this study developed assessment methods for urban flooding and summer high-temperature risks, considering multiple risk stressors and receptors based on data including land use, meteorology, soil, economy, population, and buildings. The assessment methods consider heavy rainfall and impervious surfaces as risk stressors and buildings as risk receptors and use the Urban Flood Risk Mitigation module of the Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST ) tool to simulate urban flooding. This module employs the Soil Conservation Service Curve Number (SCS-CN) method to calculate the potential economic losses of different types of buildings based on the simulated waterlogging depth. For summer high temperature risk, average and extreme high temperatures were considered risk stressors, urban population as risk receptors, and the proportion of the elderly and children as receptor vulnerability. A comprehensive index method was used to calculate risk values. The simulated and calculated risks were classified into five levels: low, relatively low, medium, relatively high, and high, using the natural breaks and equal interval methods. Hot and cold spots were identified using Getis-Ord G* analysis. We further analyzed the spatiotemporal characteristics of both risk types at the sub-district/township level. Finally, this study used road network density, population density, and GDP intensity as socioeconomic constraint indicators (socioeconomic costs of risk control) for risk control. Marxan, a systematic conservation planning tool based on a simulated annealing algorithm, identified priority risk control areas under two scenarios (with and without socioeconomic constraints) and different risk control targets and analyzed the reasons for the differences in priority risk control area identification results under different scenarios. The results showed that subdistricts/townships with relatively high and high flooding risk levels were mainly distributed in the central and northern parts of the study area, respectively. Sub-districts/townships such as the Jinrongjie Sub-district and Xinjiekou Sub-district in Xicheng District, Donghuamen Sub-district, and Jiaodaokou Sub-district in Dongcheng District, Xueyuanlu Sub-district, Huayuan Sub-district in Haidian District, and Xiaoguan Sub-district and Heping Sub-district in the Chaoyang District exhibited consistently high risk levels in both years. The overall flooding risk in 2021 was significantly lower than that in 2008, with the number of sub-districts/townships at relatively high- and high-risk levels being only 11% of that in 2008, while the number at relatively low- and low-risk levels was 220% higher than that in 2008. Sub-districts/townships with relatively high and summer high-temperature risk levels were mainly distributed in the southern part of the study area in 2008. In 2021, the number of sub-districts/townships at relatively high- and high-risk levels was 92% higher than that in 2008, showing an expansion trend toward the central and northern parts of the study area. In the scenario that did not consider the socioeconomic costs of risk control, the identified priority risk control areas were concentrated in the central part of the study area and gradually expanded outward as the risk control targets increased. In the scenario considering socioeconomic costs, the identified priority risk control areas were concentrated in the southern part of the study area, showing a trend of spreading from the western, southern, and eastern parts toward the center as the risk control targets increased. When socioeconomic constraints were not considered, the identification of priority risk control areas was primarily based on the risk values. The central part of the study area has a denser population, more concentrated buildings, and more intense human activity than the surrounding areas, thus facing relatively higher ecological risks that require priority. When considering socioeconomic constraints, although the flooding and summer high-temperature risk values in the southern part of the study area were relatively low in 2021, the lower road network density, population density, and GDP intensity compared with the central and northern areas meant lower ecological risk control costs. In this case, the Marxan model balanced the reduction in ecological risks and incurred lower socioeconomic costs, shifting the main areas identified for priority risk control from the central area to the southern areas surrounding it. This study provides effective methods for assessing flooding and summer high-temperature risks in highly urbanized areas and a scientific basis for ecological risk control in the AWFRRB.

Key words: ecological risk, stressor, spatio-temperal variation, marxan model, scenarios, control cost