Characteristics of Dianchi Lake watershed impervious surface expansion and its effect of water environment during urbanization

Abstract: Since the 20th century, urbanization is the most significant process of human activities, which is directly shown as the impervious surface area (ISA) expansion. ISA has become the main urban landscape. Although ISA rate just occupy no-iced surface about 0.2%-2.7%, the impacts on human and nature are extremely significant. Intensive research demonstrated that the relationship between urbanization and water quality was nonlinear. The foreigner researchers revealed that the deterioration of water environment could not be reversed when the ISA rate exceeded 25%. But the threshold influenced by natural and human conditions of research area, different areas have different threshold. To date analyzing the changing characteristic and correlation between water quality parameters and ISA are lees carried out. In this paper, the Dianchi lake watershed of Kunming, Yunnan province was studied, and improved normalized difference build-up index (INDBI) and linear spectral mixture analysis (LSMA) algorithm were used to analyze the characteristics of Dianchi Lake watershed impervious surface expansion in the context of rapid urbanization in the past 30 years. Secondly, the distance decay function and gravity model was used to quantitatively analyze the impervious surface area impact on water quality. Thirdly, Morlet wavelet analysis method was used to reveal water parameters periodicity. Results showed that the impervious surface area of Dianchi Lake Watershed expanded fast around Dianchi Lake (α = 0.001), nearly half of the land use/land cover has been changed. The natural surface area in 2017 was approximately 59%, the percent of high-intensity urban area was increased from 0.06% to 12.05%. Under the national strategy background of "One Road, One Belt", "One lake, four core urban area" and "Build the Urban Area on the hill", the number of main district (high-intensity urban areas) changes from 1 in 2001 to 4 in 2017 (Kunming main district, Airport main district, Chenggong main district and Jinning main district), cities are surrounding the Dianchi Lake. 35% of natural surface area was transferred into impervious surface. In the last 30 years, the growth rate of impervious surface area in the Dianchi watershed has been exceeded 21 km2 per year, and the growth rate reached 38 km2 per year from 2004 to 2017. There is a significant correlation between the spatial influence of impervious surfaces and water quality of lakes. The correlation coefficients between water quality parameters (dissolved oxygen (DO), chlorophyll a (Chla), total phosphorus (TP), permanganate index (PI), potential of hydrogen (pH) and transparency (Trans)) and the spatial influence of impervious surfaces were all passed the significance test, which shows that the high speed urbanization is the main cause of frequent eutrophication and blue-green algae bloom in Dianchi Lake Watershed.