[Characteristics of Pollutant Dynamics Under Rainfall-Runoff Events in the Chaohe River Watershed]

In semi-arid and semi-humid areas, the occurrence of non-point source nutrient pollution is mainly driven by rainfall-runoff events, and nutrient loss under rainfall events determines annual total pollution load. Therefore, research on riverine nutrient dynamics under rainfall-runoff events in flood seasons is critical for simulating and controlling pollution load in semi-arid and semi-humid areas. The Chaohe River watershed, upstream watershed of Miyun Reservoir in Beijing was considered as study area, water quantity and quality of rainfall-runoff process at Gubeikou and Xiahui stations were monitored synchronously in flood seasons in 2018 and 2019. The results indicated the following:① Among the three rainfall events (E1, E2, and E3), E1 had the highest precipitation and rainfall intensity, and the corresponding discharge and pollutant concentrations were the highest. ② Under different rainfall events, the pollutant concentrations and their variations were different. The variations of concentrations of total nitrogen (TN), ammonia (NH₄⁺-N), nitrate (NO₃^--N), total phosphorus (TP), and total suspended solids (TSS) were similar to the discharge process under the heavy rainstorm event (E1) and the rainstorm event (E3). The concentrations of total nitrogen (TN), ammonia (NH₄⁺-N), total phosphorus (TP), and total suspended solids (TSS) were similar to the discharge process under the heavy rain events (E2), but the variations of nitrate (NO₃^--N) concentrations were opposite to those in the discharge process. ③ The concentrations and variations of different forms of pollutants were different under different rainfall events. Under the event of strong rainfall erosion (E1 and E2), the concentrations of particulate pollutants varied significantly, being positively correlated with that of total suspended solids (TSS). For the rainfall event that did not cause soil erosion (E3), the forms of nitrogen and phosphorus were dominated by total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) respectively, whose variations were mainly related to discharge. ④ The discharge and pollutant concentrations at each station varied under different rainfall events. Heavy rainfall erosion was more obvious at Gubeikou station, causing significant variations in discharge, TP, and TSS. Therefore, these results can be used to determine migration patterns of non-point source pollutants caused by rainfall-runoff events and provide references for water quality prediction and control in flood seasons.