Sustainability of the use of natural capital in a city: Measuring the size and depth of urban ecological and water footprints.

The Sustainable Development Goals (SDGs) are limited in their ability to measure progress towards environmental sustainability especially at the city level. The aim of this paper is to provide insights into an integrated assessment of urban sustainability, with emphasis on the significance of the maintenance of natural capital stocks. The use of water and land as critical natural capital in Guiyang, a southeast city in China was investigated by bringing together the ecological footprint (EF), water footprint (WF) and corresponding capacity indicators into an improved three-dimensional (i3D) model. Results showed that Guiyang has long been operating in a state of overshoot due to shortage of annual natural capital flows and accumulated depletion of stocks. This is particularly true for land use, whose stocks maintained a relatively stable level of depletion between 2000 and 2014. As of 2014, an EF depth of 6.45 was accumulated. With respect to water use, a shift in the city's role from creditor to debtor was observed in 2004. Industrial use of natural capital has more than tripled over the past 15 years and replaced agriculture to be the main driver of water unsustainability. Overall, Guiyang's economic growth did not show signs of decoupling from the EF and WF. These findings highlight the need for effective policies that would help Guiyang reduce dependency on the use of critical natural capital. Finally, this paper provided an in-depth discussion of the methodological strengths and limitations of the i3D model and concluded that it is able to track the structural and characteristic dynamics of both flows and stocks while avoiding burden shifting across various components within single forms of natural capital from a strong sustainability perspective. Our study enhances understanding of the critical role of natural capital in ensuring urban sustainability and improving human welfare in connection with SDGs.