Modeling phosphorus retention in vegetative filter strips with concentration flow mechanisms: A case study in Northeast China

Vegetative filter strips (VFS) are widely recognized as one of the Best Management Practices (BMPs) for controlling diffuse pollution in agricultural ecosystems, particularly for reducing phosphorus runoff into nearby water bodies. However, concentrated flow (CF), which is common in Northeast China's black soil region due to intense rainfall and widespread sloped farmland, significantly compromises the effectiveness of VFS in pollution control. To address the limitations of current VFS models in representing key phosphorus transport mechanisms, this study proposes a semi-mechanistic equation for total phosphorus (TP) removal. The equation integrates runoff reduction rate (ΔQ), sediment retention rate (ΔE), phosphorus distribution parameter between water and soil (Fph), and soil clay content (%C). Model parameters were calibrated and validated using field data from standard plots (Alfalfa buffer strip, with slopes ranging from 3° to 8°) collected between 2015 and 2024. The equation outperformed existing empirical approaches, achieving higher predictive accuracy (R² = 0.88, NSE = 0.63). When integrated into the VFSMOD-W model, it demonstrated strong stability and broad applicability. Scenario analysis showed that when the concentrated flow ratio (CFR) exceeds 90%, TP removal efficiency drops below 10%, indicating a risk of "filter breakthrough." However, extending the filter strip length to 4 m significantly improves TP removal efficiencies by 17–28% under high CFR conditions. This study offers a quantitative framework for simulating VFS‘s phosphorus removal efficiency under CF disturbance and establishes a scalable optimization strategy. It is particularly valuable for the precision design of vegetative barriers in highly disturbed sloped farmland, providing a robust tool for managing agricultural non-point source pollution.