A problem-oriented approach to integrated planning of urban constructed wetlands

Urban hydrology has evolved in response to the degradation of flow and water quality caused by urban expansion. To enhance urban hydrology and improve urban resilience, the 'Ramsar Wetland City' scheme accredits cities that successfully integrate wetland conservation with sustainable development. According to the Ramsar Convention's wetland city accreditation criteria (2023), Criterion A4 emphasises the wetland restoration and creation for flood regulation and water quality improvement, and Criterion A5 highlights spatial planning and integrated city management. However, current planning for constructed wetlands (CWs) at the catchment scale lacks a framework that coherently links performance goals, adjustable design factors, and the spatial allocation of interventions. This study introduces a problem-oriented approach for planning integrated constructed wetland systems (ICWS) at the catchment scale. This problem-oriented approach is defined as a planning framework in which management questions are organised along three dimensions: flow and water quality indicators, wetland design parameters, and spatial allocation. Our simulation is based on the Water Systems Integrated Modelling framework (WSIMOD). From the perspective of flow and water quality indicators, the strongest co-benefits were found between nitrate and phosphate, with phosphate reductions generally achieving twice the improvement as nitrate reductions. Moderate co-benefits were identified between high flow and pollutant load reduction, while minor co-benefits were observed between low flow and other indicators. In terms of wetland design parameters, surface area shows strong spatial variability, whereas the decay constants for nitrate and phosphate are consistent across the catchment. In terms of the spatial allocation of interventions, downstream CWs tended to enhance these cumulative effects rather than produce isolated improvements, reinforcing their role in system-wide optimisation. The novelty of this study lies in proposing a problem-oriented approach for the integrated planning of urban constructed wetlands across three dimensions, providing a reference for future efforts to develop and assess new wetland cities under the Ramsar Convention.