Driving forces of impervious surface in a world metropolitan area, Shanghai: threshold and scale effect.

Shanghai is one of the largest metropolitan areas in the world, during the rapid urbanization of the past decades, impervious surface expanded dramatically and became a main factor influencing surface water quality. Thus, exploring the driving forces of impervious surface has great implications in such metropolitan area. In this study, an impervious surface coefficient method (ISC) was used to measure the percentage of total impervious area (PTIA) of Shanghai; regression analysis was conducted to define the relationship between PTIA and three socio-economic factors, population density, unit area gross domestic product, and unit area industrial output at the city and district scale. Results showed that the industrial land use generated the highest ISC value, followed by high-density residential. Strong correlations were showed between PTIA and socio-economic indicators, in which population density was the most significant. Threshold effect was presented that when population density was higher than 15000 per/km2, this relationship would become less significant and PTIA remained stable. Similar effects were found when unit area gross domestic product exceeded 125 million yuan/km2. Scale effect was also discussed that the relationship was more significant at city scale than district. An improved understanding of the threshold effect and scale effect will help guide future urban planning and design new urban ecosystem policies.