Improving Nitrogen and Phosphorus Modeling in Agricultural Drainage Systems: Field-Based Mass Balance and Derivation of Correction Coefficients for Vegetated and Dredged Channels

Abstract Agricultural drainage water from paddy fields is a major nonpoint source of nitrogen (N) and phosphorus (P) pollution, posing serious risks to downstream water bodies. This study investigated the effectiveness of natural vegetated drainage channels compared to dredged channels in reducing nitrogen and phosphorus losses under different nutrient loading conditions in rice-growing areas of northern Iran. A field-scale experiment was conducted using two third-order earthen drainage channels: a dredged channel lacking vegetation and a nondredged channel densely covered with aquatic plants, including Phragmites australis , Typha , and Glyceria . Two fertilizer input levels (high and low concentrations of urea and triple superphosphate) were applied, resulting in four experimental treatments. A comprehensive mass-balance approach was employed to quantify nutrient transport pathways, including surface outflow losses, deep percolation (leaching), sediment adsorption, and biologically mediated processes such as phytoremediation and denitrification. Results demonstrated that, despite its shorter length and smaller cross-sectional area, the vegetated nondredged channel significantly reduced nitrogen and phosphorus losses at the outlet compared to the dredged channel. Phosphorus losses were reduced by up to 74%, while nitrogen losses decreased by approximately 4%, primarily due to increased hydraulic residence time, reduced flow velocity, and enhanced biological activity induced by vegetation. When normalized per 100 m of channel length, the vegetated channel exhibited substantially higher nutrient removal efficiencies, achieving up to 75% nitrogen and 68% phosphorus removal. In contrast, the dredged channel showed higher deep leaching losses, indicating an increased risk of subsurface nutrient transport and potential groundwater contamination. Mass-balance analysis revealed that sediment adsorption dominated nutrient removal in the dredged channel, whereas phytoremediation and denitrification were the primary removal mechanisms in the vegetated channel, offering more permanent nutrient retention. Overall, the findings highlight the critical role of vegetation in enhancing the water-quality function of agricultural drainage systems. Minimizing routine dredging and promoting vegetated drainage channels, combined with optimized fertilizer management, represent effective and sustainable strategies for reducing nutrient export from paddy field networks.