Background: Among the many types of lakes, the barrier lake attracts special attention. China’s Lake Jingpo is one of the world’s rare such alpine lakes. Spring freshet flooding and summer rainfall flooding are factors that significantly increase barrier lake instability while also impacting water quality. Results: This study constructed a hydrodynamic water-quality model to simulate the impacts of spring freshet flooding, summer rainfall flooding, and 30-year frequency rainfall flooding on the flow-field and water quality at Lake Jingpo. Results showed that the lake-area flow field was generally weak (mostly lower than 0.015 m/s), but spring freshet flooding in April and summer rainfall flooding in August-September increase flow velocities to 0.045 m/s and above, much higher than in other months. Mudanjiang is the largest river that enters Lake Jingpo. Its flow reaches 4.81×10 8 m 3 , 29.77×10 8 m 3 , and 58.4×10 8 m 3 during the spring freshet flooding, summer rainfall flooding and 30-year frequency rainfall flooding period, respectively. The longest diffusion distances were measured from the lake mouth to the point of impact; these were 16.3 km, 33.1km, and 43.6km for the above periods, respectively. Our research revealed that precipitation played an important role in seasonal water quality at Lake Jingpo, and larger amounts of precipitation, longer diffusion distances, and increased pollutant concentrations. Conclusions: Compared to the effects of the spring freshet flooding period, the summer rainfall flooding and 30-year frequency rainfall flooding period more significantly affected water quality at Lake Jingpo. There was more overall precipitation, longer diffusion distances, and increased pollutant concentrations in the lake area. Alpine barrier lake environments are very fragile, which need greater care efforts and more stringent measures to control pollution sources throughout entire catchment area.

Background: Barrier lakes, formed by river interception, are particularly influenced by the upstream river. As such, spring freshet flooding and summer rainfall flooding due to wet precipitation (e.g. snow and rainfall) may sharply increase the concentrations of nitrogen, phosphorous and carbon compounds in barrier lakes. Too much nutrients will lead to lake eutrophication. In this study, we used Lake Jingpo, the world’s second largest alpine barrier lake, to examine the impact of spring freshet flooding and summer rainfall flooding on its water quality by building a hydrodynamic water-quality model with MIKE 21. Results:The MIKE 21 HD hydrodynamic model and MIKE 21 AD convection and diffusion module were calibrated using meteorological data, hydrological data, and water quality data collected in 2018. All errors were in the acceptable range. According to model simulation results, the flow velocity in Lake Jingpo is generally weak (mostly lower than 0.015 m/s), but it increases to 0.045 m/s and above during spring freshet flooding (April–May) and summer rainfall flooding (August–September), which is much higher than in other months. The flow volume of its largest inflowing river reaches 4.81×108 m3, 29.77×108 m3, and 58.4×108 m3 during spring freshet flooding, summer rainfall flooding, and 30-year frequency rainfall flooding period, respectively. The longest diffusion distances from the lake mouth to the downstream way are 16.3 km, 33.1 km, and 43.6 km during the spring freshet flooding, summer rainfall flooding, and 30-year frequency rainfall flooding period, respectively. A larger amount of precipitation leads to longer diffusion distances and increased concentrations of total nitrogen (TN), total phosphorous (TP), chemical oxygen demand (CODMn), and ammonia-nitrogen (NH4-N) in the lake. Conclusions: The sudden increase in water volume during spring flooding and summer rainfall flooding led to the rapid spread of nutrients and pollutants carried by the water into the barrier lake, resulting in a deterioration of lake water quality. In addition to pollution source control measures, ecological restoration and the construction of a buffer system in the catchment are very important measures to effectively improve the buffer capacity of barrier lakes in light of spring freshet flooding and summer rainfall flooding.