Spatial gradient of urban green field influenced by soil sealing.

Rapid urbanization and land expansion persistently shrink urban green field, which accelerates soil sealing and land degradation. Spatio-tempral pattern analysis of green field caused by soil sealing contributes to its protection but quantitative tools are rare. Taking Shanghai-Hangzhou Bay Urban Agglomeration (SHBUA) as an example, we interpreted Landsat imagery into three categories: green filed (such as farmland, grass, forest etc.), gray field (impervious surface) and water bodies in 1994, 2003, 2009, and 2015. We first analyzed swallowed green field by soil sealing and then calculated density (proportion) of green field in concentric rings using gradient analysis. Results show that green field density increases slowly around the city center followed by a sharp increase from urban core areas to urban fringe, and then slowly increases again until at a stable level, presenting an S-shape overall. We proposed an S-shaped function that can fit the spatial gradient of green field density well in nine represented cities. We further compare spatial gradients of densities of green field and gray field. This study provides a quantitative tool to characterize the spatial distribution of green field within cities, which supports to find hotspots of green field loss due to soil sealing and further identify prior areas for green field protection.