Evolution characteristics of total nitrogen in an urban river network under different rainfall and hydraulic conditions

Rainfall-driven total nitrogen (TN) pollution is a persistent threat to urban river networks with low hydraulic-gradients. A coupled model was used for runoff and TN simulation in a typical river network. Rainfall intensity exerted a dual influence on TN dynamics. As rainfall intensity increased, TN peaks occurred earlier, with a stronger temporal response upstream than downstream. Enhanced dilution and export reduced the terminal TN concentration by 26.3% network-wide under the 100-year rainfall relative to the 10-year rainfall. Peak TN concentration responded nonlinearly, decreasing by 5.2% under the 50-year rainfall and increasing by 1.4% under the 100-year rainfall relative to the 10-year event, reflecting competition between pollutant buildup-washoff and hydraulic dilution. Channel addition alone did not improve network-wide TN control: terminal and peak concentrations increased from 11.19 to 18.33 mg/L and from 87.84 to 90.14 mg/L, respectively. In contrast, sluice-controlled diversion substantially mitigated TN. Fully opening all five upstream sluices reduced terminal and peak concentrations from 18.33 to 2.37 mg/L and from 90.14 to 22.79 mg/L, respectively. Under comparable diversion volumes, distributed half-opening produced a lower network-wide peak than fully opening two sluices locally. These findings distinguish structural connectivity from effective hydraulic mobility and provide a mechanistic basis for water-quality regulation in gated river networks.