Simulation of spatiotemporal variability in nitrate leaching from farmland in the vadose zone of coastal alluvial basin of Dagu River, China

Understanding the spatiotemporal dynamics of groundwater recharge and nitrate leaching driven by agricultural activities is essential for the sustainable management of groundwater resources in agricultural regions. In this study, we developed a regional-scale process-based modeling framework using Hydrus-1D to simulate long-term water flow and nitrate transport in the vadose zone of the Dagu River Basin in eastern coastal China. Groundwater recharge fluxes （21.21–832.31 mm） exhibited strong interannual variability and were significantly correlated with precipitation and irrigation. Nitrate leaching fluxes (9.33–1049.94 kg ha−1 yr−1) closely followed the temporal patterns of groundwater recharge, indicating that recharge is the dominant driver of nitrate leaching dynamics. Spatial variations in groundwater recharge and nitrate leaching were primarily controlled by land-use types and vadose zone thickness. In the vadose zone, water inputs were primarily supplied by precipitation (71.22 %) and irrigation (26.03 %), whereas water outputs were dominated by evapotranspiration (67.77 %) and groundwater recharge (21.40 %). Nitrogen inputs were overwhelmingly derived from chemical fertilizers (94.19 %), with major nitrogen losses occurring through nitrate leaching (29.50 %), root uptake (27.60 %), and denitrification (26.60 %). The deep vadose zone served as an important reservoir for nitrate storage and exerted a significant influence on groundwater quality. These findings provide critical insights for evaluating long-term nitrate contamination risks and developing sustainable nutrient and water management strategies in agricultural landscapes.