Scenario-based evaluation of nutrient pollution control in agricultural drainage ditch–pond systems using the QUAL2K model

Agricultural drainage ditches are vital for farmland water management but often contribute to nutrient enrichment and downstream eutrophication. In this study, the QUAL2K model was calibrated with 2022 field data and validated using 2023 monitoring results to simulate nitrogen and phosphorus transport under steady-state conditions. The model demonstrated good overall agreement with the observed nutrient concentrations, with R² values ranging from 0.78 to 0.98 and NSE values ranging from 0.70 to 0.96 across the calibration and validation datasets, confirming its suitability for scenario analysis. Four management scenarios were evaluated based on the validated 2023 baseline: (S1) 20% flow reduction, (S2) 20% nutrient load reduction, (S3) addition of a treatment pond/wetland, and (S4) combined management. Results showed that all scenarios improved water quality, with reductions in total nitrogen (TN), ammonia nitrogen (NH₄-N), total phosphorus (TP), and nitrate nitrogen (NO₃-N) ranging from modest (≤15% in S1) to substantial (up to 54% in S4). The combined scenario achieved the greatest overall improvement, demonstrating that integrated hydraulic, pollution control, and ecological measures are the most effective and sustainable strategy for mitigating nutrient pollution in agricultural drainage systems.