Hydrodynamic and water quality modeling of a large floodplain lake (Poyang Lake) in China

Floodplain lakes are valuable to humans because of their various functions and are characterized by dramatic hydrological condition variations. In this study, a two-dimensional coupled hydrodynamic and water quality model was applied in a large floodplain lake (i.e., Poyang Lake), to investigate spatial and temporal water quality variations. The model was established based on detailed data such as lake terrain, hydrological, and water quality. Observed lake water level and discharge and water quality parameters (TN, TP, COD_Mn, and NH₄-N) were used to assess model performance. The hydrodynamic model results showed satisfactory results with R² and MRE values ranging between 0.96 and 0.99 and between 2.45 and 6.14%, respectively, for lake water level simulations. The water quality model basically captured the temporal variations in water quality parameters with R² of TN, TP, COD_Mn, and NH₄-N simulation ranges of 0.56-0.91, 0.44-0.66, 0.64-0.67, and 0.44-0.57, respectively, with TP of Xingzi Station and COD_Mn of Duchang Station excluded, which may be further optimized with supplementation of sewage and industrial discharge data. The modeled average TN, TP, COD_Mn, and NH₄-N concentrations across the lake were 1.36, 0.05, 1.99, and 0.48 mg/L, respectively. The modeled spatial variations of the lake showed that the main channel of the lake acted as a main pollutant passageway, and the east part of the lake suffered high level of pollution. In addition, consistent with previous water quality evaluations based on field investigations, water quality was the highest (average TN = 1.35 mg/L) during high water level periods and the poorest (average TN = 1.96 mg/L) during low water level periods. Scenario analysis showed that by decreasing discharge of upstream flow by 20% could result in the increase of TN and TP concentrations by 25.6% and 23.2% respectively. In summary, the model successfully reproduced the complex water and pollutant exchange processes in the systems involving upstream rivers, the Poyang Lake, and the Yangtze River. The model is beneficial for future modeling of the impact of different load reduction and other hydrological regime changes on water quality variation and provides a relevant example for floodplain lake management.