Seasonally dynamic thresholds enhance identification of hydrologic and water-quality critical source areas

Agricultural phosphorus and sediment losses remain major water quality concerns in the Great Lakes Basin, necessitating improved identification of Critical Source Areas (CSAs). This study evaluates a modified SWAT-VSA framework against the standard Soil and Water Assessment Tool (SWAT) for seasonal CSA delineation in an agricultural watershed in Southern Ontario, Canada. The SWAT-VSA model integrates Variable Source Area (VSA) hydrology using the Soil Topographic Index (STI) and applies seasonally dynamic thresholds supported by a storm erosion index to capture the timing of sediment and phosphorus export. Both models reasonably reproduced observed streamflow and water-quality trends, though total water yield was underestimated; subsurface flow dominated the hydrologic balance (∼73%). Compared with SWAT, and SWAT-VSA generated higher surface runoff consistent with VSA-driven processes and produced lower and conceptually more consistent estimates of sediment (3.4 t/ha yr^-1) and phosphorus (2.08 kg/ha yr^-1), with statistically significant differences (p < 0.05). Seasonal CSA mapping identified winter and spring as peak risk periods, minimal contributions during summer, and moderate risks in fall. The results demonstrate that coupling VSA hydrology with season-specific thresholds improves the spatial and temporal identification of CSAs and provides a more effective basis for targeting best management practices. The results demonstrate that coupling VSA hydrology with season-specific thresholds improves the spatial and temporal identification of CSAs and provides a more effective basis for targeting best management practices.