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.