The nonlinear effect of new urbanization on water pollutant emissions: Empirical analysis based on the panel threshold model

Rapid urbanization has led to a significant increase in water consumption and wastewater discharge. Balancing the relationship between urbanization development and water pollutants emissions is crucial for the sustainable development of the country. Given the uneven regional economic development and resource distribution in China, exploring the relationship between new urbanization and water pollution emissions cannot be limited to a single perspective such as population urbanization. This study developed a comprehensive evaluation index system for new urbanization level. Based on data from 30 provincial-level regions in China from 2006 to 2020, a Panel Threshold Regression Model (PTRM) was used to explore the nonlinear relationship between the new urbanization level and water pollution discharge. The research results show that China's new urbanization level (NUBL) and its subsystems, including population urbanization (P-NUBL), economic urbanization (E-NUBL), and spatial urbanization (SP-NUBL), all have a double threshold effect on chemical oxygen demand (COD) emissions. The promoting effect of NUBL and E-NUBL on COD emissions gradually increased in the later stage of the study. P-NUBL and SP-NUBL show a trend of inhibiting COD emissions after crossing the dual threshold values. Social urbanization (S-NUBL) and ecological urbanization (EL-NUBL) had no threshold effect, but they also had a promoting effect on COD emissions. In addition, the speed of new urbanization in eastern China was significantly faster than that in central and western China, with provinces such as Beijing, Shanghai, and Jiangsu being the first to enter the high threshold stage. The central region began to gradually enter the middle threshold stage, but provinces such as Hebei, Henan, and Anhui are still in the high pollution and high emission stage. The level of new urbanization in western China is relatively low, and future development should prioritize economic construction. Provinces with high thresholds and low water pollution emissions still need to be developed. The results of this study have important implications for promoting the harmonious development of water-saving and sustainable urban development in China.