Developing nutrient pollution management strategies on a watershed scale under climate change

The existing management frameworks were not precisely tailored to effectively reduce nutrient pollution in specific watersheds, where both nonpoint source (NPS) and point source (PS) discharges necessitate simultaneous treatment. Such situations are prevalent in developing countries, where the spatial distribution of PS is often characterized by fragmentation and overlapping with NPS. To bridge the gap, an Integrated Watershed Water Quality Management (IWWQM) framework was developed in this study. The critical areas (CAs) for priority governance of PS and NPS were identified separately, thus generating candidate plans at different size scales of CAs. Optimal plans were further obtained under various future climate scenarios, designed to address the impact of future climate uncertainties on water quality. Three robustness metrics, namely, expected value (EV), second worst case (SW) and regret index (RI), were introduced to examine the effectiveness of the optimal plans. On the premise of meeting the water quality thresholds, the three packed metrics and the total cost were used to select representative plans indicative of both climate-robustness and cost-effectiveness. This framework was implemented and validated in a typical watershed in China. The case study revealed that out of 1,149 candidate plans, a total of 19 plans were determined to meet water quality thresholds. Among these plans, 2 were ultimately recommended due to their relatively low-cost budget (<100 million CNY) and substantial marginal benefits. The results suggested that attaining optimal plans that are more in line with reality is inseparable from the approaches of considering PS control and climate uncertainty. Thus, the proposed IWWQM framework can serve as a powerful tool for meeting sustainable water quality requirements in water environmental management.