Flood dynamics of large lake floodplain systems are typically complex. This paper analyses the characteristics and factors that influence the hysteresis of water area–stage curves for Poyang Lake, the largest freshwater lake in China characterized by complex geomorphology and upstream–downstream exchange conditions. For this purpose, a two-dimensional hydrodynamic model (EFDC) based on seven scenarios is established. The results indicate that the area–stage curve presents significant hysteretic characteristics due to different water surface gradients that emerge during the water-rising and water-falling periods. Counter-clockwise, clockwise, and splayed hysteresis directions observed at the northern, southern, and central hydrometric stations, respectively, are found in Poyang Lake for the first time. Upstream catchment inflows and Hukou stage reflecting the downstream condition are the main factors that influence hysteresis. The temporal fluctuation of catchment inflows and Hukou stage has a remarkably positive impact on hysteresis, namely, an increase in fluctuation brings about a larger hysteresis. The effects of magnitude change in the two factors on hysteresis are opposing. Catchment inflows are positively related, while the decline of the Hukou stage will produce a more pronounced hysteresis. The outcomes of this study will benefit the water management of Poyang Lake and other similar large lakes.

The Danjiangkou Reservoir in China is characterized by significantly high concentrations of total nitrogen (TN), and the sources are not clear. Recently, research on this reservoir has focused on the N cycle, the spatial and temporal distribution characteristics of N, and the factors influencing N concentration. Significant temporal and spatial differences in TN concentrations exist, both in the reservoir area and the tributaries. N concentration in the area is affected by numerous factors, including N transported by tributaries, nonpoint source pollution around the reservoir, internal N release, and atmospheric N deposition. Moreover, a dam heightening project led to a larger water-fluctuation zone and more bays in the reservoir, directly affecting its N cycle. However, further research is required to explore the N cycle on a large watershed scale in the Danjiangkou Reservoir and upper stream areas, determine N pollution sources using satellite remote sensing, and conduct simulations of a water body N cycle model based on data fusion. Although the issue of excessive TN has been alleviated to some extent by the South-North Water Diversion Project, the excessively high TN concentrations require more research to aid the implementation of N-reducing strategies.

Lake eutrophication is a key point in water environmental problems in the world. Spatiotemporal variations of nutrients and chlorophyll-a and eutrophication index in Poyang Lake, the largest freshwater lake in China, are analyzed in this paper basing on field observation data at 17 sampling points from 2011 to 2016. The results show that nutrient concentrations have obvious seasonality characteristics and present bigger values in the low water period than the high water period. The peak value of chlorophyll-a concentration appears in July and October respectively. As a whole, the eutrophication index in the low water period is higher than the high water period, and the maximum value is found in October which mainly due to the high chlorophyll-a concentration. Poyang Lake is at light eutrophication level from 2011 to 2012, and mesotrophic from 2013 to 2016. From the perspective of space, nutrient concentrations in the southern part of the lake is higher than the northern part in general, and chlorophyll-a and eutrophication index show the similar law. This paper makes a quantitative analysis for spatial and temporal variations of eutrophication which benefit the water management especially water pollution control in Poyang Lake