Integrated Watershed Models for Assessing Reservoir Water Quality Responses to Vegetative Filter Strip Deployment

An integrated water quality modeling framework that considers both watershed and reservoir scales is essential for evaluating the effectiveness of low impact development, such as vegetative filter strips (VFS), in reducing pollutant loads. In this study, an integrated SWAT–Vollenweider water quality model was developed to identify priority areas for VFS deployment, using the Techi Reservoir watershed as a case study. This integrated approach enables the simulation of changes in sediment and nutrient concentrations in the reservoir in response to land-use changes within the watershed. The model was calibrated and validated using long-term historical data on weather, hydrology, and water quality, demonstrating satisfactory performance in simulating streamflow, sediment, and total phosphorus (TP) loads. Agricultural farmlands were identified as the main sources of sediment and TP contributions per unit area. Ten hydrologic response units with elevated sediment and TP loads were selected for VFS deployment along farmland field edges or riparian zones, with widths of 5 or 10 m. The 5 m riparian VFS scenario achieved the highest specific reduction of nonpoint source pollution in the watershed, i.e., decreasing sediment load by 2.7–38.8 t per ha of VFS and TP load by 4.0–34.4 kg-P per ha of VFS. At the reservoir scale, the riparian VFS scenario reduced TP concentrations in water bodies by 0.5–9.1%, with greater marginal reductions achieved by the 5 m VFS compared to the 10 m VFS. Cost-effective reductions were attained with riparian VFS deployment areas of 9, 13, and 23 ha. Overall, the developed SWAT–Vollenweider model effectively simulates VFS performance in controlling nonpoint source pollution, supporting VFS as a viable strategy to reduce sediment and nutrient discharge in the Techi Reservoir watershed.