The influence of river‐to‐lake backflow on the hydrodynamics of a large floodplain lake system (Poyang Lake, China)

Abstract

Backflow, the temporary reversal of discharge at the outlet of a lake, is an important mechanism controlling flow and transport in many connected river-lake systems. This study used statistical methods to examine long-term variations and primary causal factors of backflow from the Yangtze River to a laterally connected, large floodplain lake (Poyang Lake, China). Additionally, the effects of backflow on the lake hydrology were explored using a physically based hydrodynamic model and a particletracking model. Although backflow into Poyang Lake occurs frequently, with an average of 16 backflow events per year, and varies greatly in magnitude between years, statistical analysis indicates that both the frequency and magnitude of backflow reduced significantly during 2001-2010 relative to the previous period of 1960-2000. The ratio of Poyang Lake catchment inflows to Yangtze River discharge can be used as an indication of the daily occurrence of backflow, which is most likely to occur during periods when this ratio is lower than 5%. Statistical analysis also indicates that the Yangtze River discharge is the main controlling factor of backflow during July to October, rather than catchment inflows to the lake. Hydrodynamic modelling reveals that, in general, backflow disturbs the normal northward water flow direction in Poyang Lake and transports mass~20 km southward into the lake. The effects of backflow on flow direction, water velocities and water levels propagate to virtually its upstream extremity. The current study represents a first attempt to explore backflow and causal factors for a highly dynamic floodplain lake system. An improved understanding of Poyang Lake backflow is critical for guiding future strategies to manage the lake, its water quality and ecosystem value, given proposals to modify the lake-river connectivity.

Investigation of Water Temperature Variations and Sensitivities in a Large Floodplain Lake System (Poyang Lake, China) Using a Hydrodynamic Model

“…It is reasonable to expect some amount of water temperature stratification during high water-level seasons. Although previous work has concluded that Poyang Lake is mixed well in most areas (i.e., 85% of the lake) throughout different seasons [36,39], the depth-averaged approach may not be quite adequate for the prediction of water temperature in some potentially stratified regions. As such, the extension of a 3D representation in the lake is useful to account for the vertical distribution in the water temperature.…”

“…Compared to the Landsat data, the simulated water temperature in the main flow channels across the lake length is better reproduced than that shown in the western floodplain areas. The reason can be attributed to the influence of the complex hydrodynamic field in the lake, which likely exerts more influence on the prediction of floodplains than main flow channels [36,42]. Additionally, the simulated water temperature in some lake center regions was not in agreement with that shown by Landsat (e.g., 26 April and 11 October), with an average temperature difference up to approximately 3 • C (Figure 6d,f).…”

“…The shorelines of Poyang Lake are tortuous, with a maximum length of 170 km and a mean width of 17 km. The bottom morphology of the lake includes islands, shallow floodplains and deep flow channels (Figure 1), exhibiting complex topographical features [36]. The lake is characterized by lacustrine and riverine morphological processes that exhibit distinct seasonal variation [37].…”

“…The lake is characterized by lacustrine and riverine morphological processes that exhibit distinct seasonal variation [37]. In general, 85% of the lake is shallow with a water depth of less than 6 m during wet seasons [36]. The Lake appears to exhibit an isothermal mixed layer [38] for the entire waterbody for approximately 95% of the depth profiles (i.e., water temperature differences <1 • C at 35 sites), and corresponding velocity differences are generally less than 0.2 m/s [39], indicating that the vertical stratification of the lake can be neglected.…”

“…Model calibration and validation are described in detail by Li et al [42]. The MIKE 21 model reproduces the dynamics of water levels, water surface areas and water flow patterns of the lake and has already been applied to various cases of Poyang Lake [28,36,40,43,47]. Other aspects of the model construction can be found in the above references, and therefore, only a brief description is given in this section.…”