Cross-Scaling Approach for Water-Flow-Regulating Ecosystem Services: A Trial in Bochum, Germany

Water-flow-regulating ecosystem services (ESs) determine the regulation of hydrological flows on the ground’s surface. A lack of water-flow-regulating ESs would cause environmental problems such as heavy rainfall runoff and urban water logging, leading to floods affecting well-being, especially in dense urban areas. Research on water-flow-regulating ES supply–demand relationships in urban areas is urgently needed to better support the management of urban surface runoff. However, matching the supply–demand relationships of water-flow-regulating ESs remains challenging. In this contribution, a cross-scale approach linking the supply–demand assessment of water-flow-regulating ESs on a macroscale and the evaluation of the constructed urban environment on a microscale was developed. The approach was applied in the city of Bochum, Germany, as a trial of bridging the “science–practice gap”. Our findings show that the supply–demand budget of water-flow-regulating ESs in Bochum exhibits an urban–rural difference and is also partially influenced by land cover transformations such as vegetation degradation. In addition, further assessment of the constructed urban environment confirmed the result from the assessment of water-flow-regulating ESs based on the understanding of the urban hydrological cycle in Bochum. To account for the mismatch in the supply–demand budget, we classified the typical superior and inferior forms of urban water-flow-regulating ESs through field research on the same extreme areas to summarize the operable optimization, enhancement, and protection suggestions for urban construction decision makers. Finally, the cross-scale approach was approved as a possible way to bridge the “science–practice” gap for water-flow-regulating ES research in urban areas.