2 Study Area
Poyang Lake, the largest freshwater lake in China, is located in the south bank of middle reach of Yangtze River (28°24′ ~ 29°46′N, 115°49′ ~ 116°46′E). It has a drainage basin area of 162,225 km2, occupying about 9% of the Yangtze River basin (Tan, Tao, Jiang, & Zhang, 2015 ). Poyang lake receives inflow via five tributaries, i.e., Xiushui, Ganjiang, Fuhe, Xinjiang, Raohe, and discharges into Yangtze river through a narrow outlet located in Hukou (Fig. 1a). With seasonal changes of inflow from the five tributary rivers, the water level in Poyang Lake varies dramatically through the year and the maximum inundation area could be 13 times larger than the minimum (Feng et al., 2012 ). Extreme variability in water level provides favorable condition for the growth of various types of vegetation, making a unique wetland ecosystem formed in Poyang Lake. The vegetation distribution in the floodplains of Poyang Lake is characterized with ringed pattern along the elevation gradient (Wang, Han, Xu, Wan, & Chen, 2014 ). For example, submerged vegetations (e.g., Vallisneria natans ,Potamogeton malaianus ) are distributed in the lower elevation of floodplains with its surface inundated all year round. In the seasonal-inundated floodplains with its elevation between the water level of dry and flood seasons, some emergent aquatic vegetations (e.g.,Carex cinerascens , Artemisia selengensis , and reeds) grow.
The study area (black square in Fig. 1a) was chosen at the southwest part of Poyang Lake. Two sites, located at floodplains formed in Ganjiang River, were selected and named sites A and B from upstream to downstream the Ganjiang River, respectively (Fig. 1b). Water flows from the south to the north (shown by the black arrows in Fig. 1b, 1c, and 1d). As the water surface was also influenced by wind, the hydrodynamic conditions of our study sites were dominated by combined currents and waves. For each site, five cases (Fig. 1c and 1d) with their distribution vertical to the flow direction were selected for velocity measurements in the elevation between dry and flood water level, so that stem densities can be estimated when the floodplain was not flooded. In order to make comparison, a bare-bed case (i.e., S0) located in the mainstream of Ganjiang River was also considered (Fig. 1c). The dominant vegetations of cases in sites A (A1-A5) and B (B1-B5) were different. For site A, the cases were dominated by Carex cinerascens and Artemisia selengensis , and all the five cases in site B were in thePhalaris arundinacea communities. The specific dominant vegetation of each case was listed in Table 1. Velocity measurements were conducted from August 18 to August 24 in the year of 2015. As Duchang Station (with its location shown in Fig. 1a) is the nearest hydrological station to our study area, variation of water level in our measurement sites was assumed to be same with that of Duchang Station. Just as shown in Figure 2a, water level varied within 0.8% from Aug. 18 to 24, indicating that the hydrological condition was stable during the velocity measurements. The wind directions in our study area were mainly toward north during measurements, which can be indicated from the meteorological records (Fig. 2b) observed by the Poyang Lake Wetland Observation Station, Chinese Academy of Sciences, in Xingzi (Fig. 1a). Therefore, surface waves propagated in the same direction with the flow during measurements.