Identifying city-scale potential and priority areas for retrofitting green roofs and assessing their runoff reduction effectiveness in urban functional zones

Green roof is a popular green infrastructure type implemented in highly urbanized areas, but it is difficult to reliably translate local results to urban or watershed scales. It is necessary to develop appropriate methods to quantitatively identify prioritization of green roof retrofitting at urban functional zones (UFZs) where green roof implementation is most needed and suited. In urban regions, various UFZ types may have diverse demands and suitability levels of green roof retrofitting, thus they should be given different spatial prioritization in location planning of green roof practices. In this paper, the spatial potential of green roof retrofitting at UFZs scale was estimated and mapped, a priority index considered the demand for green space, runoff risk and suitability for roof greening of UFZs was developed to identify the priority areas for retrofitting green roofs, and runoff reduction effectiveness of hypothetical green roof retrofitting scenarios under three storm return periods were assessed. Results indicated that the total potential area for green roof retrofitting is 143.62 km2 within the Fifth Ring Road of Beijing, which accounts for 21.52% of the study area; The developed priority index was able to appropriately reflecting the differences on demands and suitability levels of green roof retrofitting in various UFZs; Compared with the baseline scenario (0% retrofitting), the average runoff volume was reduced by 1.34%–2.73% with 10% green roof retrofitting; The maximum average reduction effectiveness achieved was 9.17%–19.05% under a 100% green roof retrofitting scenario. The runoff coefficient within COZ (commercial zone), EGZ (education & government zone) and HRZ (high-density residential zone) was significantly reduced as green roof retrofitting percentages increased. These results can inform urban planners of a meaningful estimator of UFZs scale green roof potential and support for identifying the priority areas for partially implementing green roofs.