Spatial-temporal variations in urbanization in Kunming and their impact on urban lake water quality

As a radiation centre or hub in Southeast Asia and South Asia and the only Chinese metropolis bordering other ASEAN countries, Kunming has experienced extensive urbanization since the 1980s. This type of human activity has been accompanied by environmental deterioration, especially water quality pollution, in Dianchi Lake, China's sixth largest freshwater lake. This study reveals that the urbanization process has led to land degradation over the past 30 years. First law of geography was introduced to analyze the effects of impervious surface area (ISA) expansion on the lake water environment. The results indicate that the ISA encompassed the lake and radiated outwards. Since the 1990s, approximately 35% of natural surfaces transformed to ISA, and the urban district density exploded from one in 2001 to four in 2017. The urban district area density exploded from 0.06% to 12.05% over the past 17 years. During the past 30 years, the growth rate of ISA exceeded 21 km(2) a(-1) and even reached 38 km(2) a(-1) between 2004 and 2017. The spatial pattern of lake water quality was determined by the spatial influences of ISA (G). Then the relationship between water quality indicators and G was analyzed, and the threshold between G and water quality was investigated using an exponential regression analysis method. The results of the correlation analysis indicated that a significant correlation exists between G and lake water quality; although the lake water quality showed a cyclical change, the G impact on the lake water quality was notable. This research proposed an effective method to calculate the spatial influence of the impervious surface on the water quality to reveal the effect of urbanization on the water environment quantitatively and helps us to enhance the understanding of ISA expansion impact on lake environment. Moreover, this research provides decision supports for the environmental protection and management of the lake and empirical evidence regarding urban development impacts on watershed planning.